ONET8531T

SLLS891B

– FEBRUARY 2008 – REVISED AUGUST 2011

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DETAILED DESCRIPTION

SIGNAL PATH

A transimpedance amplifier serves as the signal path first stage that converts the photodiode current into a

voltage. If the input signal current exceeds a certain value, the transimpedance gain is reduced by means of 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

FILTER CIRCUITRY

The FILTER pins provide a filtered VCC for a PIN photodiode bias. The on-chip low pass filter for the photodiode

is implemented using a filter resistor of 220

Ω and a capacitor. The corresponding corner frequency is below

5 MHz. The supply voltages for the transimpedance amplifier are filtered by means of on-chip capacitors, thus

avoiding the need for an external supply filter capacitor. The input stage has a separate VCC supply (VCC_IN)

that is not connected on the chip to the supply of the limiting and CML stages (VCC_OUT).

AGC AND RSSI

The voltage drop across the internal photodiode supply-filter resistor is monitored by the bias and RSSI control

circuit block, in the situation where a PIN diode is biased using the FILTER pins.

If the DC input current exceeds a specified level then it is partially cancelled by means of a controlled current

source. This 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 resistor 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 ground (GND) using an external resistor. For proper operation, ensure that the voltage at the RSSI_IB pad

does not exceed VCC-0.65 V.

If an APD or PIN photodiode is used with an external bias then the RSSI_EB pin should be used. However, for

greater accuracy under external photo diode biasing conditions, it is recommended to derive the RSSI from the

external bias circuitry.

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