FUNCTIONAL BLOCK DIAGRAM
REV. C
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
a
14-Bit 128 kSPS
Complete Sampling ADC
AD679*
FEATURES
AC and DC Characterized and Specified
(K, B, T Grades)
128k Conversions per Second
1 MHz Full Power Bandwidth
500 kHz Full Linear Bandwidth
80 dB S/N+D (K, B, T Grades)
Twos Complement Data Format (Bipolar Mode)
Straight Binary Data Format (Unipolar Mode)
10 M Input Impedance
8-Bit Bus Interface (See AD779 for 16-Bit Interface)
On-Board Reference and Clock
10 V Unipolar or Bipolar Input Range
Pin Compatible with AD678 12-Bit, 200 kSPS ADC
MIL-STD-883 Compliant Versions Available
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
PRODUCT HIGHLIGHTS
1. COMPLETE INTEGRATION: The AD679 minimizes
external component requirements by combining a high
speed sample-hold amplifier (SHA), ADC, 5 V reference,
clock and digital interface on a single chip. This provides a
fully specified sampling A/D function unattainable with
discrete designs.
2. SPECIFICATIONS: The AD679K, B and T grades provide
fully specified and tested ac and dc parameters. The AD679J,
A and S grades are specified and tested for ac parameters; dc
accuracy specifications are shown as typicals. DC specifica-
tions (such as INL, gain and offset) are important in control
and measurement applications. AC specifications (such as
S/N+D ratio, THD and IMD) are of value in signal process-
ing applications.
3. EASE OF USE: The pinout is designed for easy board lay-
out, and the two read output provides compatibility with 8-
bit buses. Factory trimming eliminates the need for calibration
modes or external trimming to achieve rated performance.
4. RELIABILITY: The AD679 utilizes Analog Devices' mono-
lithic BiMOS technology. This ensures long term reliability
compared to multichip and hybrid designs.
5. UPGRADE PATH: The AD679 provides the same pinout as
the 12-bit, 200 kSPS AD678 ADC.
6. The AD679 is available in versions compliant with MIL-
STD-883. Refer to the Analog Devices Military Products
Databook or current AD679/883B data sheet for detailed
specifications.
GENERAL DESCRIPTION
The AD679 is a complete, multipurpose 14-bit monolithic
analog-to-digital converter, consisting of a sample-hold ampli-
fier (SHA), a microprocessor compatible bus interface, a voltage
reference and clock generation circuitry.
The AD679 is specified for ac (or "dynamic") parameters such
as S/N+D ratio, THD and IMD which are important in signal
processing applications. In addition, the AD679K, B and T
grades are fully specified for dc parameters which are important
in measurement applications.
The 14 data bits are accessed in two read operations (8+6), with
left justification. Data format is straight binary for unipolar
mode and twos complement binary for bipolar mode. The input
has a full-scale range of 10 V with a full power bandwidth of
1 MHz and a full linear bandwidth of 500 kHz. High input im-
pedance (10 M
) allows direct connection to unbuffered
sources without signal degradation. Conversions can be initiated
either under microprocessor control or by an external clock
asynchronous to the system clock.
This product is fabricated on Analog Devices' BiMOS process,
combining low power CMOS logic with high precision, low
noise bipolar circuits; laser-trimmed thin-film resistors provide
high accuracy. The converter utilizes a recursive subranging al-
gorithm which includes error correction and flash converter cir-
cuitry to achieve high speed and resolution.
The AD679 operates from +5 V and
12 V supplies and dissi-
pates 560 mW (typ). 28-pin plastic DIP, ceramic DIP and 44
J-leaded ceramic surface mount packages are available.
*Protected by U.S. Patent Nos. 4,804,960; 4,814,767; 4,833,345; 4,250,445;
4,808,908; RE 30,586
AD679SPECIFICATIONS
AC SPECIFICATIONS
AD679J/A/S
AD679K/B/T
Parameter
Min
Typ
Max
Min
Typ
Max
Units
SIGNAL-TO-NOISE AND DISTORTION (S/N+D) RATIO
2
0.5 dB Input (Referred to 0 dB Input)
78
79
80
81
dB
20 dB Input (Referred to 20 dB Input)
58
59
60
61
dB
60 dB Input (Referred to 60 dB Input)
18
19
20
21
dB
TOTAL HARMONIC DISTORTION (THD)
3
@ +25
C
90
84
90
84
dB
0.003
0.006
0.003
0.006
%
T
MIN
to T
MAX
88
82
88
82
dB
0.004
0.008
0.004
0.008
%
PEAK SPURIOUS OR PEAK HARMONIC COMPONENT
90
84
90
84
dB
FULL POWER BANDWIDTH
1
1
MHz
FULL LINEAR BANDWIDTH
500
500
kHz
INTERMODULATION DISTORTION (IMD)
4
2nd Order Products
90
84
90
84
dB
3rd Order Products
90
84
90
84
dB
DIGITAL SPECIFICATIONS
Parameter
Test Conditions
Min
Max
Units
LOGIC INPUTS
V
IH
High Level Input Voltage
2.0
V
DD
V
V
IL
Low Level Input Voltage
0
0.8
V
I
IH
High Level Input Current
V
IN
= 5 V
10
+10
A
I
IL
Low Level Input Current
V
IN
= 0 V
10
+10
A
C
IN
Input Capacitance
10
pF
LOGIC OUTPUTS
V
OH
High Level Output Voltage
I
OH
= 0.1 mA
4.0
V
I
OH
= 0.5 mA
2.4
V
V
OL
Low Level Output Voltage
I
OL
= 1.6 mA
0.4
V
I
OZ
High Z Leakage Current
V
IN
= 0 or 5 V
10
+10
A
C
OZ
High Z Output Capacitance
10
pF
NOTES
1
f
lN
amplitude = 0.5 dB (9.44 V p-p) bipolar mode full scale unless otherwise indicated. All measurements referred to a 0 dB (9.997 V p-p) input signal
unless otherwise noted.
