M02006 Datasheet, PDF (13/16 Pages) M/A-COM Technology Solutions, Inc. – CMOS Pre-amplifier with AGC for Long-reach 155 Mbps Fiber-optics Based Transmitter

M02006
CMOS Pre-amplifier with AGC for Long-reach
155 Mbps Fiber-optics Based Transmitter
Rev V4
In the positive direction after about 700 mV, the impedance of the input is relatively high. After the TIA is turned on,
the DC servo and AGC circuits attempt to null any input currents (up to the absolute maximum stated in the device
table) as shown by the I/V curve in Figure 4-5.
Figure 4-5. TIA Use with Externally Biased Detectors, Powered On
-300
-200
1000
800
600
400
200
-100
0
0
-200
-400
-600
-800
-1000
PINA biased
100
200
300
400
500
600
700
mV
It can be seen that any negative voltage below 200 mV is nulled and that any positive going voltage above the
PINA standing voltage is nulled by the DC servo. The DC servo upper bandwidth varies from part to part, but is
generally at least 10 kHz.
When externally biasing a detector such as an APD with a voltage in the 30-80V range, care should be taken to
power up the TIA first and to keep the TIA powered up until after the power supply voltage on the APD is removed.
Failure to do this with the TIA unpowered may result in damage to the input FET gate at PINA. In some cases the
damage may be very subtle, in that nearly normal operation may be experienced with the damage causing slight
reductions in bandwidth, and corresponding reductions in input sensitivity.
4.4.1
Treatment of PINK
PINK still requires bypassing to ground with a high quality 220-1000 pf (470 pf recommended) capacitor, even with
no other connection to it. The capacitor stabilizes the internal voltage regulator of the TIA.
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