REV. 0

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 2002

AD9975

Broadband Modem

Mixed-Signal Front End

FEATURES
Low Cost, 3.3 V-CMOS, Mixed Signal, Front End

Converter for Broadband Modems

10-Bit D/A Converter (TxDAC+

)

50 MSPS Input Word Rate
2 Interpolating Low-Pass Transmit Filter
100 MSPS DAC Output Update Rate
Wide (21 MHz) Transmit Bandwidth
Power-Down Modes

10-Bit, 50 MSPS A/D Converter

Fourth Order LPF with Selectable Cutoff Frequency
Dual Mode Programmable Gain Amplifier

Internal Clock Multiplier (PLL)
Two Auxiliary Clock Outputs
48-Lead LQFP Package

APPLICATIONS
Powerline Networking
Home Phone Networking

FUNCTIONAL BLOCK DIAGRAM

TXEN

RXEN

TXCLK

RXCLK

ADIO[9:0]

SPORT

OSCIN

XTAL

AD9975

PGA

LPF

PGA

ADC

REGISTER

CONTROL

CLK-GEN

TxDAC+

AGC[2:0]

TX+

TX

CLK1

CLK2

RX+

RX

GENERAL DESCRIPTION

The AD9975 is a single-supply, broadband modem, mixed
signal, front end (MxFETM) IC. The device contains a transmit
path interpolation filter and DAC and a receive path PGA,
LPF, and ADC required for a variety of broadband modem
applications. Also on-chip is a PLL clock multiplier that pro-
vides all required clocks from a single crystal or clock input.

The TxDAC+ uses a digital 2

× interpolation low-pass filter to

oversample the transmit data and ease the complexity of analog
reconstruction filtering. The transmit path bandwidth is 21 MHz
when sampled at 100 MSPS. The 10-bit DAC provides differen-
tial current outputs. The DAC full-scale current can be adjusted
from 2 to 20 mA by a single resistor, providing 20 dB of additional
gain range.

The receive path consists of a PGA, LPF, and ADC. The program-
mable gain amplifier (PGA) has two modes of operation. One
mode allows programming through the serial port and provides a
gain range from 6 dB to +36 dB in 2 dB steps. The other mode
allows the gain to be controlled through an asynchronous 3-pin
port and offers a gain range from 0 dB to 48 dB in 8 dB steps
with the use of an external gain stage. The receive path LPF
cutoff frequency can be selected to either 12 MHz or 26 MHz.

The filter cutoff frequency can also be tuned or bypassed where
filter requirements differ. The 10-bit ADC uses a multistage
differential pipeline architecture to achieve excellent dynamic
performance with low power consumption.

The digital transmit and receive ports are multiplexed onto a
10-bit databus and have individual TX/RX clocks and TX/RX
enable lines. This interface connects directly to Homelug 1.0
PHY/MAC chips from Intellon and Conexant.

The AD9975 is available in a space-saving 48-lead LQFP pack-
age. The device is specified over the commercial (40

°C to

+85

°C) temperature range.

TxDAC+ is a registered trademark and MxFE is a trademark of Analog Devices, Inc.

REV. 0

AD9975SPECIFICATIONS

Test

Parameter

Temp

Level

Min

Typ

Max

Unit

OSC IN CHARACTERISTICS

Frequency Range

Full

MHz

Duty Cycle

°C

Input Capacitance

°C

III

Input Impedance

°C

III

100

CLOCK OUTPUT CHARACTERISTICS

CLKA Jitter (F

CLKA

Derived from PLL)

°C

ps rms

CLKA Duty Cycle

°C

III

±5

TX CHARACTERISTICS

× Interpolation Filter Characteristics

TX Path Latency, 2

× Interpolation

Full

DAC

Cycles

Pass-Band Flatness 0 MHz to 20.7 MHz

Full

0.8

Stop-Band Rejection @ 29.3 MHz

Full

TxDAC

Resolution

Full

Bits

Conversion Rate

Full

100

MHz

Full-Scale Output Current

Full

Voltage Compliance Range (TX+ or TX AVSS) Full

0.5

+1.5

Gain Error

°C

5.5

±2

+5.0

%FS

Output Offset

°C

TBD

µA

Differential Nonlinearity

°C

III

0.5

LSB

Integral Nonlinearity

°C

LSB

Output Capacitance

°C

III

Phase Noise @ 1 kHz Offset, 10 MHz Signal

°C

III

100

dBc/Hz

Signal-to-Noise and Distortion (SINAD)

5 MHz Analog Out (20 MHz BW)

°C

60.6

Wideband SFDR (to Nyquist, 50 MHz max)

°C

III

5 MHz Analog Out

°C

III

76.2

dBc

Narrowband SFDR (3 MHz Window)

5 MHz Analog Out

°C

III

77.9

dBc

IMD (f1 = 6.25 MHz, f2 = 7.8125 MHz)

°C

III

dBFS

RX PATH CHARACTERISTICS (LFP Bypassed)

Resolution

N/A

Bits

Conversion Rate

Full

MHz

Pipeline Delay, ADC Clock Cycles

N/A

5.5

Cycles

Dynamic Performance (A

= 0.5 dBFS, f = 5 MHz)

@ F

OSCIN

= 50 MHz, RX LPF Bypassed

Signal-to-Noise and Distortion Ratio (SINAD)

Full

III

56.6

Effective Number of Bits (ENOB)

Full

III

9.1

Bits

Signal-to-Noise Ratio (SNR)

Full

III

59.2

Total Harmonic Distortion (THD)

Full

III

60.1

Spurious-Free Dynamic Range (SFDR)

Full

III

RX PATH GAIN/OFFSET

Minimum Programmable Gain

°C

Maximum Programmable Gain

Narrow Band Rx LPF or Rx LPF Bypassed

°C

+36

Wideband Rx LPF

°C

+30

Gain Step Size

°C

Gain Step Accuracy

°C

±0.4

Gain Range Error

Full

±1.0

Absolute Gain Error, PGA Gain = 0 dB

Full

±0.8

RX PATH INPUT CHARACTERISTICS

Input Voltage Range (Gain = 6 dB)

Full

III

Vppd

Input Capacitance

°C

III

Differential Input Resistance

°C

III

270

Input Bandwidth (3 dB) (Rx LPF Bypassed)

°C

III

MHz

Input Referred Noise (at +36 dB Gain with Filter)

25ºC

III

µV rms

Input Referred Noise (at 6 dB Gain with Filter)

25ºC

III

684

µV rms

Common-Mode Rejection

25ºC

III

= 3.3 V 10%, F

OSCIN

= 50 MHz, F

DAC

= 100 MHz, Gain = 6 dB,

SET

= 4.02 k

, 100

DAC Load.)