2
See Figure 15 for higher frequencies and other input amplitudes.
3
See Figures 13 and 14 for higher frequencies and other input amplitudes.
4
f
A
= 9.08 kHz, f
B
= 9.58 kHz, with f
SAMPLE
100 kSPS. See Definition of Specifications section.
Specifications subject to change without notice.
REV. C
2
(All device types T
MIN
to T
MAX
, V
CC
= +12 V 5%, V
EE
= 12 V 5%, V
DD
= +5 V 10%)
(T
MIN
to T
MAX
, V
CC
= +12 V 5%, V
EE
= 12 V 5%, V
DD
= +5 V 10%, f
SAMPLE
= 128 kSPS, f
IN
= 10.009 kHz
unless otherwise noted)
1
DC SPECIFICATIONS
AD679J/A/S
AD679K/B/T
Parameter
Min
Typ
Max
Min
Typ
Max
Units
TEMPERATURE RANGE
J, K Grades
0
+70
0
+70
C
A, B Grades
40
+85
40
+85
C
S, T Grades
55
+125
55
+125
C
ACCURACY
Resolution
14
14
Bits
Integral Nonlinearity (INL)
2
1
2
LSB
Differential Nonlinearity (DNL)
14
14
Bits
Unipolar Zero Error
1
(@ +25
C)
0.08
0.05
0.07
% FSR*
Bipolar Zero Error
1
(@ +25
C)
0.08
0.05
0.07
% FSR
Gain Error
1, 2
(@ +25
C)
0.12
0.09
0.11
% FSR
Temperature Drift
Unipolar Zero
3
J, K Grades
0.04
0.04
0.05
% FSR
A, B Grades
0.05
0.05
0.07
% FSR
S, T Grades
0.09
0.09
0.10
% FSR
Bipolar Zero
3
J, K Grades
0.02
0.02
0.04
% FSR
A, B Grades
0.04
0.04
0.05
% FSR
S, T Grades
0.08
0.08
0.09
% FSR
Gain
3
J, K Grades
0.09
0.09
0.11
% FSR
A, B Grades
0.10
0.10
0.16
% FSR
S, T Grades
0.20
0.20
0.25
% FSR
Gain
4
J, K Grades
0.04
0.04
0.05
% FSR
A, B Grades
0.05
0.05
0.07
% FSR
S, T Grades
0.09
0.09
0.10
% FSR
ANALOG INPUT
Input Ranges
Unipolar Mode
0
+10
0
+10
V
Bipolar Mode
5
+5
5
+5
V
Input Resistance
10
10
M
Input Capacitance
10
10
pF
Input Settling Time
1.5
1.5
s
Aperture Delay
10
10
ns
Aperture Jitter
150
150
ps
INTERNAL VOLTAGE REFERENCE
Output Voltage
5
4.98
5.02
4.98
5.02
V
External Load
Unipolar Mode
+1.5
+1.5
mA
Bipolar Mode
+0.5
+0.5
mA
POWER SUPPLIES
Power Supply Rejection
V
CC
= +12 V
5%
6
6
LSB
V
EE
= 12 V
5%
6
6
LSB
V
DD
= +5 V
10%
6
6
LSB
Operating Current
I
CC
18
20
18
20
mA
I
EE
25
34
25
34
mA
I
DD
8
12
8
12
mA
Power Consumption
560
745
560
745
m
W
NOTES
1
Adjustable to zero. See Figures 5 and 6.
2
Includes internal voltage reference error.
3
Includes internal voltage reference drift.
4
Excludes internal voltage reference drift.
5
With maximum external load applied.
*% FSR = percent of full-scale range.
Specifications shown in boldface are tested on all devices at final electrical test with worst case supply voltages at T
MIN,
25
C and T
MAX
. Results from those tests are used to
calculate outgoing quality levels. All min and max specifications are guaranteed, although only those shown in boldface are tested.
Specifications subject to change without notice.
