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BLOCK DIAGRAM

B0066-01

OUT+

OUT–

RSSI

V

CC

FILTER

220 W

200 pF

275 pF

DC Input Current

Cancellation,

AGC, and RSSI

Band-Gap Voltage

Reference and

Bias Current

Generation

GND

R

F

IN

Voltage Amplifier

CML Output Buffer

Transimpedance Amplifier

SIGNAL PATH

ONET4291TA

SLLS670 – SEPTEMBER 2005

The ONET4291TA is a high-performance, 4.25-Gbps transimpedance amplifier consisting of the signal path,

supply filter, a control block for dc input current cancellation, automatic gain control (AGC), received signal

strength indication (RSSI), and a band-gap voltage reference and bias current generation block.

The signal path comprises a transimpedance amplifier stage, a voltage amplifier, and a CML output buffer.

input current cancellation and AGC use internal low-pass filters to cancel the dc current on the input and to

adjust the transimpedance amplifier gain. Furthermore, circuitry to monitor the received signal strength is

provided.

A simplified block diagram of the ONET4291TA is shown in Figure 1.

Figure 1. Simplified Block Diagram of the ONET4291TA

The first stage of the signal path is a transimpedance amplifier that takes the photodiode current and converts it

into a voltage signal.

If the input signal current exceeds a certain value, the transimpedance gain is reduced by means of AGC

circuitry.

The second stage is a voltage amplifier that provides additional gain and converts its single-ended input voltage

into a differential data signal.

The third signal-path stage is the output buffer, which provides CML outputs with on-chip, 50-

Ω back-termination

2

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