REV. 0

AD9975

Test

Parameter

Temp

Level

Min

Typ

Max

Unit

RX PATH LPF (Low Cutoff Frequency)

Cutoff Frequency

Full

III

MHz

Cutoff Frequency Variation

Full

III

±7

Attenuation @ 22 MHz

Full

III

Pass-Band Ripple

Full

±1.0

Group Delay Variation

Full

Settling Time (to 1% FS, Min to Max Gain Change)

°C

150

Total Harmonic Distortion at Max Gain (THD)

Full

dBc

RX PATH LPF (High Cutoff Frequency)

Cutoff Frequency

Full

III

MHz

Cutoff Frequency Variation

Full

III

±7

Attenuation @ 35 MHz

Full

III

Pass-Band Ripple

Full

±1.2

Group Delay Variation

Full

Settling Time (to 1% FS, Min to Max Gain Change)

°C

Total Harmonic Distortion at Max Gain (THD)

Full

dBc

RX PATH DIGITAL HPF

Latency (ADC Clock Source Cycles)

Cycle

Roll-Off in Stop Band

dB/Octave

3 dB Frequency

ADC

/400

POWER-DOWN/DISABLE TIMING

Power-Down Delay (Active-to-Power-Down)

DAC

°C

200

Interpolator

°C

200

Power-Up Delay (Power-Down-to-Active)

DAC

°C

µs

PLL

°C

µs

ADC

°C

1000

µs

PGA

°C

µs

LPF

°C

µs

Interpolator

°C

200

Minimum RESET Pulsewidth Low (t

)

Full

III

OSCIN

Cycles

ADIO PORT INTERFACE

Maximum Input Word Rate

°C

100

MHz

TX-Data Setup Time (t

)

°C

3.0

TX-Hold Hold Time (t

)

°C

RX-Data Valid Time(t

)

°C

3.0

RX-Data Hold Time (t

)

°C

1.5

REV. 0

AD9975

SPECIFICATIONS

(continued)

Test

Parameter

Temp

Level

Min

Typ

Max

Unit

SERIAL CONTROL BUS

Maximum SCLK Frequency (f

SCLK

)

Full

MHz

Clock Pulsewidth High (t

PWH

)

Full

Clock Pulsewidth Low (t

PWL

)

Full

Clock Rise/Fall Time

Full

µs

Data/Chip-Select Setup Time (t

)

Full

Data Hold Time (t

)

Full

Data Valid Time (t

)

Full

CMOS LOGIC INPUTS

Logic "1" Voltage

°C

DRVDD

0.7

Logic "0" Voltage

°C

0.4

Logic "1" Current

°C

µA

Logic "0" Current

°C

µA

Input Capacitance

°C

III

CMOS LOGIC OUTPUTS (1 mA Load)

Logic "1" Voltage

Full

DRVDD

0.6

Logic "0" Voltage

°C

0.4

Digital Output Rise/Fall Time

Full

1.5

2.5

POWER SUPPLY

All Blocks Powered Up

S_TOTAL

(Total Supply Current)

°C

210

227

Digital Supply Current (I

DRVDD

+ I

DVDD

)

°C

III

22.5

Clock Supply Current (I

CLKVDD

)

°C

III

5.5

Analog Supply Current (I

AVDD

)

°C

III

182

Power Consumption of Functional Blocks

Rx LPF

°C

III

110

ADC and SPGA

°C

III

Rx Reference

°C

III

Interpolator

°C

III

DAC

°C

III

PLL-A

°C

III

All Blocks Powered Down

S_TOTAL

(Total Supply Current)

°C

Digital Supply Current (I

DRVDD

+ I

DVDD

)

°C

III

Clock Supply Current (I

CLKVDD

)

°C

III

Analog Supply Current (I

AVDD

)

°C

III

Specifications subject to change without notice.

ABSOLUTE MAXIMUM RATINGS

Power Supply (V

) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 V

Digital Output Current . . . . . . . . . . . . . . . . . . . . . . . . . . 5 mA
Digital Inputs . . . . . . . . . . . . . . . . 0.3 V to DRVDD + 0.3 V
Analog Inputs . . . . . . . . . . . . . . . . . 0.3 V to AVDD + 0.3 V
Operating Temperature . . . . . . . . . . . . . . . . . . 40

°C to +85°C

Maximum Junction Temperature . . . . . . . . . . . . . . . . . 150

°C

Storage Temperature . . . . . . . . . . . . . . . . . . 65

°C to +150°C

Lead Temperature (Soldering 10 sec) . . . . . . . . . . . . . . 300

°C

*Absolute Maximum Ratings are limiting values to be applied individually and

beyond which the serviceability of the circuit may be impaired. Functional
operability under any of these conditions is not necessarily implied. Exposure to
absolute maximum rating conditions for extended periods of time may affect
device reliability.

EXPLANATION OF TEST LEVELS

Devices are 100% production tested at 25

°C and guaranteed

by design and characterization testing for the commercial
operating temperature range (40

°C to +85°C).

II. Parameter is guaranteed by design and/or characterization

testing.

III. Parameter is a typical value only.

THERMAL CHARACTERISTICS
Thermal Resistance

48-Lead LQFP

= 57ºC/W

= 28ºC/W

REV. 0

AD9975

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although the
AD9975 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended
to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

PIN CONFIGURATION

OSC IN

SENABLE

SCLK

SDATA

AVDD

AVSS

TX+

TX

AVSS

FS ADJ

REFIO

CLKVDD

DRVSS

DRVDD

RXBOOST/SDO

CLKOUT2

RXCLK

TXCLK

TXEN

RXEN

ADIO0

ADIO1

ADIO2

ADIO3

VSS

GC2

GC1

GC0

CLK

OUT1

ADIO9

ADIO8

ADIO7

ADIO6

ADIO5

ADIO4

VSS

RX

RX+

VSS

REFT

REFB

VSS

RESET

AD9975

48-PIN LQFP

TOP VIEW

(Not to Scale)

PIN FUNCTION DESCRIPTION

Pin No.

Mnemonic

Function

OSC IN

Crystal Oscillator Inverter Input

SENABLE

Serial Bus Enable Input

SCLK

Serial Bus Clock Input

SDATA

Serial Bus Data I/O

5, 38, 47

AVDD

Analog 3.3 V Power Supply

6, 9, 39, 42,

AVSS

Analog Ground

43, 46

Tx+

Transmit DAC + Output

Transmit DAC Output

FS ADJ

DAC Full-Scale Output Current
Adjust with External Resistor

REFIO

DAC Band Gap Decoupling Node

CLKVDD

Power Supply for CLKOUT1

DVSS

Digital Ground

DVDD

Digital 3.3 V Power Supply

1517

AGC[2:0]

AGC Control Inputs

CLKOUT1

Auxiliary Clock Output

1928

ADIO[9:0]

Digital Data I/O Port

RXEN

ADIO Direction Control Input

TXEN

TX Path Enable

TXCLK

ADIO Sample Clock Input

RXCLK

ADIO Request Clock Input

CLKOUT2

Auxiliary Clock Output

RXBOOST/

External Gain Control Output/

SDO

Serial Data Output

DRVDD

Digital I/O 3.3 V Power Supply

DRVSS

Digital I/O Ground

RESET

Reset Input

40, 41

REFB, REFT ADC Reference Decoupling Node

Rx+

Receive Path + Input

Receive Path Input

XTAL

Crystal Oscillator Inverter Output

ORDERING GUIDE

Model

Temperature Range

Package Description

Package Option

AD9975ABST

40ºC to +85ºC

48-Lead LQFP

ST-48

AD9975ABSTEB

40ºC to +85ºC

AD9975 EVAL Board

AD9975ABSTRL

40ºC to +85ºC

AD9975ABST Reel

REV. 0

AD9975

DEFINITIONS OF SPECIFICATIONS
Clock Jitter

The clock jitter is a measure of the intrinsic jitter of the PLL
generated clocks. It is a measure of the jitter from one rising
edge of the clock with respect to another edge of the clock nine
cycles later.