(T
MIN
to T
MAX
, V
CC
= +12 V 5%, V
EE
= 12 V 5%, V
DD
= +5 V 10% unless otherwise noted)
AD679
REV. C
3
AD679
REV. C
4
TIMING SPECIFICATIONS
(All device types T
MIN
to T
MAX
, V
CC
= +12 V
5%, V
EE
= 12 V
5%,
V
DD
= +5 V
10%)
Parameter
Symbol
Min
Max
Units
SC
Delay
t
SC
50
ns
Conversion Time
t
C
6.3
s
Conversion Rate
1
t
CR
7.8
s
Convert Pulse Width
t
CP
0.097
3.0
s
Aperture Delay
t
AD
5
20
ns
Status Delay
t
SD
0
400
ns
Access Time
2, 3
t
BA
10
100
ns
10
57
4
ns
Float Delay
5
t
FD
10
80
ns
Output Delay
t
OD
0
ns
Format Setup
t
FS
100
ns
OE
Delay
t
OE
20
ns
Read Pulse Width
t
RP
195
ns
Conversion Delay
t
CD
400
ns
EOCEN
Delay
t
EO
50
ns
NOTES
1
Includes Acquisition Time.
2
Measured from the falling edge of OE/EOCEN (0.8 V) to the time at which the
data lines/EOC cross 2.0 V or 0.8 V. See Figure 4.
3
C
OUT
= 100 pF.
4
C
OUT
= 50 pF.
5
Measured from the rising edge of OE/EOCEN (2.0 V) to the time at which the
output voltage changes by 0.5. See Figure 4; C
OUT
= 10 pF.
Specifications subject to change without notice.
ORDERING GUIDE
1
Tested
Temperature
and
Package
Model
2
Package
Range
Specified
Option
3
AD679JN
28-Pin Plastic DIP
0
C to +70
C
AC
N-28
AD679KN 28-Pin Plastic DIP
0
C to +70
C
AC + DC
N-28
AD679JD
28-Pin Ceramic DIP
0
C to +70
C
AC
D-28
AD679KD 28-Pin Ceramic DIP
0
C to +70
C
AC + DC
D-28
AD679AD 28-Pin Ceramic DIP
40
C to +85
C
AC
D-28
AD679BD 28-Pin Ceramic DIP
40
C to +85
C
AC + DC
D-28
AD679SD 28-Pin Ceramic DIP
55
C to +125
C
AC
D-28
AD679TD 28-Pin Ceramic DIP
55
C to +125
C
AC + DC
D-28
AD679AJ
44-Lead Ceramic JLCC
40
C to +85
C
AC
J-44
AD679BJ
44-Lead Ceramic JLCC
40
C to +85
C
AC + DC
J-44
AD679SJ
44-Lead Ceramic JLCC
55
C to +125
C
AC
J-44
AD679TJ
44-Lead Ceramic JLCC
55
C to +125
C
AC + DC
J-44
NOTES
1
For parallel read (14-bits) interface to 16-bit buses, see AD779.
2
For details grade and package offerings screened in accordance with MIL-STD-
883, refer to the Analog Devices Miliary Products Databook or current AD679/
883B data sheet.
3
N = Plastic DIP; D = Ceramic DIP; J = J-Leaded Ceramic Chip Carrier.
Figure 1. Conversion Timing
Figure 2. Output Timing
Figure 3. EOC Timing
Figure 4. Load Circuit for Bus Timing Specifications
AD679
REV. C
5
ABSOLUTE MAXIMUM RATINGS*
With
Respect
Specification
To
Min
Max
Units
V
CC
AGND
0.3
+18
V
V
EE
AGND
18
+0.3
V
V
CC
(Note 1)
V
EE
0.3
+26.4
V
V
DD
DGND
0
+7
V
AGND
DGND
1
+1
V
AIN, REF
IN
AGND
V
EE
V
CC
V
Digital Inputs
DGND
0.5
+7
V
Digital Outputs
DGND
0.5
V
DD
+ 0.3 V
Max Junction
Temperature
175
C
CAUTION
The AD679 features input protection circuitry consisting of large "distributed" diodes and
polysilicon series resistors to dissipate both high energy discharges (Human Body Model) and fast,
low energy pulses (Charged Device Model). Per Method 3015.2 of MIL-STD-883C, the AD679 has
been classified as a Category 1 device.
Proper ESD precautions are strongly recommended to avoid functional damage or performance
degradation. Charges as high as 4000 volts readily accumulate on the human body and test equip-
ment and discharge without detection. Unused devices must be stored in conductive foam or shunts,
and the foam should be discharged to the destination socket before devices are removed. For further
information on ESD precautions, refer to Analog Devices' ESD Prevention Manual.
WARNING!
ESD SENSITIVE DEVICE
With
Respect
Specification
To
Min
Max
Units
Operating
Temperature
J and K Grades
0
+70
C
A and B Grades
40
+85
C
S and T Grades
55
+125
C
Storage Temperature
65
+150
C
Lead Temperature
(10 sec max)
+300
C
NOTES
*Stresses above those listed under "Absolute Maximum Ratings" may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those indicated in
the operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
1
The AD679 is not designed to operate from
15 V supplies.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
44-Lead J-Leaded Chip Carrier (J-44)
28-Pin Ceramic DIP Package (D-28)
28-Lead Plastic DIP Package (N-28)
AD679
REV. C
6
PIN DESCRIPTION
28-Pin
44-Lead
DIP
JLCC
Symbol
Pin No.
