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APPLICATION INFORMATION

S0097-02

6

10

8

7

9

2

3

1

4

5

V

CC

GND

OUT–

OUT+

RSSI

ONET

4291TA

220 W

200 pF

275 pF

C

0 to 2 pF

Optional

NBW

C

0.1 F

2

m

C

0.1 F

1

m

ASSEMBLY RECOMMENDATIONS

ONET4291TA

SLLS670 – SEPTEMBER 2005

Figure 15 shows an application circuit for an ONET4291TA being used in a typical fiber-optic receiver. The

ONET4291TA converts the electrical current generated by the PIN photodiode into a differential output voltage.

The FILTER input provides a dc bias voltage for the PIN that is low-pass filtered by the combination of the

internal 220-

Ω resistor and 200-pF capacitor. Because the voltage drop across the 220-Ω resistor is sensed and

used by the AGC circuit, the photodiode must be connected to a FILTER pad for the AGC to function correctly.

The RSSI output is used to mirror the photodiode average current and must be connected via a resistor to GND.

The voltage gain can be adjusted for the intended application by choosing the external resistor. However, for

output is not used, it must be grounded.

The OUT+ and OUT– pads are internally terminated by 50-

Ω pullup resistors to V

ac-coupled (e.g., using C1 = C2 = 0.1

µF) to the succeeding device. An additional capacitor, C

differentially connected between the two output pins OUT+ and OUT–, can be used to limit the noise bandwidth

and thus optimize the noise performance.

Figure 15. Basic Application Circuit

When packaging the ONET4291TA, careful attention to parasitics and external components is necessary to

achieve optimal performance. Recommendations that optimize performance include:

1. Minimize total capacitance on the IN pad by using a low-capacitance photodiode and paying attention to

stray capacitances. Place the photodiode close to the ONET4291TA die 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+

on-chip. Filtering can be improved by using an additional external capacitor.

11

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