ONET8551T_16 Datasheet, PDF (6/16 Pages) Texas Instruments – 11.3-Gbps Limiting Transimpedance Amplifier

ONET8551T
SLLSEI5 – OCTOBER 2013
DETAILED DESCRIPTION
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SIGNAL PATH
The first stage of the signal path is a transimpedance amplifier, which converts the photodiode current into a
voltage. If the input signal current exceeds a certain value, the transimpedance gain is reduced by a nonlinear
AGC circuit to limit the signal amplitude.
The second stage is a limiting voltage amplifier that provides additional limiting gain and converts the single-
ended input voltage into a differential data signal. The output stage provides CML outputs with an on-chip 50-Ω
back-termination to V_CC.
FILTER CIRCUITRY
The FILTER pins provide a regulated and filtered VCC for a PIN photodiode bias. The supply voltages for the
transimpedance amplifier are filtered by on-chip capacitors, thus an external supply filter capacitor is not
necessary. The input stage has a separate VCC supply (VCC_IN), which is not connected on chip to the supply
of the limiting and CML stages (VCC_OUT).
AGC AND RSSI
The voltage drop across the regulated FILTER FET is monitored by the bias and RSSI control circuit block in the
case where a PIN diode is biased using the FILTER pins.
If the DC input current exceeds a certain level, then it is partially cancelled by a controlled current source. This
process keeps the transimpedance amplifier stage within sufficient operating limits for optimum performance.
The automatic gain control circuitry adjusts the voltage gain of the AGC amplifier to ensure limiting behavior of
the complete amplifier.
Finally this circuit block senses the current through the FILTER FET and generates a mirrored current that is
proportional to the input signal strength. The mirrored current is available at the RSSI_IB output and can be sunk
to GND using an external resistor. For proper operation, ensure that the voltage at the RSSI_IB pad does not
exceed V_CC – 0.65 V.
If an APD or PIN photodiode is used with an external bias, then the RSSI_EB pin can be used. However, for
greater accuracy under external photodiode biasing conditions, TI recommends deriving the RSSI from the
external bias circuitry.
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