Pin No.
Type
Name and Function
AGND
7
11
P
Analog Ground. This is the ground return for AIN only.
AIN
6
10
AI
Analog Signal Input.
BIPOFF
10
15
AI
Bipolar Offset. Connect to AGND for +10 V input unipolar mode and straight
binary output coding. Connect to REF
OUT
for
5 V input bipolar mode and
twos complement binary output coding.
CS
4
6
DI
Chip Select. Active LOW.
DGND
12, 14
23
P
Digital Ground.
DB7DB0
2619
40, 39, 37, 36,
DO
Data Bits. These pins provide all 14 bits in two bytes (8+6 bits). Active HIGH.
35, 34, 33, 31
EOC
27
42
DO
End-of-Convert. EOC goes LOW when a conversion starts and goes HIGH
when the conversion finishes. In asynchronous mode, EOC is an open drain
output and requires an external 3 k
pull-up resistor. See EOCEN and SYNC
pins for information on EOC gating.
EOCEN
1
1
DI
End-of-Convert Enable. Enables EOC pin. Active LOW.
HBE
15
25
DI
High Byte Enable. If LOW, output contains high byte. If HIGH, output
contains low byte (corresponding to the most recently read high byte).
OE
2
3
DI
Output Enable. A down-going transition on OE enables DB7DB0. Gated with
CS
. Active LOW.
REF
IN
9
14
AI
Reference Input. +5 V input gives 10 V full-scale range.
REF
OUT
8
12
AO
+5 V Reference Output. Tied to REF
IN
for normal operation.
SC
3
5
DI
Start Convert. Active LOW. See SYNC pin for gating.
SYNC
13
21
DI
SYNC Control. If tied to V
DD
(synchronous mode), SC and EOCEN are gated
by CS. If tied to DGND (asynchronous mode), SC and EOCEN are indepen-
dent of CS, and EOC is an open drain output. EOC requires an external 3 k
pull-up resistor in asynchronous mode.
V
CC
11
17
P
+12 V Analog Power.
V
EE
5
8
P
12 V Analog Power.
V
DD
28
43
P
+5 V Digital Power.
--
16
U
Tie to DGND.
--
1718
2, 4, 7, 9, 13,
U
These pins are unused and should be connected to DGND or V
DD
.
16, 18, 19, 20,
22, 24, 26, 27,
28, 29, 30, 32,
38, 41, 44
Type: AI = Analog Input. AO = Analog Output. DI = Digital Input (TTL and 5 V CMOS compatible). DO = Digital Output (TTL and 5 V CMOS compatible).
All DO pins are three-state drivers. P = Power. U = Unused.
PIN CONFIGURATION
DIP Package
JLCC Package
AD679
REV. C
7
NYQUIST FREQUENCY
An implication of the Nyquist sampling theorem, the "Nyquist
Frequency" of a converter is that input frequency which is one-
half the sampling frequency of the converter.
SIGNAL-TO-NOISE AND DISTORTION (S/N+D) RATIO
S/N+D 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.
TOTAL HARMONIC DISTORTION (THD)
THD is the ratio of the rms sum of the first six harmonic com-
ponents to the rms value of a full-scale input signal and is ex-
pressed as a percentage or in decibels. For input signals or
harmonics that are above the Nyquist frequency, the aliased
component is used.
PEAK SPURIOUS OR PEAK HARMONIC COMPONENT
The peak spurious or peak harmonic component is the largest
spectral component excluding the input signal and dc. This
value is expressed in decibels relative to the rms value of a full-
scale input signal.
INTERMODULATION DISTORTION (IMD)
With inputs consisting of sine waves at two frequencies, fa and
fb, any device with nonlinearities will create distortion products,
of order (m + n), at sum and difference frequencies of mfa
nfb, where m, n = 0, 1, 2, 3. . . . Intermodulation terms are
those for which m or n is not equal to zero. For example, the
second order terms are (fa + fb) and (fa fb) and the third or-
der terms are (2 fa + fb), (2 fa fb), (fa + 2 fb) and (fa 2 fb).
The IMD products are expressed as the decibel ratio of the rms
sum of the measured input signals to the rms sum of the distor-
tion terms. The two signals applied to the converter are of equal
amplitude and the peak value of their sum is 0.5 dB from full-
scale (9.44 V p-p). The IMD products are normalized to a 0-dB
input signal.
BANDWIDTH
The full-power bandwidth is that input frequency at which the
amplitude of the reconstructed fundamental is reduced by 3 dB
for a full-scale input.
The full-linear bandwidth is the input frequency at which the
slew rate limit of the sample-hold-amplifier (SHA) is reached.
At this point, the amplitude of the reconstructed fundamental
has degraded by less than 0.1 dB. Beyond this frequency, dis-
tortion of the sampled input signal increases significantly.
The AD679 has been designed to optimize input bandwidth, al-
lowing it to undersample input signals with frequencies signifi-
cantly above the converter's Nyquist frequency.
APERTURE DELAY
Aperture delay is a measure of the SHA's performance and is
measured from the falling edge of Start Convert (SC) to when
the input signal is held for conversion. In synchronous mode,
Chip Select (CS) should be LOW before SC to minimize aper-
ture delay.