Differential Nonlinearity Error (DNL, No Missing Codes)

An ideal converter exhibits code transitions that are exactly 1 LSB
apart. DNL is the deviation from this ideal value. Guaranteed no
missing codes to 10-bit resolution indicate that all 1024 codes,
respectively, must be present over all operating ranges.

Integral Nonlinearity Error (INL)

Linearity error refers to the deviation of each individual code from
a line drawn from "negative full scale" through "positive full
scale." The point used as negative full scale occurs 1/2 LSB
before the first code transition. Positive full scale is defined as a
level 1 1/2 LSB beyond the last code transition. The deviation is
measured from the middle of each particular code to the true
straight line.

Phase Noise

Single-sideband phase noise power density is specified relative
to the carrier (dBc/Hz) at a given frequency offset (1 kHz) from
the carrier. Phase noise can be measured directly on a generated
single tone with a spectrum analyzer that supports noise marker
measurements. It detects the relative power between the carrier
and the offset (1 kHz) sideband noise and takes the resolution
bandwidth (rbw) into account by subtracting 10 log(rbw). It also
adds a correction factor that compensates for the implementation
of the resolution bandwidth, log display, and detector characteristic.

Output Compliance Range

The range of allowable voltage at the output of a current-output
DAC. Operation beyond the maximum compliance limits may
cause either output stage saturation, resulting in nonlinear per-
formance, or breakdown.

Spurious-Free Dynamic Range (SFDR)

The difference, in dB, between the rms amplitude of the DAC's
output signal (or ADC's input signal) and the peak spurious
signal over the specified bandwidth (Nyquist bandwidth, unless
otherwise noted).

Pipeline Delay (Latency)

The number of clock cycles between conversion initiation and
the associated output data being made available.

Offset Error

First transition should occur for an analog value 1/2 LSB above
negative full scale. Offset error is defined as the deviation of the
actual transition from that point.

Gain Error

The first code transition should occur at an analog value 1/2 LSB
above negative full scale. The last transition should occur for an
analog value 1 1/2 LSB below the nominal full scale. Gain error
is the deviation of the actual difference between first and last
code transitions and the ideal difference between first and last
code transitions.

Input Referred Noise

The rms output noise is measured using histogram techniques.
The ADC output code's standard deviation is calculated in LSB
and converted to an equivalent voltage. This results in a
noise figure that can directly be referred to the RX input of
the AD9975.

Signal-to-Noise and Distortion Ratio (SINAD)

SINAD is the ratio of the rms value of the measured input signal
to the rms sum of all other spectral components below the Nyquist
frequency, including harmonics but excluding dc. The value for
SINAD is expressed in decibels.

Effective Number of Bits (ENOB)

For a sine wave, SINAD can be expressed in terms of the number
of bits. Using the following formula,

SINAD

= (

)

/ .

1 76

6 02

it is possible to get a measure of performance expressed as N,
the effective number of bits.

Signal-to-Noise Ratio (SNR)

SNR is the ratio of the rms value of the measured input signal to
the rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in decibels.

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of the first six harmonic com-
ponents to the rms value of the measured input signal and is
expressed as a percentage or in decibels.

Power Supply Rejection

Power supply rejection specifies the converter's maximum
full-scale change when the supplies are varied from nominal to
minimum and maximum specified voltages.

REV. 0

Typical Performance CharacteristicsAD9975

SAMPLE

0.2

0.4

0.6

0.8

1.0

GNITUDE dB

20

60

80

100

10

40

70

90

0.1

0.3

0.5

0.7

0.9

50

30

INTERPOLATION FILTER

INCLUDING
SIN(X)/X

TPC 1. 2

× Low-Pass Interpolation Filter

 MHz

0.2

0.4

0.6

0.8

1.0

GNITUDE dB

20

60

80

100

10

40

70

90

0.1

0.3

0.5

0.7

0.9

50

30

INTERPOLATION FILTER

INCLUDING
SIN(X)/X

TPC 2. 2

× Band-Pass Interpolation Filter, F

Modulation, Adjacent Image Preserved

FREQUENCY MHz

GNITUDE dBc

20

60

80

100

10

40

70

90

50

30

TPC 3. Single Tone Spectral Plot @ f

DATA

= 50 MSPS,

OUT

= 5 MHz, 2

× LPF

FREQUENCY MHz

GNITUDE dBc

20

60

80

100

10

40

70

90

50

30

TPC 4. Single Tone Spectral Plot @ f

DATA

= 50 MSPS,

OUT

= 11 MHz, 2

× LPF

FREQUENCY MHz

10.0

10.4

10.8

11.2

11.6

12.0

dBc

20

60

80

100

10

40

70

90

1.02

10.6

11.0

11.4

11.8

50

30

TPC 5. Dual Tone Spectral Plot @ f

DATA

= 50 MSPS,

OUT

= 6.7 MHz and 7.3 MHz, 2

× LPF

FREQUENCY MHz

70

GNITUDE dBc

74

76

72

11 12 13 14 15 16 17 18 19 20

78

80

82

84

86

88

90

THIRD ORDER

HARMONIC

SECOND ORDER

HARMONIC

TPC 6. Harmonic Distortion vs. f

OUT

DATA

= 50 MSPS

REV. 0

AD9975

FREQUENCY MHz

10.003

10.005

10.007

10.009

10.011

10.013

dBc

20

60

80

100

10

40

70

90

10.004

10.006

10.008

10.010

10.012

50

30

TPC 7. Phase Noise Plot @f

DATA

50 MSPS,

OUT

= 10 MHz, 2

× LPF

FREQUENCY MHz

dBc

30

50

70

10

40

60

20

TPC 8. "In-Band" Multitone Spectral Plot
@f

DATA

= 50 MSPS, f

OUT

= k

× 195 kHz, 2 × LPF

FREQUENCY MHz

dBc

30

50

70

10

40

60

20

101

TPC 9. "Wide-Band" Multitone Spectral Plot
@f

DATA

= 50 MSPS, f

OUT

= k

× 195 kHz, 2 × LPF

TUNING TARGET Decimal

FREQ

UENCY MHz

192

208

224

240

255

TPC 10. F

vs. Tuning Target, F

ADC

= 50 MHz,

LPF = Wideband Rx Filter

TUNING TARGET Decimal

FREQ

UENCY MHz

192

208

224

240

255

TPC 11. F

vs. Tuning Target, F

ADC

= 50 MHz,

LPF = Narrowband Rx Filter

0.60

0.20

0.80

0.20

0.60

0.40

GAIN A

CCURA

CY dB

VGA GAIN dB

4 2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

TPC 12. PGA Gain Error vs. Gain

REV. 0

AD9975

TRANSMIT PATH

The AD9975 transmit path consists of a digital interface port,
a bypassable 2

× interpolation filter, and a transmit DAC. The

clock signals required by these blocks are generated by the inter-
nal PLL. The block diagram below shows the interconnection
between the major functional components of the transmit path.

