www.ti.com

APPLICATION INFORMATION

0.1 F

m

0.1 F

m

RSSI

OUT–

OUT+

VCC_IN

GND

VCC_OUT

220 W

R

RSSI

3

1

4

5

6

7

8

9

10

11

12

18

17

16

15

14

13

2

S0332-01

8511T

ASSEMBLY RECOMMENDATIONS

ONET8511T

SLLS895 – MARCH 2008

Figure 15 shows the ONET8511T being used in a typical fiber optic receiver circuit using the internal photodiode

bias. The ONET8511T 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 220

Ω resistor and a capacitor. Because the voltage drop across the 220 Ω resistor is sensed and

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

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

The voltage gain can be adjusted for the intended application by choosing the external resistor; however, for

output must be grounded if it is unused.

The OUT+ and OUT– pins are internally terminated by 50

Ω pull-up resisters to VCC. The outputs must be AC

coupled, for example by using 0.1

µF capacitors, to the succeeding device.

Figure 15. Basic Application Circuit

You need to concentrate on assembly parasitics and external components 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 compensating for

stray capacitances. Place the photodiode close to the ONET8511T die in order to minimize the bond wire

length and associated parasitic inductance.

2. Use identical termination and symmetrical transmission lines at the AC coupled differential output pins OUT+

and OUT–.

3. Use short bond wire connections for the supply terminals VCC_IN, VCC_OUT and GND. Supply voltage

filtering is provided on chip but filtering may be improved by using an additional external capacitor.

10

Submit Documentation Feedback

Copyright © 2008, Texas Instruments Incorporated

Product Folder Link(s): ONET8511T