ONET8551T Datasheet, PDF (7/16 Pages) Texas Instruments – 11.3-Gbps Limiting Transimpedance Amplifier With RSSI

ONET8551T
www.ti.com
APPLICATION INFORMATION
SLLSEI5 – OCTOBER 2013
Figure 3 shows the ONET8551T device used in a typical fiber optic receiver with the internal photodiode bias.
The ONET8551T device converts the electrical current generated by the PIN photodiode into a differential output
voltage. The FILTER output provides a low-pass filtered DC bias voltage for the PIN. The photodiode must be
connected to the FILTER pad for the bias to function correctly, because the bias circuit senses and uses the
voltage drop across the FET.
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 ONET8551T, ensure that the voltage at RSSI never exceeds V_CC – 0.65 V. Leave the
RSSI output open, if the RSSI output is not used while operating with internal PD bias.
The OUT+ and OUT– pins are internally terminated by 50-Ω pullup resistors to VCC. The outputs must be AC
coupled, for example by using 0.1-μF capacitors, to the succeeding device.
For PIN diode applications, TI recommends grounding the BW0 pin. However, for higher bandwidth, the BW1
pin, or both the BW0 and BW1 pins, can be grounded. To reduce the bandwidth, the BW0 and BW1 pins can be
left open.
VCC_OUT
0.1 F ^OUT+
VCC_IN
0.1 F _OUT±
RRSSI
RSSI
GND
Figure 3. Basic Application Circuit for PIN Receivers
Figure 4 shows the ONET8551T device used in a typical fiber-optic receiver using an external photodiode bias
for an avalanche photodiode. To increase the bandwidth using APDs, ground the BW0 and BW1 pins. This
configuration can also be used for a PIN diode. However, it may be beneficial to reduce the bandwidth, and
therefore the noise, by grounding only the BW0 pin. The external bias RSSI signal is based on the DC offset
value and is not as accurate as the internal bias RSSI, which is based on the photodiode current.
Copyright © 2013, Texas Instruments Incorporated
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