3

Rev. 6/02/03

SP8024, 25, 26 200V/µs Integrated APC Amplifier

© Copyright 2003 Sipex Corporation

PIN DESCRIPTION

PIN NUMBER

NAME

FUNCTION

Supply Voltage

2

GAIN

Gain Select

4

GND

Power Ground

Gain Adjust 1

Gain Adjust 2

Output Voltage -

Output Voltage +

Internal Operation

The SP8024/25/26 APC circuits have an inte-

grated photo detector and are designed with

three nominal sensitivities of 1mV/

µW, 2mV/

µW and 3mV/µW respectively. Each part’s sen-

sitivity can also be adjusted continuously and

independently for two different gain modes via

two external resistors over a range of 12dB. The

two gain modes are controlled by a TTL com-

patible logic input. This logic input also normal-

izes the internal photo detector’s responsivity

for 650nm and 780nm laser wavelengths. The

logic pin selects between the two external gain

setting resistors to allow independent control

and settings for the two gain functions.

The 8024 APC family uses two stages of gain to

optimize for speed and offset. The two stages

consist of a differential trans-impedance ampli-

fier (TIA), and a differential gain adjust ampli-

fier.

TIA

The first stage is a differential trans-impedance

amplifier (TIA) for converting the photo detec-

tor output current to a balanced differential

voltage. This topology allows for fast settling of

the photo detector and also cancels offset ef-

fects. The TIA has no external components.

Variable Gain Amplifier

This stage is used to vary the gain of the system.

It provides selection for two different gain set-

6 via internal MOSFET switches S1 and S2. The

logic input at pin 2 controls the selection of the

two external gain set resistors.

Table 1: Gain Select Logic Truth Table

Gain

Sensitivity

Select

Rexternal,

Gain

(mV/µW)

Pin 2

Pin 3 to:

Factor (x)

SP8024

SP8025

SP8026

0 or Open

6.25

1

2

3

6.25

1

2

3

The gain of this balanced amplifier topology is

given by:

GAIN = 1 +

Rf1 + Rf2

in feedback.

The nominal gain is defined as 6.25.

THEORY OF OPERATION