1.25Gbps/2.5Gbps, +3V to +5.5V, Low-Noise

Transimpedance Preamplifiers for LANs

8

_______________________________________________________________________________________

Optical Linear Range

The MAX3266 has high gain, which limits the output

when the input signal exceeds 30µAp-p (40µAp-p for

MAX3267). The MAX3266 operates in a linear range for

inputs not exceeding:

Layout Considerations

Use good high-frequency design and layout tech-

niques. The use of a multilayer circuit board with sepa-

rate ground and power planes is recommended.

Connect the GND pins to the ground plane with the

shortest possible traces.

Noise performance and bandwidth will be adversely

affected by capacitance at the IN pin. Minimize capaci-

tance on this pin and select a low-capacitance photodi-

ode. Assembling the MAX3266 in die form using chip

and wire technology provides the best possible perform-

ance. Figure 6 shows a suggested layout for a TO

header.

The SO package version of the MAX3266 is offered as

an easy way to characterize the circuit and become

familiar with the circuit’s operation, but it does not offer

optimum performance. When using the SO version of

the MAX3266, the package capacitance adds approxi-

mately 0.3pF at the input. The PC board between the

MAX3266 input and the photodiode also adds parasitic

capacitance. Keep the input line short, and remove

power and ground planes beneath it.

Photodiode Filter

Supply voltage noise at the cathode of the photodiode

∆V/∆t, which reduces the

tance.) The filter resistor of the MAX3266, combined

with an external capacitor, can be used to reduce this

noise (see the Typical Application Circuit). Current gen-

erated by supply noise voltage is divided between

noise is (assuming the filter capacitor is much larger

than the photodiode capacitance):

If the amount of tolerable noise is known, the filter

capacitor can be easily selected:

For example, with maximum noise voltage = 100mVp-p,

be 100nA (1/2 of the MAX3266’s input noise):

Ω)(100nA) = 570pF

Wire Bonding

For high current density and reliable operation, the

MAX3266 uses gold metalization. Connections to the

die should be made with gold wire only, using ball-

bonding techniques. Wedge bonding is not recom-

mended. Die thickness is typically 15mils (0.375mm).

Linear Range

Ar

r

dBm

e

e

=

+

()

−

()













10

30

1

21

1000

log

µ

ρ

Figure 5. Optical Power Relations

PO

PI

TIME

PARAMETER

Average Power

Extinction Ratio

Optical Power

of a 1

P1

SYMBOL

RELATION

Optical Power

of a 0

P0

Signal

Amplitude

Table 1. Optical Power Relations