INTERPOLATION FILTER

The interpolation filter can be programmed to run at a 2

upsampling ratio in either a low-pass filter or band-pass filter
mode. The transfer functions of these two modes are shown in
TPC 1 and TPC 2, respectively. The y-axes of the figures show
the magnitude response of the filters in dB, and the x-axes show
the frequency normalized to F

DAC

. The top trace of the plot shows

the discrete time transfer function of the interpolation filter. The
bottom trace shows the TX path transfer function including the
sin(x)/x transfer function of the DAC. In addition to the two
upsampling modes, the interpolation filter can be programmed
into a pass-through mode if no interpolation filtering is desired.

The table below shows the following parameters as a function of
the mode in which it is programmed.

Latency The number of clock cycles from the time a digital
impulse is written to the DAC until the peak value is output at
the TX+ and TX Pins.

Flush The number of clock cycles from the time a digital
impulse is written to the DAC until the output at the TX+ and
TX Pins settles to zero.

PASS

The frequency band over which the pass-band ripple is

less than the stated magnitude (i.e., 0.1 dB or 1.0 dB).

STOP

The frequency band over which the stop-band attenuation

is greater than the stated magnitude (i.e., 40 dB or 50 dB).

Table I. Interpolation Filters vs. Mode

Register 7[7:4]

0x1

0x5

Mode

× LPF

× BPF, Adj. Image

Latency, F

DAC

Clock Cycles

Flush, F

DAC

Clock Cycles

PASS

, 0.1 dB

<0.204

>0.296, <0.704

PASS

, 1.0 dB

<0.207

>0.293, <0.707

STOP

, 40 dB

<0.296

>0.204, <0.796

STOP

, 50 dB

<0.302

<0.198, >0.802

DPLL-A CLOCK DISTRIBUTION

Figure 1 shows the clock signals used in the transmit path. The
DAC sampling clock, f

DAC

, is generated by DPLL-A. f

DAC

has a

frequency equal to L

× f

OSCIN

, where L is the PLL clock multiplier

value and f

OSCIN

is the frequency of the input to PLL-A. The value

of L is programmed through the serial interface port and can be
set to 1, 2, 4, or 8. The transmit path expects a new input sample
at the ADIO interface at a rate of f

DAC

/2 if the interpolation

filter is being used. If the interpolation filter is bypassed, the
transmit path expects a new input sample at the ADIO interface
at a rate of f

DAC.

D/A CONVERTER

The AD9975 DAC provides differential output current on the
TX+ and TX pins. The values of the output currents are comple-
mentary, meaning they will always sum to I

, the full-scale

current of the DAC. For example, when the current from TX+
is at full scale, the current from TX is zero. The two currents
will typically drive a resistive load that will convert the output
currents to a voltage. The TX+ and TX output currents are
inherently ground seeking and should each be connected to
matching resistors, R

, that are tied directly to AGND.

The full-scale output current of the DAC is set by the value of
the resistor placed from the FS ADJ pin to AGND. The rela-
tionship between the resistor, R

SET

, and the full-scale output

current is governed by the following equation:

SET

= 39 4

. /

The full-scale current can be set from 2 to 20 mA. Generally, there
is a trade-off between DAC performance and power consumption.
The best DAC performance will be realized at an I

of 20 mA.

However, the value of I

adds directly to the overall current

consumption of the device.

The single-ended voltage outputs appearing at the TX+ and
TX nodes are:

Note that the full-scale voltage of V

TX+

and V

should not

exceed the maximum output compliance range of 1.5 V to pre-
vent signal compression. To maintain optimum distortion and
linearity performance, the maximum voltages at V

TX+

and V

should not exceed

±0.5 V.

The single-ended full-scale voltage at either output node will be:

The differential voltage, V

DIFF

, appearing across V

TX+

and V

is:

DIFF

(

)

and

DIFF

It should be noted that the differential output impedance of the
DAC is 2

× R

and any load connected across the two output

resistors will load down the output voltage accordingly.

REV. 0

AD9975

RECEIVE PATH DESCRIPTION

The receive path consists of a two stage PGA, a continuous
time, 4-pole LPF, an ADC, and a digital HPF. Also working
in conjunction with the receive path is an offset correction
circuit and a digital phase-locked loop. Each of these blocks
will be discussed in detail in the following sections.

PROGRAMMABLE GAIN AMPLIFIER

The PGA has a programmable gain range from 6 dB to +36 dB
if the narrower (approximately 12 MHz) LPF bandwidth is selected,
or if the LPF is bypassed. If the wider (approximately 29 MHz)
LPF bandwidth is selected, the gain range is 6 dB to +30 dB.
The PGA is comprised of two sections, a continuous time PGA
(CPGA), and a switched capacitor PGA (SPGA). The CPGA
has possible gain settings of 0, 6, 12, 18, 24 and 30. The SPGA
has possible gain settings of 6 dB, 4 dB, 2 dB, 0 dB, +2 dB,
+4 dB, and +6 dB. Table II shows how the gain is distributed
for each programmed gain setting.

The CPGA input appears at the device RX+ and RX input pins.
The input impedance of this stage is nominally 270

differential

and is not gain dependent. It is best to ac-couple the input signal
to this stage and let the inputs self-bias. This will lower the
offset voltage of the input signal, which is important at higher
gains, since any offset will lower the output compliance range of
the CPGA output. When the inputs are driven by direct coupling,
the dc level should be AVDD/2. However, this could lead to
larger dc offsets and reduce the dynamic range of the RX path.

There are two modes for selecting the RX path gain. The first
mode is to program the PGA through the serial port. A 5-bit
word determines the gain with a resolution of 2 dB per step.
More detailed information about this mode is included in the
Register Programming Definitions section of this data sheet.

The second mode sets the gain through the asynchronous
AGC[2:0] pins. These three pins set the PGA gain and state of
the RXBOOST pin according to Table II.

Table II. AGC[2:0] Gain Mapping

AGC

Path

CPGA

SPGA

[2:0]

Gain

RXBOOST

0x00

0x01

0x02

0x03

0x04

0x05

0x06

0x07

LOW-PASS FILTER

The low-pass filter (LPF) is a programmable, three-stage, fourth
order low-pass filter. The first real pole is implemented within
the CPGA. The second filter stage implements a complex pair
of poles. The last real pole is implemented in a buffer stage that
drives the SPGA.

There are two pass band settings for the LPF. Within each pass
band, the filters are tunable over about a

±15% frequency range.

The formula for the cutoff frequency is:

Target

ADC

/ (

)

Where Target is the decimal value programmed as the tuning
target in Register 5.

This filter may also be bypassed. In this case, the bandwidth of
the RX path will be gain dependent and will be around 50 MHz
at the highest gain settings.