APERTURE JITTER
Aperture jitter is the variation in aperture delay for successive
samples and is manifested as noise on the input to the A/D.
INPUT SETTLING TIME
Settling time is a function of the SHA's ability to track fast slew-
ing signals. This is specified as the maximum time required in
track mode after a full-scale step input to guarantee rated con-
version accuracy.
DIFFERENTIAL NONLINEARITY (DNL)
In an ideal ADC, code transitions are 1 LSB apart. Differential
linearity is the deviation from this ideal value. It is often speci-
fied in terms of resolution for which no missing codes (NMC)
are guaranteed.
INTEGRAL NONLINEARITY (INL)
The ideal transfer function for a linear ADC is a straight line
drawn between "zero" and "full scale." The point used as
"zero" occurs 1/2 LSB before the first code transition. "Full
scale" is defined as a level 1 1/2 LSB beyond the last code tran-
sition. Integral linearity error is the worst case deviation of a
code from the straight line. The deviation of each code is mea-
sured from the middle of that code.
Note that the linearity error is not user adjustable.
POWER SUPPLY REJECTION
Variations in power supply will affect the full-scale transition,
but not the converter's linearity. Power Supply Rejection is the
maximum change in the full-scale transition point due to a
change in power supply voltage from the nominal value.
TEMPERATURE DRIFT
This is the maximum change in the parameter from the initial
value (@ +25
C) to the value at T
MIN
or T
MAX
.
UNIPOLAR ZERO ERROR
In unipolar mode, the first transition should occur at a level
1/2 LSB above analog ground. Unipolar zero error is the devia-
tion of the actual transition from that point. This error can be
adjusted as discussed in the Input Connections and Calibration
section.
BIPOLAR ZERO ERROR
In the bipolar mode, the major carry transition (11 1111 1111
1111 to 00 0000 0000 0000 ) should occur at an analog value
1/2 LSB below analog ground. Bipolar zero error is the devia-
tion of the actual transition from that point. This error can be
adjusted as discussed in the Input Connections and Calibration
section.
GAIN ERROR
The last transition should occur at an analog value 1 1/2 LSB
below the nominal full scale (9.9991 volts for a 0 V10 V range,
4.9991 volts for a
5 V range). The gain error is the deviation of
the actual level at the last transition from the ideal level with the
zero error trimmed out. This error can be adjusted as shown in
the Input Connections and Calibration section.
AD679
REV. C
8
CONVERSION CONTROL
In synchronous mode (SYNC = HIGH), both Chip Select (CS)
and Start Convert (SC) must be brought LOW to start a con-
version. CS should be LOW t
SC
before SC is brought LOW. In
asynchronous mode (SYNC = LOW), a conversion is started by
bringing SC low, regardless of the state of CS.
Before a conversion is started, End-of-Convert (EOC) is HIGH
and the sample-hold is in track mode. After a conversion is
started, the sample-hold goes into hold mode and EOC goes
LOW, signifying that a conversion is in progress. During the
conversion, the sample-hold will go back into track mode and
start acquiring the next sample.
In track mode, the sample-hold will settle to
0.003% (14 bits)
in 1.5
s maximum. The acquisition time does not affect the
throughput rate as the AD679 goes back into track mode more
than 2
s before the next conversion. In multichannel systems,
the input channel can be switched as soon as EOC goes LOW.
Bringing OE LOW t
OE
after CS goes LOW makes the output
register contents available on the output data bits (DB7DB0).
A period of time t
CD
is required after OE is brought HIGH be-
fore the next SC instruction is issued.
If SC is held LOW, conversion accuracy may deteriorate. For
this reason, SC should not be held low in an attempt to operate
in a continuously converting mode.
START CONVERSION TRUTH TABLE
INPUTS
SYNC
CS
SC
STATUS
1
1
X
No Conversion
Synchronous
1
0
f
Start Conversion
Mode
1
f
0
Start Conversion
(Not Recommended)
1
0
0
Continuous Conversion
(Not Recommended)
0
X
1
No Conversion
Asynchronous
0
X
f
Start Conversion
Mode
0
X
0
Continuous Conversion
(Not Recommended)
NOTES
1 = HIGH voltage level.
0 = LOW voltage level.
X = Don't care.
= HIGH to LOW transition. Must stay low for t = t
CP
.
14-BIT MODE CODING FORMAT (1 LSB = 0.61 mV)
Unipolar Coding
Bipolar Coding
(Straight Binary)
(Twos Complement)
V
IN
*
Output Code
V
IN
*
Output Code
0.00000 V
000 . . . 0
5.00000 V
100 . . . 0
5.00000 V
100 . . . 0
0.00061 V
111 . . . 1
9.99939 V
111 . . . 1
0.00000 V
000 . . . 0
+2.50000 V
010 . . . 0
+4.99939 V
011 . . . 1
*Code center.
