ATA06212AD1C
AGC Transimpedance Amplifier
Preliminary - Rev 1
D1
FEATURES
Single +5 Volt Supply
Automatic Gain Control
Excellent Sensitivity (- 34 dBm)
0 dBm Optical Overload
APPLICATIONS
SONET OC-12/SDH STM-4 Receiver
Low Noise RF Amplifier
Electrical Characteristics (1)
(T
A
= 25C, V
DD
=+5.0V + 10%, C
DIODE
+C
STRAY
= 0.7 pF, Detector Cathode to I
IN
)
NOTES
(1) f = 50 MHz
(2) Measured with I
IN
below AGC Threshold.
During AGC, input impedance will decrease
proportionally to I
IN.
(3) Defined as the I
IN
where Transresistance has
decreased by 50%.
(4) See note on "Indirect Measurement of Optical
Overload."
(5) See note on "Input Referred Noise Current."
(6) C
AGC
= 56 pF
(7) Parameter is guaranteed (not tested) by
design and characterization data @ 622 Mb/s,
assuming detector responsively of 0.95.
PARAMETER
MIN
TYP
MAX
UNIT
Transresistance(R
L
=
,I
DC
< 500 nA)
7
11
Transresistance (R
L
=50
W
) (1)
3.5
5
Bandwidth - 3dB
400
425
MHz
Input Resistance (2)
300
Output Resistance
30
43
60
Input Offset Voltage
1.0
1.3
Volts
Output Offset Voltage
1.0
1.4
Volts
AGC Threshold (I
IN
) (3)
40
AGC Time Constant (6)
4.2
Optical Overload (4)
0
+ 1
dBm
Optical Sensitivity (7)
- 34
dBm
Input Noise Current (RMS) (5)
40
50
nA
Supply Current
30
45
mA
Operating Voltage Range
+ 4.5
+ 5.0
+ 5.5
Volts
Operating Temperature Range
- 40
85
k
k
A
sec
C
Vdd2
GND
GND
GND
GND
GND
GND
GND
GND
V out
I in
Vdd2
CBY
CBY
CAGC
Vdd1
2
ATA06212AD1C
V
DD1
7.0 V
V
DD2
7.0 V
I
IN
5 mA
T
A
T
S
Absolute Maximum Ratings
Operating Temp. -40
C to 125
C
Storage Temp. -65
C to 150
C
ATA06212AD1C Pad Description
PAD
Description
Comment
V
DD1
VDD1
Positive supply for input gain stage
V
DD2
VDD2
Positive supply for second gain stage
I
IN
TIA Input Current
Connect to detector cathode for proper operation
V
OUT
TIA Output Voltage
Requires external DC block
C
AGC
External AGC Capacitor
70k
W
x (4.5pF+ C
AGC
) = AGC time constant
C
BY
Input gain stage bypass capacitor
>56 pF
ATA06212AD1C Equivalent Ciruit
AGC
I
IN
GND
or
neg. supply
C
AGC
V
OUT
V
DD
Photodetector cathode must be connected to
I
IN
for proper AGC operation
GND
3
ATA06212AD1C
Bonding Pads
Typical Bonding Diagram
Note: Bond to either V
DD2
Bond Pad
56 pF
56 pF
56 pF
56 pF
PIN
OUT
V
DD
V
DD2
GND
GND
GND
GND
GND
GND
GND
CAGC
V
OUT
I
IN
V
DD2
CBY
CBY
GND
V
DD1
AN14T
V
DD2
GND
GND
GND
GND
GND
GND
GND
CAGC
V
OUT
I
IN
V
DD2
CBY
CBY
GND
V
DD1
AN14T
925um
1250um
100um
100um
TYPICAL CHARACTERISTICS (V
DD
= +5V, T
A
= +25C, UNLESS OTHERWISE NOTED)
AGC Bandwidth vs. Temperature
1400.0
1500.0
1600.0
1700.0
1800.0
1900.0
2000.0
2100.0
2200.0
2300.0
2400.0
2500.0
-40
-20
0
20
40
60
80
100
Temperature (C)
(MHz)
C
T
= 0.6 pF; I
IN
= 1.5 mA (Average)
VDD = 5.0V
VDD = 5.5V
VDD = 4.5V
3dB Bandwidth vs. Temperature
350.0
400.0
450.0
500.0
550.0
600.0
-40
-20
0
20
40
60
80
100
Temperature (C)
Bandwidth (MHz)
C
T
= 0.6 pF
VDD = 5.5V
VDD = 4.5V
VDD = 5.0V
Input Offset Voltage vs. Temperature
1.2
1.3
1.4
1.5
1.6
1.7
-40
-20
0
20
40
60
80
100
Temperature (C)
(V)
VDD = 5.0V
VDD = 4.5V
VDD = 5.5V
Input Referred Noise vs. Temperature
30.0
35.0
40.0
45.0
50.0
-40
-20
0
20
40
60
80
100
Temperature (C)
Input Referred Noise (nA RMS)
CT = 0.5 pF; 467 MHz LPF
4
ATA06212AD1C
POWER SUPPLIES AND GENERAL LAYOUT CONSIDERATIONS
The ATA06212AD1C may be operated from a positive supply as low as + 4.5 V and as high as + 6.0 V.