ADC

The AD9975's analog-to-digital converter implements pipelined
multistage architecture to achieve high sample rates while consum-
ing low power. The ADC distributes the conversion over several
smaller A/D subblocks, refining the conversion with progressively
higher accuracy as it passes the results from stage to stage. As a
consequence of the distributed conversion, ADCs require a small
fraction of the 2

comparators used in a traditional n-bit flash-

type A/D. A sample-and-hold function within each of the stages
permits the first stage to operate on a new input sample while the
remaining stages operate on preceding samples. Each stage of
the pipeline, excluding the last, consists of a low resolution
Flash A/D connected to a switched capacitor DAC and interstage
residue amplifier (MDAC). The residue amplifier amplifies the
difference between the reconstructed DAC output and the flash
input for the next stage in the pipeline. One bit of redundancy is
used in each one of the stages to facilitate digital correction of
flash errors. The last stage simply consists of a Flash A/D.

SHA

GAIN

A/D

D/A

AINP

AINN

SHA

GAIN

A/D

D/A

A/D

CORRECTION LOGIC

Figure 1. ADC Theory of Operation

The digital data outputs of the ADC are represented in straight
binary format. They saturate to full scale or zero when the input
signal exceeds the input voltage range.

The maximum value will be output from the ADC when the
RX+ input is 1 V or more greater than the RX input. The mini-
mum value will be output from the ADC when the RX input is
1 V or more greater than the RX+ input. This results in a full-scale
ADC voltage of 2 Vppd.

The data can be translated to straight binary data format by
simply inverting the most significant bit.

The timing of the interface is fully described in the Digital Inter-
face Port Timing section.

REV. 0

AD9975

DIGITAL HPF

Following the ADC, there is a bypassable digital HPF. The
response is a single pole IIR HPF. The transfer function is
approximately:

H z

( )

(

) / (

)

0 99994

0 98466

where the sampling period is equal to the ADC clock period.
This results in a 3 dB frequency approximately 1/400th of the
ADC sampling rate. The transfer function of the digital HPF with
an ADC sample rate of 50 MSPS is plotted in TPC 23.

The digital HPF introduces a 1 ADC clock cycle latency. If the
HPF function is not desired, the HPF can be bypassed and the
latency will not be incurred.

CLOCK AND OSCILLATOR CIRCUITRY

The AD9975 generates all internally required clocks from a single
clock source. This source can be supplied in one of two ways.
The first method uses the on-chip oscillator by connecting a
fundamental frequency quartz crystal between the OSC IN (Pin 1)
and XTAL (Pin 48) with parallel resonant load capacitors as
specified by the crystal manufacturer. Alternatively, a TTL-level
clock applied to OCS IN with the XTAL pin left unconnected
can overdrive the internal oscillator circuit.

The PLL has a frequency capture range between 10 MHz and
50 MHz.

AGC TIMING CONSIDERATIONS

When implementing the AGC timing loop, it is important to
consider the delay and settling time of the RX path in response
to a change in gain. Figure 2 shows the delay the receive signal
experiences through the blocks of the RX path. Whether the gain
is programmed through the serial port or via the AGC[2:0] pins,
the gain takes effect immediately with the delays shown in Figure 2.
When gain changes do not involve the CPGA, the new gain will
be evident in samples after about 7 ADC clock cycles. When the
gain change does involve the CPGA, it takes an additional 45 ns
to 70 ns due to the propagation delays of the buffer, LPF and PGA.
Table VI in the Register Programming section details the PGA
programming map.

PGA

DIGITAL HPF

SHA

A/D

BUFFER

LPF

DECODE LOGIC

5ns

1 CLK CYCLE

5 CLK CYCLE 1/2 CLK CYCLE

10ns

25ns OR 50ns

10ns

GAIN REGISTER

Figure 2. AGC Loop Timing

AGC PROGRAMMING

The gain in the receive path can be programmed in two ways.
The default method is through the AGC[2:0] pins. In this mode,
the gain is achieved using a combination of internal and external
gain. The external gain is controlled by the RXBOOST output
pin, which is determined by the decode of the 3-bit AGC gain value.

DIGITAL INTERFACE PORT OPERATION

The digital interface port is a 10-bit bidirectional bus shared in
burst fashion between the transmit path and receive path. The
MxFE acts as a slave to the digital ASIC, accepting two input
enable signals, TXEN and RXEN, as well as two input clock
signals, TXCLK and RXCLK. Because the sampling clocks for
the DAC and ADC are derived internally from the OSC IN
signal, it is required that the TXCLK and RXCLK signals are
exactly the same frequency as the OSC IN signal. The phase
relationships between the TXCLK, RXCLK, and OSC IN signal
are arbitrary.

In order to add flexibility to the digital interface port, there are
several programming options available. The data input format is
straight binary by default. It is possible to independently change
the data format of the transmit path and receive path to twos
complement. Also, the clock timing can be independently changed
on the transmit and receive paths by selecting either the rising
or falling clock edge as the validating/sampling edge of the clock.
The digital interface port can also be programmed into a three-
state output mode allowing it to be connected onto a shared bus.

The timing of the interface is fully described in the Digital Inter-
face Port Timing section.

CLOCK DISTRIBUTION

The DAC sampling clock, f

DAC

, is generated by the internal

digital phase-locked loop (DPLL). f

DAC

has a frequency equal to

× f

OSCIN

, where f

OSCIN

is the internal signal generated either by

the crystal oscillator when a crystal is connected between the
OSC IN and XTAL pins or by the clock that is fed into the
OSC IN pin, and L is the multiplier programmed through the
serial port. L can have the values of 1, 2, 4, or 8.

When the interpolation filter is enabled (either 2

× LPF or 2× BPF

is selected), the data rate is upsampled by a factor of two. In this
case, the transmit path expects a new data input word at the rate
of f

DAC

/2. When the interpolation filter is bypassed, the transmit

path expects a new input word at the same frequency as DAC
sampling clock, f

DAC

Therefore, in terms of f

OSCIN

, the TXCLK

frequency should be:

TXCLK

OSCIN

= ×

where K is the interpolation factor. The interpolation factor, K, is
equal to 2 when the interpolator is enabled and is equal to 1 when
the interpolator is bypassed.

The ADC sampling clock is derived from f

OSCIN

and a new

output sample is available every f

OSCIN

clock cycle. The ADC

sampling lock can be programmed to be equal to f

OSCIN

if desired.

The timing of the digital interface port is illustrated in the
Figures 3 and 4.

REV. 0

AD9975

DIGITAL INTERFACE PORT TIMING

The ADIO[9:0] bus accepts input data-words into the transmit
path when the TXEN pin is high, the RXEN pin is low, and a
clock is present on the TXCLK pin. Figure 3 illustrates the
transmit path input timing.

TXCLK

TXEN

ADIO[9:0]

RXEN

TX0

TX1

TX2

TX3

TX4

Figure 3. Transmit Data Input Timing Diagram

It should be noted that to clear the transmit path input buffers,
an additional six clock cycles on the TXCLK input are required
after TXEN goes low. The interpolation filters will be "flushed"
with zeros if the clock signal into the TXCLK pin is present for
48 clock cycles after TXEN goes low (the data on the ADIO
bus being irrelevant over this interval).

