Output Buffer

The output buffer is designed to drive a 150Ω differen-

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

noise rejection, the MAX3658 should be terminated with

a differential load. The MAX3658 single-ended outputs

do not drive a DC-coupled grounded load. The outputs

ended output is required, both the used and the unused

outputs should be terminated in a similar manner (see

the Interface Schematics section).

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 MAX3658 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.

The DC cancellation current is drawn from the input and

creates noise. For low-level signals with little or no DC

component, the added noise is insignificant. However,

amplifier noise increases for signals with significant DC

component (see the Typical Operating Characteristics).

Photocurrent Monitor

The MAX3658 includes an average photocurrent moni-

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

mately equal to the DC current at IN.

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 capac-

itance on this pin. The MAX3658 is optimized for 0.5pF of

capacitance on the input. Assembling the MAX3658 in die

form using chip and wire technology provides the lowest

capacitance input and the best possible performance.

Supply voltage noise at the cathode of the photodiode

tance). The filter resistor of the MAX3658 combined

with an external capacitor, can be used to reduce the

effect of supply noise on performance (see the Typical

Operating Circuit). Current generated by supply noise

Select Supply Filter

Sensitive optical receivers require wide-band power-

supply decoupling. Power-supply bypassing should

frequencies between 10kHz and 700MHz. Isolate the

MAX3658 from noise sources with LC supply filters and

MAX3658 as possible.

If photocurrent monitoring is desired, connect a resistor

between MON and ground to monitor the average pho-

observed. An ammeter can also monitor the current out

of the MON pin.

Select Coupling Capacitors

A receiver built with the MAX3658 will have a bandpass

frequency response. The low-frequency cutoff due to

the coupling capacitors and load resistors is:

LFC

RC

TERM

LOAD

COUPLE

=

××

1

2π

R

V

I

MON

MONMAX

.

=

22

622Mbps, Low-Noise, High-Gain

Transimpedance Preamplifier

_______________________________________________________________________________________

7

AMPLITUDE

INPUT FROM PHOTODIODE

TIME

INPUT AFTER DC CANCELLATION

Figure 2. DC Cancellation Effect on Input