ONET8541T_16 Datasheet, PDF (10/17 Pages) Texas Instruments – 11.3 Gbps Limiting Transimpedance Amplifier

ONET8541T
SLLSE85A – JULY 2011 – REVISED AUGUST 2011
APPLICATION INFORMATION
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Figure 15 shows the ONET8541T used in a typical fiber optic receiver using the internal photodiode bias. The
ONET8541T 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 ONET8541T, 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-
GND
RRSSI
Figure 15. Basic Application Circuit for PIN Receivers
RSSI
ASSEMBLY RECOMMENDATIONS
Careful attention to assembly parasitics and external components is necessary to achieve optimal performance.
Recommendations that optimize performance include:
1. Minimize the total capacitance on the IN pad by using a low capacitance photodiode and paying attention to
stray capacitances. Place the photodiode close to the ONET8541T die in order to minimize the bond wire
length and thus the parasitic inductance.
2. Use identical termination and symmetrical transmission lines at the ac coupled differential output pins OUT+
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