The output from the receive path will be driven onto the
ADIO[9:0] bus when the RXEN pin is high, and a clock is present
on the RXCLK pin. When both TXEN and RXEN are low, the
ADIO[9:0] bus is three-stated. Figure 4 illustrates the receive
path output timing.

RXCLK

RXEN

ADIO[9:0]

RX0

RX1

RX2

RX3

Figure 4. Receive Data Output Timing Diagram

SERIAL INTERFACE FOR REGISTER CONTROL

The serial port is a 3-wire serial communications port consisting
of a clock (SCLK), chip select (

SENABLE), and a bidirectional

data (SDATA) signal. The interface allows read/write access to
all registers that configure the AD9975 internal parameters.
Single or multiple byte transfers are supported as well as MSB
first or LSB first transfer formats.

General Operation of the Serial Interface

Serial communication over the serial interface can be from 1 byte
to 5 bytes in length. The first byte is always the instruction byte.
The instruction byte establishes whether the communication is
going to be a read or write access, the number of data bytes to be
transferred, and the address of the first register to be accessed.
The instruction byte transfer is complete immediately upon the
eighth rising edge of SCLK after

SENABLE is asserted. Likewise,

the data registers change immediately upon writing to the eighth
bit of each data byte.

Instruction Byte

The instruction byte contains the information shown in Table III.

Table III. Instruction Byte Bit Definitions

MSB

LSB

R/W

Bit I7 R/W
This bit determines whether a read or a write data transfer will
occur after the instruction byte write. Logic high indicates a read
operation, and Logic 0 indicates a write operation.

Bits I6:I5 N1:N0
These two bits determine the number of bytes to be transferred
during the data transfer cycle. The bit decodes are shown in
Table IV.

Table IV. N1:N0 Bit Map

N1:N0

Description

0:0

Transfer 1 Byte

0:1

Transfer 2 Bytes

1:0

Transfer 3 Bytes

1:1

Transfer 4 Bytes

Bits I4:I0 - A4:A0
These bits determine which register is accessed during the data
transfer portion of the communications cycle. For multibyte
transfers, this address is the starting byte address. The remaining
register addresses are generated by the AD9975.

Serial Interface Port Pin Description

SCLK--Serial Clock
The serial clock pin is used to synchronize data transfers to and
from the AD9975 and to run the internal state machines. SCLK
maximum frequency is 25 MHz. All data transmitted to the
AD9975 is sampled on the rising edge of SCLK. All data read
from the AD9975 is validated on the rising edge of SCLK and is
updated on the falling edge.

SENABLE--Serial Interface Enable
The

SENABLE Pin is active low. It enables the serial communi-

cation to the device.

SENABLE select should stay low during

the entire communication cycle. All input on the serial port is
ignored when

SENABLE is inactive.

SDATA--Serial Data I/O
The signal on this line is sampled on the first eight rising edges
of SCLK after

SENABLE goes active. Data is then read from or

written to the AD9975 depending on what was read.

Figures 5 and 6 show the timing relationships between the three
SPI signals.

INSTRUCTION BIT 7

INSTRUCTION BIT 6

SENABLE

SCLK

PWL

PWH

SCLK

SDATA

Figure 5. Timing Diagram Register Write to AD9975

REV. 0

AD9975

INSTRUCTION BIT n

INSTRUCTION BIT n1

SCLK

SDATA

Figure 6. Timing Diagram Register Read from AD9975

MSB/LSB Transfers

The AD9975 serial port can support both most significant bit
(MSB) first or least significant bit (LSB) first data formats. The
bit order is controlled by the SPI LSB First Bit (Register 0, Bit 6).
The default value is 0, MSB first. Multibyte data transfers in
MSB format can be completed by writing an instruction byte
that includes the register address of the last address to be accessed.
The AD9975 will automatically decrement the address for each
successive byte required for the multibyte communication cycle.

When the SPI LSB First Bit (Register 0, Bit 6) is set high, the
serial port interprets both instruction and data bytes LSB first.
Multibyte data transfers in LSB format can be completed by
writing an instruction byte that includes the register address of
the first address to be accessed. The AD9975 will automatically
increment the address for each successive byte required for the
multibyte communication cycle.

Figures 7a and 7b show how the serial port words are built for
each of these modes.

SCLK

SDATA

SENABLE

INSTRUCTION CYCLE

DATA TRANSFER CYCLE

R/W

I6(n) I5(n) I4

D7n D6n

D6n D7n

Figure 7a. Serial Register Interface Timing MSB First

SCLK

SDATA

SENABLE

INSTRUCTION CYCLE

DATA TRANSFER CYCLE

I5(n) I6(n)

D00 D10 D20

D6n D7n

R/W

Figure 7b. Serial Register Interface Timing LSB First

Notes on Serial Port Operation

The serial port is disabled and all registers are set to their default
values during a hardware reset. During a software reset, all regis-
ters except Register 0 are set to their default values. Register 0
will remain at the last value sent, with the exception that the
Software Reset Bit will be set to 0.

Table V. Register Layout

Address

Default

(hex)

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

(hex)

Type

Select 4-Wire LSB/MSB

Software

R/W

SPORT

First

Reset

Power-

Power-Down Power-Down Power-

Power-Down 00

R/W

Down

Interpolators RX Reference Down ADC Receive Filter

PLL-A

DAC

and SPGA

and CPGA

R/W

ADC Clock

PLL-A (xL)

R/W

Source

Multiplier [1:0]

OSC IN/2

RX LPF

RX Path

RX Digital Fast ADC

Wideband RX Enable

RX LPF

R/W

Tuning

DC Offset HPF

Sampling

LPF

1-Pole

Bypass

Update

Update in

Correction Bypass

RX LPF

Disable

Progress

RX LPF Filter Tuning Target [7:0]

R/W

RXBOOST

RXBOOST PGA Gain

RX PGA Gain [4:0]

R/W

Active Low

Setting
through
Register

TX Interpolation Filter Select

Sample TX TX Data

R/W

on Falling

Input Twos

TXCLKIN Complement

CLK-B Equal CLK-A

CLK-B

CLK-A

Three-State

ADC Output RX Data

R/W

to OSC IN/4

Equal

Output

RX Port

on Falling

Output Twos

to OSC IN Disable

Disable

RXCLK

Complement

Version [3:0]

REV. 0

AD9975

The serial port is operated by an internal state machine and is
dependent on the number of SCLK cycles since the last time
SENABLE went active. On every eighth rising edge of SCLK, a
byte is transferred over the SPI. During a multibyte write cycle,
this means the registers of the AD9975 are not simultaneously
updated but occur sequentially. For this reason, it is recom-
mended that single byte transfers be used when changing the
SPI configuration or performing a software reset.

Bit 5: Software Reset
Setting this bit high resets the chip. The PLLs will relock to the
input clock and all registers (except Register 0x0, Bit 6) revert
to their default values. Upon completion of the reset, Bit 5 is
reset to 0.

The content of the interpolator stage is not cleared by software
or hardware resets. It is recommended to "flush" the transmit
path with zeros before transmitting data.