END-OF-CONVERT
In asynchronous mode, End-of-Convert (EOC) is an open drain
output (requiring a minimum 3 k
pull-up resistor) enabled by
End-of-Convert Enable (EOCEN). In synchronous mode, EOC
is a three-state output which is enabled by EOCEN and CS. See
Conversion Status Truth Table. Access (t
BA
) and float (t
FD
)
timing specifications do not apply in asynchronous mode where
they are a function of the time constant formed by the external
load capacitance and the pull-up resistor.
OUTPUT ENABLE OPERATION
The data bits (DB7DB0) are three-state outputs that are en-
abled by Chip Select (CS) and Output Enable (OE). CS should
be LOW t
OE
before OE is brought LOW.
When EOC goes HIGH, the conversion is completed and the
output data may be read. The output is read in two steps as a
16-bit word, with the high byte read first, followed by the low
byte. High Byte Enable (HBE) controls the output sequence.
The 14-bit result is left justified within the 16-bit field.
In unipolar mode (BIPOFF tied to AGND), the output coding
is straight binary. In bipolar mode (BIPOFF tied to REF
OUT
),
output coding is twos-complement binary.
POWER-UP
The AD679 typically requires 10
s after power-up to reset in-
ternal logic.
CONVERSION STATUS TRUTH TABLE
INPUTS
OUTPUT
SYNC CS EOCEN EOC
STATUS
1
0
0
0
Converting
1
0
0
1
Not Converting
Synchronous
1
1
X
High Z
Either
Mode
1
X
1
High Z
Either
0
X
0
0
Converting
Asynchronous
0
X
0
High Z
Not Converting
Mode*
0
X
1
High Z
Either
NOTES
1 = HIGH voltage level.
0 = LOW voltage level.
X = Don't care.
*EOC requires a pull-up resistor in asynchronous mode.
OUTPUT ENABLE TRUTH TABLE
INPUTS
OUTPUTS
HBE
(CS U OE)
DB7 . . . DB0
X
1
High Z
Unipolar or
0
0
a b c d e f g h
Bipolar
1
0
i j k l m n 0 0
NOTES
1 = HIGH voltage level.
a = MSB.
0 = LOW voltage level.
n = LSB.
X = Don't care.
U = Logical OR.
Data coding is binary for Unipolar Mode and 2s Complement Binary for Bipolar
Mode.
AD679
REV. C
9
INPUT CONNECTIONS AND CALIBRATION
The high (10 M
) input impedance of the AD679 eases the
task of interfacing to high source impedances or multiplexer
channel-to-channel mismatches of up to 300
. The 10 V p-p
full-scale input range accepts the majority of signal voltages
without the need for voltage divider networks which could dete-
riorate the accuracy of the ADC.
The AD679 is factory trimmed to minimize offset, gain and lin-
earity errors. In unipolar mode, the only external component
that is required is a 50
1% resistor. Two resistors are re-
quired in bipolar mode. If offset and gain are not critical (as in
some ac applications), even these components can be eliminated.
In some applications, offset and gain errors need to be trimmed
out completely. The following sections describe the correct pro-
cedure for these various situations.
BIPOLAR RANGE INPUTS
The connections for the bipolar mode are shown in Figure 5. In
this mode, data output coding will be twos complement binary.
This circuit will allow approximately
25 mV of offset trim
range (
40 LSB) and
0.5% of gain trim range (
80 LSB).
Either or both of the trim pots can be replaced with 50
1%
fixed resistors if the AD679 accuracy limits are sufficient for ap-
plication. If the pins are shorted together, the additional offset
and gain errors will be approximately 80 LSB.
To trim bipolar zero to its nominal value, apply a signal 1/2 LSB
below midrange (0.305 mV for a
5 V range) and adjust R1
until the major carry transition is located (11 1111 1111 1111 to
00 0000 0000 0000). To trim the gain, apply a signal 1 1/2 LSB
below full scale (+4.9991 V for a
5 V range) and adjust R2 to
give the last positive transition (01 1111 1111 1110 to 01 1111
1111 1111). These trims are interactive so several iterations may
be necessary for convergence.
A single pass calibration can be done by substituting a bipolar
offset trim (error at minus full scale) for the bipolar zero trim
(error at midscale), using the same circuit. First, apply a signal
1/2 LSB above minus full scale (4.9997 V for a
5 V range)
and adjust R1 until the minus full-scale transition is located
(10 0000 0000 0000 to 10 000 000 0001). Then perform the
gain error trim as outlined above.
Figure 5. Bipolar Input Connections with Gain and
Offset Trims
UNIPOLAR RANGE INPUTS
Offset and gain errors can be trimmed out by using the configu-
ration shown in Figure 6. This circuit allows approximately
25 mV of offset trim range (
40 LSB) and
0.5% of gain trim
range (
80 LSB).
The nominal offset is 1/2 LSB so that the analog range that cor-
responds to each code will be centered in the middle of that
code (halfway between the transitions to the codes above and
below it). Thus the first transition (from 00 0000 0000 0000 to
00 0000 0000 0001) should nominally occur for an input level
of +1/2 LSB (0.305 mV above ground for a 10 V range). To
trim unipolar zero to this nominal value, apply a 0.305 mV sig-
nal to AIN and adjust R1 until the first transition is located.
