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MAX3793

1Gbps to 4.25Gbps Multirate Transimpedance Amplifier with Photocurrent Monitor

Maxim Integrated 

1Gbps to 4.25Gbps Multirate Transimpedance

Amplifier with Photocurrent Monitor

AMPLITUDE

AMPLITUDE

INPUT FROM PHOTODIODE

TIME

OUTPUT (SMALL SIGNALS)

OUTPUT (LARGE SIGNALS)

TIME

INPUT AFTER DC CANCELLATION

Figure 1. MAX3793 Limited Output

Figure 2. DC-Cancellation Effect on Input

Detailed Description

The MAX3793 transimpedance amplifier is designed for

1Gbps to 4.25Gbps fiber optic applications. The

MAX3793 is comprised of a transimpedance amplifier,

a voltage amplifier, an output buffer, a DC-cancellation

circuit, a photodiode biasing resistor, and a photocur-

rent monitor (see the Functional Diagram).

Transimpedance Amplifier

The signal current at the input flows into the summing

node of a high-gain amplifier. Shunt feedback through

resistor RF converts this current into a voltage. Schottky

diodes clamp the output signal for large input currents

(Figure 1).

Voltage Amplifier

The voltage amplifier provides additional gain and con-

verts the transimpedance amplifier single-ended output

signal into a differential signal.

Output Buffer

The output buffer is designed to drive a 100Ω differen-

tial load between OUT+ and OUT-. For optimum supply

noise rejection, the MAX3793 should be terminated with

a matched load. The MAX3793 outputs do not drive a

DC-coupled grounded load. The outputs should always

be AC-coupled. Refer to Application Note HFAN-01.1:

Choosing AC-Coupling Capacitors for a more detailed

discussion on selecting capacitors. If a single-ended

output is required, both the used and the unused out-

puts should be terminated in a similar manner. See

Figure 6.

DC-Cancellation Circuit

The DC-cancellation circuit uses low-frequency feed-

back to remove the DC component of the input signal

(Figure 2). This feature centers the input signal within

the transimpedance amplifier’s linear range, thereby

reducing pulse-width distortion.

The DC-cancellation circuit is internally compensated

and does not require external capacitors. This circuit

minimizes pulse-width distortion for data sequences

that exhibit a 50% mark density. A mark density signifi-

cantly different from 50% causes the MAX3793 to gen-

erate pulse-width distortion. Grounding the FILT pin

disables the DC-cancellation circuit. For normal opera-

tion, the DC-cancellation circuit must be enabled.

Photocurrent Monitor

The MAX3793 includes an average photocurrent moni-

tor. The current sourced from MON to ground is propor-

tionally equal to the average RFILT current (see the

Typical Operating Characteristics). This RFILT is used

to bias the photodiode, see Figure 7.

RFILT is an internal 750Ω resistor between VCC and

FILT. This resistor is to be used in conjunction with an

external CFILT to bias the photodiode. The current

through this resistor is monitored and creates the pho-

tocurrent monitor output. For test purposes, driving

FILT below 0.5V disables the DC-cancellation circuit.

Design Procedure

Select Photodiode

Noise performance and bandwidth are adversely

affected by capacitance on the TIA input node. Select

a low-capacitance photodiode to minimize the total

input capacitance on this pin. The MAX3793 is opti-

mized for 0.6pF of capacitance on the input.

Select CFILT

The filter resistor of the MAX3793, combined with an

external capacitor, can be used to reduce noise (see

the Typical Application Circuit). Current generated by

supply noise voltage is divided between CFILT and CIN.

Assuming the filter capacitor is much larger than the

photodiode capacitance, the input noise current due to

supply noise is:

INOISE = (VNOISE x CIN) / (RFILT x CFILT)

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