Bit 6: LSB/MSB First

Setting this bit high causes the serial port to send and receive
data least significant bit (LSB) first. The default low state con-
figures the serial port to send and receive data most significant
bit (MSB) first.

Bit 7: Select 4-Wire SPORT

Setting this bit high puts the serial port into a four-line mode.
The SCLK and

SENABLE retain their normal functions,

SDATA becomes an input only line, and the RXBOOST/SDO
pin becomes the serial port output. When in 4-wire mode, the
data on the RXBOOST/SDO pin will change on the falling edge
of SCLK and should be sampled on the rising edge of SCLK.

Register 1, Power-Down

Bit 0: Power-Down Receive Filter and CPGA
Setting this bit high powers down and bypasses the RX LPF
and continuous time programmable gain amplifier.

Bit 1: Power-Down ADC and SPGA
Setting this bit high powers down the ADC and the switched
capacitor programmable gain amplifier (SPGA).

Bit 2: Power-Down RX Reference
Setting this bit high powers down the ADC reference. This bit
should be set if an external reference is applied.

Bit 3: Power-Down Interpolators

Setting this bit high powers down the transmit digital interpolator.
It does not clear the content of the data path.

Bit 4: Power-Down DAC

Setting this bit high powers down the transmit DAC.

Bit 5: Power-Down PLL-A
Setting this bit high powers down the on-chip phase-locked loop
that generates the transmit path clocks and the auxiliary clock
CLK-A. When powered down, the CLK-A output goes to a high
impedance state.

Register 3, Clock Source Configuration

The AD9975 contains a programmable PLL referred to as PLL-A.
The output of the PLL is used to generate the internal clocks for
the TX path and the auxiliary clock, CLK-A.

Bit 1,0: PLL-A Multiplier

Bits 1 and 0 determine the multiplication factor (L) for PLL-A
and the DAC sampling clock frequency, F

DAC

DAC

= L

× F

CLKIN

Bit 1,0

0,0: L = 1
0,1: L = 2
1,0: L = 4
1,1: L = 8

Bit 6: ADC Clock Source OSC IN/2

Setting Bit 6 high selects the the OSC IN clock signal divided by
2 as the ADC sampling clock source. Setting Bit 6 low selects
the OSC IN clock to be used directly as the ADC sampling
clock source. The best ADC performance is achieved by using
an external crystal or by driving the OSC IN pin with a low jitter
clock source.

Register 4, Receive Filter Selection

The AD9975 receive path has a continuous time 4-pole LPF
and a 1-pole digital HPF. The 4-pole LPF has two selectable
cutoff frequencies. Additionally, the filter can be tuned around
those two cutoff frequencies. These filters can also be bypassed
to different degrees as described below.

The continuous time 4-pole low-pass filter is automatically
calibrated to one of two selectable cutoff frequencies. The cutoff
frequency, F

cutoff

, is described as a function of the ADC sampling

frequency F

ADC

and can be influenced

±15% by the RX filter

tuning target word in Register 5.

Target

cutoff low

ADC

(

)

Target

cutoff high

ADC

(

)

158

Bit 0: RX LPF Bypass
Setting this bit high bypasses the 4-pole LPF. The filter is auto-
matically powered down when this bit is set.

Bit 1: Enable 1-Pole RX LPF

The AD9975 can be configured with a 1-pole filter when the
4-pole receive low-pass filter is bypassed. The 1-pole filter is
untrimmed and subject to cutoff frequency variations of

±20%.

Bit 2: Wideband RX LPF
This bit selects the nominal cutoff frequency of the 4-pole LPF.
Setting this bit high selects a nominal cutoff frequency of 28.8 MHz.
When the wideband filter is selected, the RX path gain is limited
to 30 dB.

Bit 3: Fast ADC Sampling
Setting this bit increases the quiescent current in the SVGA block.
This may provide some performance improvement when the ADC
sampling frequency is greater than 40 MSPS.

Bit 4: RX Digital HPF Bypass
Setting this bit high bypasses the 1-pole digital HPF that follows
the ADC. The digital filter must be bypassed for ADC sampling
above 50 MSPS.

Bit 5: RX Path DC Offset Correction
Writing a 1 to this bit triggers an immediate receive path offset
correction and reads back 0 after the completion of the offset
correction.

Bit 6: RX LPF Tuning Update in Progress
This bit indicates when receive filter calibration is in progress.
The duration of a receive filter calibration is about 500

µs.

Writing to this bit has no effect.

REV. 0

AD9975

Bit 7: RX LPF Tuning Update Disable
Setting this bit high disables the automatic background receive
filter calibration. The AD9975 automatically calibrates the
receive filter on reset and every few (~2) seconds thereafter to
compensate for process and temperature variation, power supply,
and long term drift. Programming a 1 to this bit disables this
function. Programming a 0 triggers an immediate first calibration
and enables the periodic update.

Register 5, Receive Filter Tuning Target

This register sets the filter tuning target as a function of F

OSCIN

See Register 4 description.

Register 6, RX Path Gain Adjust

The AD9975 uses a combination of a continuous time PGA
(CPGA) and a switched capacitor PGA (SPGA) for a gain range
of 6 dB to +36 dB with a resolution of 2 dB. The RX path gain
can be programmed over the serial interface by writing to the
RX path Gain Adjust Register or directly using the GAIN and
MSB aligned TX[5:1] Bits. The register default value is 0x00 for
the lowest gain setting (6 dB). The register always reads back the
actual gain setting irrespective of which of the two programming
modes was used.

Bits [4:0]: RX PGA Gain
Table VI describes the gains and how they are achieved as a
function of the RX path adjust bits. It should be noted that the
value of these bits will read back the actual gain value to which
the PGA is set. If Bit 5 of this register is low, then the value read
back will be that set by the AGC[2:0] Pins.

Bit 5: PGA Gain Set through Register
Setting this bit high will result in the RX path gain being set by
writing to the PGA Gain Control Register. Default is zero, which
selects writing the gain through the AGC[2:0] pins in conjunction
with the RXBOOST pin.

Bit 6: RXBOOST
This bit is read-only. It reflects the level of the RXBOOST pin.

Bit 7: RXBOOST Active Low
Setting this bit high results in the value mapped to the RXBOOST
pin by the AGC inputs being inverted.

Table VI. PGA Programming Map

RX Path

CPGA

SPGA

Gain [4:0]

Gain

0x00

0x01

0x02

0x03

0x04

0x05

0x06

0x07

0x08

0x09

0x0A

0x0B

0x0C

0x0D

0x0E

0x0F

0x10

0x11

0x12

30/30

24/30

6/0

0x13

30/32

24/30

6/2

0x14

30/34

24/30

6/4

0x15

30/36

24/30

6/6

*When the wideband RX filter bit is set high, the RX path gain is limited to 30 dB.

The first of the two values refers to the mode when the lower RX LPF cutoff
frequency is chosen, or when the RX LPF filter is bypassed.

Register 7, Transmit Path Settings

Bit 0: TX Data Input Twos Complement

Setting this bit high changes the TX path input data format to
twos complement. When this bit is low, the TX data format is
straight binary.

