ONET2591TA_17 Datasheet, PDF (3/16 Pages) Texas Instruments – 2.5-Gbps Transimpedance Amplifier With AGC and RSSI

ONET2591TA
www.ti.com
SLLS669 – SEPTEMBER 2005
FILTER CIRCUITRY
The filter pin provides filtered V_CC for the photodiode bias. The on-chip, low-pass filter for the photodiode V_CC is
implemented using a filter resistor of 220 Ω and an internal 200-pF capacitor. The corresponding corner
frequency is below 4 MHz.
If a lower cutoff frequency is required for the intended application, an external capacitor can be connected to one
of the FILTER pins.
The supply voltage for the whole amplifier is filtered by means of an on-chip, 275-pF capacitor as well, thus
avoiding the necessity to use an external supply-filter capacitor.
DC INPUT CURRENT CANCELLATION, AGC, AND RSSI
The voltage drop across the internal photodiode supply-filter resistor is monitored by means of a dc input current
cancellation, AGC, and RSSI control circuit block.
If the dc input current exceeds a certain level, it is partially cancelled by means of a controlled current source.
This measure keeps the transimpedance amplifier stage within sufficient operating point limits for optimum
performance. Furthermore, disabling the dc input cancellation at low input currents leads to superior noise
performance.
The AGC circuitry lowers the effective transimpedance feedback resistor R_F by means of a MOSFET device
acting as a controlled shunt. This prevents the transimpedance amplifier from being overdriven at high input
currents, which leads to improved jitter behavior within the complete input-current dynamic range. Because the
voltage drop across the supply-filter resistor is sensed and used by the AGC circuit, the photodiode must be
connected to a FILTER pad for the AGC to function correctly.
Finally, this circuit block senses the current through the filter resistor and generates a mirrored current, which is
proportional to the input signal strength. The mirrored current is available at the RSSI output and must be sunk to
ground (GND) using an external resistor. The RSSI gain can be adjusted by choosing the external resistor;
however, for proper operation, ensure that the voltage at the RSSI pad never exceeds V_CC – 0.65 V.
BAND-GAP VOLTAGE AND BIAS GENERATION
The ONET2591TA transimpedance amplifier is supplied by a single, 3.3-V supply voltage connected to the V_CC
pad. This voltage is referred to GND.
On-chip band-gap voltage circuitry generates a supply-voltage-independent reference from which all other
internally required voltages and bias currents are derived.
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