M02014 Datasheet, PDF (10/19 Pages) M/A-COM Technology Solutions, Inc. – CMOS Transimpedance Amplifier with AGC for Fiber Optic Networks up to 2.5 Gbps

Functional Description
3.2
General Description
3.2.1
TIA (Transimpedance Amplifier)
The transimpedance amplifier consists of a high gain single-ended CMOS amplifier (TIA) with a feedback resistor.
The feedback creates a virtual ground low impedance at the input and virtually all of the input current passes
through the feedback resistor defining the voltage at the output. Advanced CMOS design techniques are employed
to maintain the stability of this stage across all input conditions.
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 the external 470 pF decoupling capacitor
connected to PINK.
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, such as the provided PINK terminal. Reverse DC
bias is applied to reduce the photodiode capacitance. Avalanche photodiodes can be connected externally to a
higher voltage.
3.2.2
AGC
The M02014 has been designed to operate over the input range of +6 dBm to –26.5 dBm. This represents a ratio of
1:1500 whereas the acceptable dynamic range of the output is only 1:30 which implies a compression of 50:1 in the
transimpedance. The design uses a MOS transistor operating as a “voltage controlled resistor” to achieve the
transimpedance variation.
Another feature of the AGC is that it only operates on signals greater than –20 dBm (@ 0.9 A/W). This knee in the
gain response is important when setting “signal detect” functions in the following post amplifier. It also aids in active
photodiode alignment.
The AGC pad allows the AGC to be forced externally through a low impedance, if desired. The AGC control voltage
can be monitored during normal operation at this pad by a high impedance (>10 MΩ) circuit.
3.2.3
Output Stage
The signal from the TIA enters a phase splitter followed by a DC-shift stage and a pair of voltage follower outputs.
These are designed to drive a differential (100Ω) load. They are stable for driving capacitive loads such as
interstage filters. Since the M02014 exhibits rapid roll-off (3 pole), simple external filtering is sufficient.
3.2.4
Monitor O/P
High impedance output sources a replica average photodiode current for photo-alignment use. The accuracy of this
signal does not meet the DDMI Receive Power Specification (SFP-8472) and it is not intended be used as such.
Alternatives such as the M02015 TIA or the use of use of the Mindspeed M0204x/50 limiting amplifiers’ RxAVG_IN
pin to bias the photodiode cathode are available to provide an SFP-8472 compliant monitoring function. Ensure
that the voltage on V_MON is in the range of 0 to 2V.
02014-DSH-001-D
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