Bit 1: Sample TX Data on Falling TXCLKIN
If Bit 1 is set high, the TX path data will be sampled on the
falling edge of TXCLKIN. When this bit is low, the data will be
sampled on the rising edge of TXCLKIN.

Bit 4 to Bit 7: Interpolation Filter Select

Bits 4 to 7 define the interpolation filter characteristic and inter-
polation rate.
Bits 7:4;

0x1; see TPC 1. 2 Interpolation, LPF.
0x2; Interpolation Bypass.
0x5; see TPC 2. 2 Interpolation, BPF, Adj image.

The interpolation factor has a direct influence on the rate at
which the TX path will read the input data-words from the input
buffer. When the interpolation filter has been bypassed, the data
will be read out of the buffer at a rate of F

OSCIN

L. When the

interpolator is configured to run in either of the 2 interpola-
tion modes, the data will be read out of the buffer at a rate of
0.5 F

OSCIN

Register 8, Receiver and Clock Output Settings

Bit 0: Rx Data Output Twos Complement

Setting this bit high changes the RX path input data format to
twos complement. When this bit is low, the RX data format is
straight binary.

REV. 0

AD9975

Bit 1: ADC Output on Falling RXCLK
If Bit 1 is set high, the TX path data will be sampled on the falling
edge of RXCLK. When this bit is low, the data will be sampled on
the rising edge of RXCLK.

Bit 3: Three-State RX Port
This bit sets the receive output RX[5:0] into a high impedance
three-state mode. It allows for sharing the bus with other devices.

Bit 4: CLK-A Output Disable
Setting Bit 4 high fixes the CLK-A output to a Logic 0 output level.

Bit 5: CLK-B Output Disable
Setting Bit 5 high fixes the CLK-A output to a Logic 0 output level.

Bit 6: CLK-A Equal to OSC IN
Setting Bit 6 high sets the CLK-A output signal frequency equal
to the OSC IN signal frequency. Otherwise, the CLK-A output
frequency is equal to F

OSCIN

× L.

Bit 7: CLK-B Equal to OSC IN/4
Setting Bit 7 high sets the CLKB output signal frequency equal
to the OSC IN/4 signal frequency. Otherwise, the CLKB output
frequency is equal to OSC IN/2.

Register F, Die Revision

This register stores the die revision of the chip. It is a read-only
register.

PCB DESIGN CONSIDERATIONS

Although the AD9975 is a mixed signal device, the part should
be treated as an analog component. The digital circuitry on-chip
has been specially designed to minimize the impact that the
digital switching noise will have on the operation of the analog
circuits. Following the power, grounding, and layout recom-
mendations in this section will help you get the best performance
from the MxFE.

Component Placement

If the three following guidelines of component placement are
followed, chances for getting the best performance from the
MxFE

are greatly increased. First, manage the path of return cur-

rents flowing in the ground plane so that high frequency switching
currents from the digital circuits do not flow on the ground plane
under the MxFE or analog circuits. Second, keep noisy digital
signal paths and sensitive receive signal paths as short as possible.
Third, keep digital (noise generating) and analog (noise suscep-
tible) circuits as far away from each other as possible.

In order to best manage the return currents, pure digital circuits
that generate high switching currents should be closest to the
power supply entry. This will keep the highest frequency return
current paths short and prevent them from traveling over the
sensitive MxFE and analog portions of the ground plane. Also,
these circuits should be generously bypassed at each device that
will further reduce the high frequency ground currents. The
MxFE should be placed adjacent to the digital circuits such that
the ground return currents from the digital sections will not flow
in the ground plane under the MxFE. The analog circuits should
be placed furthest from the power supply.

The AD9975 has several pins that are used to decouple sensitive
internal nodes. These pins are REFIO, REFB, and REFT. The decou-
pling capacitors connected to these points should have low ESR
and ESL. These capacitors should be placed as close to the MxFE
as possible and be connected directly to the analog ground plane.

The resistor connected to the FS ADJ pin should also be placed
close to the device and connected directly to the analog ground plane.

Power Planes and Decoupling

The AD9975 evaluation board demonstrates a good power supply
distribution and decoupling strategy. The board has four layers;
two signal layers, one ground plane, and one power plane. The
power plane is split into a 3VDD section, which is used for the
3 V digital logic circuits; a DVDD section, which is used to supply
the digital supply pins of the AD9975; an AVDD section, which
is used to supply the analog supply pins of the AD9975; and a
VANLG section, which supplies the higher voltage analog com-
ponents on the board. The 3VDD section will typically have the
highest frequency currents on the power plane and should be
kept the furthest from the MxFE and analog sections of the board.
The DVDD portion of the plane brings the current used to power
the digital portion of the MxFE to the device. This should be
treated similar to the 3VDD power plane and be kept from
going underneath the MxFE or analog components. The MxFE
should largely sit on the AVDD portion of the power plane.

The AVDD and DVDD power planes may be fed from the same
low noise voltage source; however, they should be decoupled
from each other to prevent the noise generated in the DVDD
portion of the MxFE from corrupting the AVDD supply. This
can be done by using ferrite beads between the voltage source and
DVDD and between the source and AVDD. Both DVDD and
AVDD should have a low ESR, bulk decoupling capacitor on the
MxFE side of the ferrite as well as a low ESR, ESL decoupling
capacitors on each supply pin (i.e., the AD9975 requires five
power supply decoupling caps, one each on Pins 5, 38, 47, 14, and
35). The decoupling caps should be placed as close to the MxFE
supply pins as possible. An example of the proper decoupling is
shown in the AD9975 evaluation board schematic.

Ground Planes

In general, if the component placing guidelines discussed earlier
can be implemented, it is best to have at least one continuous
ground plane for the entire board. All ground connections should
be made as short as possible. This will result in the lowest imped-
ance return paths and the quietest ground connections.

If the components cannot be placed in a manner that would keep
the high frequency ground currents from traversing under the
MxFE and analog components, it may be necessary to put current
steering channels into the ground plane to route the high frequency
currents around these sensitive areas. These current steering
channels should be made only when and where necessary.

Signal Routing

The digital RX and TX signal paths should be kept as short as
possible. Also, the impedance of these traces should have a con-
trolled impedance of about 50

. This will prevent poor signal

integrity and the high currents that can occur during undershoot
or overshoot caused by ringing. If the signal traces cannot be kept
shorter than about 1.5 inches, then series termination resistors
(33

to 47 ) should be placed close to all signal sources. It is

a good idea to series terminate all clock signals at their source
regardless of trace length.

The receive RX+/RX signals are the most sensitive signals on the
entire board. Careful routing of these signals is essential for good
receive path performance. The RX+/RX signals form a differential
pair and should be routed together as a pair. By keeping the traces
adjacent to each other, noise coupled onto the signals will appear
as common mode and will be largely rejected by the MxFE receive
input. Keeping the driving point impedance of the receive signal low
and placing any low-pass filtering of the signals close to the MxFE
will further reduce the possibility of noise corrupting these signals.

Document Outline

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Document Outline

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: AD9975ABSTEB