4

Rev. 6/02/03

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

© Copyright 2003 Sipex Corporation

There are internal buffering resistors and a

MOSFET switch resistance in series with the

external Rgain. These internal resistances add

up to a nominal value of 62

Ω. Therefore the true

gain equation is:

GAIN = 1 +

Rf1 + Rf2

This gives a nominal external Rgain value of

324 - 62 = 262

Ω for a gain of 6.25.

The constraints on this stage are the parasitic

capacitances associated with the input pins. The

resistors used for setting the system gain are

actually subminiature potentiometers. They are

used for calibrating out systematic variations in

the optical path.

Pins 5 and 6 connect to the summing node of the

gain stage through MOSFET switches. When a

capacitor is added to the summing node of an

inverting amplifier it creates a pole in the re-

sponse. If this pole increases enough to interfere

with the internal compensation of the amplifier,

instability occurs. This instability appears as over-

shoot in the transient response. The stray capaci-

tance at pins 5 and 6 should be kept below 1pF.

This stage also allows the part to drive high

capacitive loads. The maximum load capaci-

tance is 50pF bulk. The actual load is typically

a flexible ribbon cable (FLEX) that acts like a

transmission line. This presents a distributed

capacitive load plus inductance and resistance.

In this case care should be taken to match the

characteristic impedance of the line at the far

end to avoid standing waves and ringing. The

buffer is designed to drive 1k

Ω to ground. How-

ever, this resistor can be adjusted in value to

accommodate the characteristic impedance of

the signal trace.

Variable Gain Amplifier: continued

Figure 1. System Gain versus Gain Set Resisitor

SP8026 Differential APC Sensitivity

versus Gain Set Resistor

6.0

5.0

4.0

3.0

2.0

1.0

0.0

-1.0

-2.0

-3.0

-4.0

-5.0

-6.0

-7.0

-8.0

-9.0

-10.0

-11.0

-12.0

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

Resistance (Ω)

6.0

5.7

5.3

5.0

4.7

4.3

4.0

3.7

3.3

3.0

2.7

2.3

2.0

1.7

1.3

1.0

0.7

0.3

0.0

Sensitivity

THEORY OF OPERATION : Continued