The gain trim is done by adjusting R2. If the nominal value is
required, apply a signal 1 1/2 LSB below full scale (9.9997 V for
a 10 V range) and adjust R2 until the last transition is located
(11 1111 1111 1110 to 11 1111 1111 1111).
If offset adjustment is not required, BIPOFF should be con-
nected directly to AGND. If gain adjustment is not required, R2
should be replaced with a fixed 50
1% metal film resistor. If
REF
OUT
is connected directly to REF
IN
, the additional gain
error will be approximately 1%.
Figure 6. Unipolar Input Connections with Gain and
Offset Trims
REFERENCE DECOUPLING
It is recommended that a 10
F tantalum capacitor be con-
nected between REF
IN
(Pin 9) and ground. This has the effect
of improving the S/N+D ratio through filtering possible broad-
band noise contributions from the voltage reference.
BOARD LAYOUT
Designing with high resolution data converters requires careful
attention to board layout. Trace impedance is a significant issue.
A 1.22 mA current through a 0.5
trace will develop a voltage
drop of 0.6 mV, which is 1 LSB at the 14 bit level for a 10 V
full-scale span. In addition to ground drops, inductive and ca-
pacitive coupling need to be considered, especially when high
accuracy analog signals share the same board with digital sig-
nals. Finally, power supplies need to be decoupled in order to
filter out ac noise.
Analog and digital signals should not share a common path.
Each signal should have an appropriate analog or digital return
routed close to it. Using this approach, signal loops enclose a
small area, minimizing the inductive coupling of noise. Wide PC
tracks, large gauge wire, and ground planes are highly recom-
mended to provide low impedance signal paths. Separate analog
AD679
REV. C
10
and digital ground planes are also desirable, with a single inter-
connection point to minimize ground loops. Analog signals
should be routed as far as possible from digital signals and
should cross them at right angles.
The AD679 incorporates several features to help the user's lay-
out. Analog pins (V
EE
, AIN, AGND, REF
OUT
, REF
IN
, BIPOFF,
V
CC
) are adjacent to help isolate analog from digital signals. In
addition, the 10 M
input impedance of AIN minimizes input
trace impedance errors. Finally, ground currents have been
minimized by careful circuit architecture. Current through
AGND is 200
A, with no code dependent variation. The cur-
rent through DGND is dominated by the return current for
DB7DB0 and EOC.
SUPPLY DECOUPLING
The AD679 power supplies should be well filtered, well regu-
lated, and free from high frequency noise. Switching power sup-
plies are not recommended due to their tendency to generate
spikes which can induce noise in the analog system.
Decoupling capacitors should be used in very close layout prox-
imity between all power supply pins and analog ground. A 10
F
tantalum capacitor in parallel with a 0.1
F ceramic capacitor
provides adequate decoupling.
An effort should be made to minimize the trace length between
the capacitor leads and the respective converter power supply
and common pins. The circuit layout should attempt to locate
the AD679, associated analog input circuitry and interconnec-
tions as far as possible from logic circuitry. A solid analog
ground plane around the AD679 will isolate large switching
ground currents. For these reasons, the use of wire wrap circuit
construction is not recommended; careful printed circuit con-
struction is preferred.
GROUNDING
If a single AD679 is used with separate analog and digital
ground planes, connect the analog ground plane to AGND and
the digital ground plane to DGND keeping lead lengths as short
as possible. Then connect AGND and DGND together at the
AD679. If multiple AD679s are used or the AD679 shares ana-
log supplies with other components, connect the analog and
digital returns together once at the power supplies rather than at
each chip. This prevents large ground loops which inductively
couple noise and allow digital currents to flow through the ana-
log system.
USE OF EXTERNAL VOLTAGE REFERENCE
The AD679 features an on-chip voltage reference. For improved
gain accuracy over temperature, a high performance external
voltage reference may be used in place of the on-chip reference.
The AD586 and AD588 are popular references appropriate for
use with high resolution converters. The AD586 is a low cost
reference which utilizes a buried Zener architecture to provide
low noise and drift. The AD588 is a higher performance refer-
ence which uses a proprietary implanted buried Zener diode in
conjunction with laser-trimmed thin-film resistors for low offset
and low drift.
Figure 7 shows the use of the AD586 with the AD679 in a bipo-
lar input mode. Over the 0
C to +70
C range, the AD586
L-grade exhibits less than a 2.25 mV output change from its ini-
tial value at 25
C. REF
IN
(Pin 9) scales its input by a factor of
two; thus, this change becomes effectively 4.5 mV. When ap-
plied to the AD679, this results in a total gain drift of 0.09%
FSR, which is an improvement over the on-chip reference per-
formance of 0.11% FSR. A noise-reduction capacitor, C
N
, has
been shown.
This capacitor reduces the broadband noise of the AD586 out-
put, thereby optimizing the overall ac and dc performance of the
AD679.
Figure 7. Bipolar Input with Gain and Offset Trims
Figure 8 shows the AD679 in unipolar input mode with the
AD588 reference. The AD588 output is accurate to 0.65 mV
from its value at 25
C over the 0
C to 70
C range. This results
in a 0.06% FSR total gain drift for the AD679, which is a sub-
stantial improvement over the on-chip reference performance of
0.11% FSR. A noise-reduction network on Pins 4, 6 and 7 has
been shown. The 1
F capacitors form low pass filters with the
internal resistance of the AD588 Zener and amplifier cells and
external resistance. This reduces the high frequency (to 1 MHz)
noise of the AD588, providing optimum ac and dc performance
of the AD679.
