ONET8521T Datasheet, PDF (10/16 Pages) Texas Instruments – 11.3 Gbps Limiting Transimpedance Amplifier With RSSI

ONET8521T
SLLSE87A – JULY 2011 – REVISED AUGUST 2011
APPLICATION INFORMATION
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Figure 15 shows the ONET8521T used in a typical fiber optic receiver using the internal photodiode bias. The
ONET8521T converts the electrical current generated by the PIN photodiode into a differential output voltage.
The FILTER inputs provide a dc bias voltage for the PIN that is low pass filtered by the combination of an internal
220Ω resistor and a capacitor. Because the voltage drop across the 220Ω resistor is sensed and used by the
bias circuit, the photodiode must be connected to the FILTER pads for the bias to function correctly.
The RSSI output is used to mirror the photodiode output current and can be connected via a resistor to GND.
The voltage gain can be adjusted for the intended application by choosing the external resistor; however, for
proper operation of the ONET8521T, ensure that the voltage at RSSI never exceeds V_CC – 0.65V. If the RSSI
output is not used while operating with internal PD bias, it should be left open.
The OUT+ and OUT– pins are internally terminated by 50Ω pull-up resisters to VCC. The outputs must be ac
coupled, for example by using 0.1μF capacitors, to the succeeding device.
VCC_OUT
VCC_IN
0.1μF ^OUT+
1
0.1μF _OUT-
RRSSI
RSSI
GND
Figure 15. Basic Application Circuit for PIN Receivers
Figure 16 shows the ONET8521T being used in a typical fiber optic receiver using an external photodiode bias
for an APD photodiode. This configuration can also be used for a PIN diode if desired. The external bias RSSI
signal is based on a dc offset value and is not as accurate as the internal bias RSSI signal which is based upon
the photodiode current.
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