Layout Considerations

Noise performance and bandwidth are adversely affect-

ed by capacitance at the IN pad. Minimize capacitance

on this pad and select a low-capacitance photodiode.

Assembling the MAX3724/MAX3725 in die form using

chip and wire technology provides the best possible

performance. Figure 7 shows a suggested layout for a

TO header for the MAX3724/MAX3725. The placement

of the filter cap to minimize the ground loop of the pho-

todiode is required to achieve the specified bandwidth.

The OUT+ and OUT- bond wire lengths should also be

minimized to meet the bandwidth specification. Special

care should be taken to ensure that ESD at IN does not

exceed 500V.

Photodiode Filter

Supply voltage noise at the cathode of the photodiode

tance.) The filter resistor of the MAX3724/MAX3725,

combined with an external capacitor, can be used to

reduce this noise (see the Typical Application Circuit).

Current generated by supply noise voltage is divided

to supply 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:

be 350nA:

Wire Bonding

For high-current density and reliable operation, the

MAX3724/MAX3725 use gold metalization. Connections

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

bonding techniques. Die thickness is typically 14 mils

(0.4mm).

3.2Gbps SFP Transimpedance

Amplifiers with RSSI

_______________________________________________________________________________________

9

PO

PI

TIME

Figure 6. Optical Power Relations

TOP VIEW OF TO-46 HEADER

OUT+

OUT-

CASE

GROUND

400pF TO

1000pF

PHOTODIODE

400pF TO

1000pF

MAX3724

MAX3725

Figure 7. Suggested Layout for TO-46 Header