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SP8126

High Speed Differential APC Amplifier

Signal Processing Technologies 

LAYOUT AND ROUTING CONSIDERATIONS

A major consideration in developing the optical

pick-up head system is the extremely small form

factor, which is additionally aggravated by the

physical locations required by the light path.

Special care must be taken when designing the

Flex or PCB for this part. The output peak

current requirement is in the order of 12.5mA

when driving 50pF of capacitive load with a

slew rate of 250V/µs Therefore care must be

taken to provide low inductance, low resistance

paths for power and ground and output traces.

Supply coupling is also very important. Good

supply decoupling is important to ensure the

high frequency performance of the system by

eliminating supply lead inductance effects. The

decoupling capacitor C1, as shown in Figure 5,

should be as close to the part as possible. This

capacitor should be 0.1µF ceramic. C2 is op-

tional to improve decoupling and is recom-

mended to be 1µF tantalum. The layout of the

PCB is pictured here. Note the wide and short

traces on the supply lines.

The traces for the gain resistors RGAIN1 and

RGAIN2 are kept as short as possible to avoid

excessive parasitics. Any parasitics on these

nodes will limit the performance of the system.

RGAIN1 and RGAIN2 are subminiature potentiom-

eters in the application. This is a single layer

board using FR4 material.

In order to minimize coupling capacitance into

the gain setting resistor nodes, it is also critical

that VOUT+ and VOUT- are routed away from the

traces associated with the gain-setting resistors.

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Figure 4. Test and Evaluation PCB Layout for COB 8 Lead Package

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Rev:B Date: 3/18/04

SP8126B High Speed Differential APC Amplifier

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© Copyright 2004 Sipex Corporation

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