Figure 8. Unipolar Input with Gain and Offset Trims
INTERFACING THE AD679 TO MICROPROCESSORS
The I/O capabilities of the AD679 allow direct interfacing to
general purpose and DSP microprocessor buses. The asynchro-
nous conversion control feature allows complete flexibility and
control with minimal external hardware.
AD679
REV. C
11
The following examples illustrate typical AD679 interface
configurations.
AD679 to TMS320C25
In Figure 9 the AD679 is mapped into the TMS320C25 I/O
space. AD679 conversions are initiated by issuing an OUT in-
struction to Port 1. EOC status and the conversion result are
read in with an IN instruction to Port 1. A single wait state is in-
serted by generating the processor READY input from IS, Port
1 and MSC. Address line A0 provides HBE decoding to select
between the high and low bytes of data. This configuration sup-
ports processor clock speeds of 20 MHz and is capable of sup-
porting processor clock speeds of 40 MHz if a NOP instruction
follows each AD679 read instruction.
Figure 9. AD679 to TMS320C25 Interface
AD679 to 80186
Figure 10 shows the AD679 interfaced to the 80186 micropro-
cessor. This interface allows the 80186's built-in DMA control-
ler to transfer the AD679 output into a RAM based FIFO buffer
of any length, with no microprocessor intervention.
In this application the AD679 is configured in the asynchronous
mode, which allows conversions to be initiated by an external
trigger source independent of the microprocessor clock. After
each conversion, the AD679 EOC signal generates a DMA re-
quest to Channel 1 (DRQ1). The subsequent DMA READ se-
quences the high and low byte AD679 data and resets the
interrupt latch. The system designer must assign a sufficient pri-
ority to the DMA channel to ensure that the DMA request will
be serviced before the completion of the next conversion. This
configuration can be used with 6 MHz and 8 MHz 80186
processors.
Figure 10. AD679 to 80186 DMA Interface
AD679 to Analog Devices ADSP-2101
Figure 11 demonstrates the AD679 interfaced to an ADSP-2101.
With a clock frequency of 12.5 MHz, and instruction execution in
one 80 ns cycle, the digital signal processor supports the AD679
interface with one wait state.
The converter is configured to run asynchronously using a sam-
pling clock. The EOC output of the AD679 gets asserted at the
end of each conversion and causes an interrupt. Upon interrupt,
the ADSP-2101 immediately asserts its FO pin LOW. In the
following cycle, the processor starts a data memory read by pro-
viding an address on the DMA bus. The decoded address gener-
ates OE for the converter, and the high byte of the conversion
result is read over the data bus. The read operation is extended
with one wait state and thus started and completed within two
processor cycles (160 ns). Next, the ADSP-2101 asserts its FO
HIGH. This allows the processor to start reading the lower byte
of data. This read operation executes in a similar manner to the
first and is completed during the next 160 ns.
Figure 11. AD679 to ADSP-2101 Interface
AD679 to Analog Devices ADSP-2100A
Figure 12 demonstrates the AD679 interfaced to an ADSP-2100A.
With a clock frequency of 12.5 MHz, and instruction execution in
one 80 ns cycle, the digital signal processor will support the AD679
data memory interface with three hardware wait states.
The converter is configured to run asynchronously using a sam-
pling clock. The EOC output of the AD679 gets asserted at the
end of each conversion and causes an interrupt. Upon interrupt,
the ADSP-2100A immediately executes a data memory write in-
struction which asserts HBE. In the following cycle, the proces-
sor starts a data memory read (high byte read) by providing an
address on the DMA bus. The decoded address generates OE
for the converter. OE, together with logic and latch, is used to
force the ADSP-2100A into a one cycle wait state by generating
DMACK. The read operation is thus started and completed
within two processor cycles (160 ns). HBE is released during
"high byte read." This allows the processor to read the lower
byte of data as soon as "high byte read" is complete. The low
byte read operation executes in a similar manner to the first and
is completed during the next 160 ns.
Figure 12. AD679 to ADSP-2100A Interface
AD679
REV. C
12
Figure 13. Harmonic Distortion vs. Input Frequency
(0.5 dB Input)
Figure 14. Total Harmonic Distortion vs. Input
Frequency and Amplitude
Figure 15. S/(N+D) vs. Input Frequency and Amplitude
Figure 16. 5-Plot Averaged 2048 Point FFT at 128
kSPS, f
IN
= 10.009 kHz
Figure 17. Nonaveraged IMD Plot for f
IN
= 9.08 kHz
(f
a
), 9.58 kHz (f
b
) at 128 kSPS
Figure 18. Power Supply Rejection (f
IN
= 10 kHz,
f
SAMPLE
= 128 kSPS, V
RIPPLE
= 0.1 V p-p)
PRINTED IN U.S.A.
C1419a53/92
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