Below + 4.5 V, bandwidth, overload and sensitivity will degrade, while at + 6.0 V, bandwidth, overload
and sensitivity improve (see "Bandwidth vs. Temperature" curves). Use of surface mount (preferably
MIM type capacitors), low inductance power supply bypass capacitors (>=56pF) are essential for
good high frequency and low noise performance. The power supply bypass capacitors should be
mounted on or connected to a good low inductance ground plane.
GENERAL LAYOUT CONSIDERATIONS
Since the gain stages of the transimpedance amplifier have an open loop bandwidth in excess of 1.0
GHz, it is essential to maintain good high frequency layout practices. To prevent oscillations, a low
inductance RF ground plane should be made available for power supply bypassing. Traces that can be
made short should be made short, and the utmost care should be taken to maintain very low capacitance
at the photodiode-TIA interface (I
IN
); excess capacitance at this node will cause a degradation in
bandwidth and sensitivity.
I
IN
CONNECTION
Bonding the detector cathode to I
IN
(and thus drawing current from the ATA06212D1C) improves the
dynamic range. Although the detector may be used in the reverse direction for input currents not
exceeding 25
A, the specifications for optical overload will not be met (refer to the equivalent circuit
diagram).
V
OUT
CONNECTION
The output pad should be connected via a coupling capacitor to the next stage of the receiver channel
(filter or decision circuits), as the output buffers are not designed to drive a DC coupled 50 Ohm load
(this would require an output bias current of approximately 36 mA to maintain a quiescent 1.8 Volts
across the output load). If V
OUT
is connected to a high input impedance decision circuit (>500 Ohms),
then a coupling capacitor may not be required, although caution should be exercised since DC offsets
of the photo detector/TIA combination may cause clipping of subsequent gain or decision circuits.
SENSITIVITY AND BANDWIDTH
In order to guarantee sensitivity, the TIA is subjected to a comprehensive series of tests at the die sort
level (100% testing at 25
o
C) to verify the DC and AC parametric performance (transimpedance and
bandwidth).
5
ATA06212AD1C
INDIRECT MEASUREMENT OF OPTICAL OVERLOAD
Optical overload can be defined as the maximum optical power above which the BER (bit error rate)
increases beyond 1 error in 10
10
bits. The ATA06212AD1C is 100% tested at die sort by an AC
measurement which has excellent correlation with a PRBS optical overload measurement.
INPUT REFERRED NOISE CURRENT
The "Input Noise Current" is directly related to sensitivity . It can be defined as the output noise
voltage (V
OUT
) with no input signal, (including a 400 MHz lowpass filter at the output of the TIA) divided
by the AC transresistance.
AGC CAPACITOR
It is important to select an external AGC capacitor of high quality and appropriate size. The
ATA06212AD1C has an on-chip 70 k
resistor with a shunt 4.5 pF capacitor to ground. Without
external capacitance the chip will provide an AGC time constant of 315 ns. For the best performance
in a typical 622Mb/s SONET receiver, a minimum AGC capacitor of 56pF is recommended. This will
provide the minimum amount of protection against pattern sensitivity and pulse width distortion on
repetitive data sequences during high average optical power conditions. The AGC function can be
disabled by bonding C
AGC
to ground. Conservative design practices should be followed when selecting
an AGC capacitor, since unit to unit variability of the internal time constant and various data conditions
can lead to data errors if the chosen value is too small.
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