M02009 Datasheet, PDF (8/19 Pages) M/A-COM Technology Solutions, Inc. – Pin Pre-amplifier with AGC for 3.3V Fiber Optic Applications to 622 Mbps

M02009 Data Sheet
Single-ended amplifiers have inherently poor power supply noise rejection. For this reason, an on-chip low dropout
linear regulator has been incorporated into the design to give excellent noise rejection up to several MHz. Higher
frequency power supply noise is removed by external decoupling.
The circuit is designed for PIN photodiodes in the “grounded cathode” configuration, with the anode connected to
the input of the TIA and the cathode connected to AC ground. Reverse DC bias is applied to reduce the photodiode
capacitance.
1.3.2
AGC
The MC2009 has been designed to operate over the input range of +3 dBm to –32 dBm at long wavelengths. This
represents a ratio of 1:3000, whereas the acceptable dynamic range of the output is only 1:100 which implies a
compression of 30:1 in the transimpedance. The design uses a MOS transistor to achieve the transimpedance
variation.
Another feature of the AGC is that it is only operates on signals greater than –20 dBm (@ 0.9A/W). This knee in the
gain response is important when setting “signal detect” functions in the following postamplifier. It also aids in active
photodiode alignment.
The AGC pad allows the AGC to be disabled during photodiode alignment by grounding the pad through a low
impedance. The AGC control voltage can be monitored during normal operation at this pad by a high impedance
(>10 MΩ) circuit. In addition, taking this pad to V_CC +1.2V enables an internal test oscillator which supplies a 1
MHz 10 uA pk(approximate) square wave current internally between the PIN K and PIN A pads to emulate a pho-
todiode for test purposes.
1.3.3
Output Stage
The signal from the TIA enters a phase splitter and a pair of voltage follower outputs. These are designed to drive
a high impedance (>500Ω) load. They are stable for driving capacitive loads such as interstage filters. Each output
has its own GND pad, all four GND pads on the chip should be connected for proper operation.
Since the MC2009 exhibits rapid rolloff, external filtering is not required.
1.3.4
Monitor Output
High impedance output sinks a 1:1 replica of average photodiode current for monitoring purposes.
Note that this output is provided because in this device it is not possible to connect the photodiode cathode to Vcc.
For the correct operation of the AGC and DC restore, the photodiode cathode must be connected to the PINK pin.
The MC2009 measures the photodiode current and uses this information to set the transimpedance and reduce the
DCoffset of the outputs. To convert this output to a voltage, a resistor to Vcc should be used. Note that for linearity,
ensure that Vmon is always > 1V.
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02009-DSH-001-D
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