SP8025 SIPEX [Sipex Corporation], SP8025 Datasheet - Page 4

• Manufacturer Part Number: SP8025
• Description: 200V/us Integrated APC Amplifier with Gain Adjust & Differential Output
• Manufacturer: SIPEX [Sipex Corporation]

Variable Gain Amplifier: continued
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 62Ω. Therefore the true
gain equation is:
This gives a nominal external Rgain value of
324 - 62 = 262Ω 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
Rev. 6/02/03
Figure 1. System Gain versus Gain Set Resisitor
-10.0
-11.0
-12.0
-1.0
-2.0
-3.0
-4.0
-5.0
-6.0
-7.0
-8.0
-9.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0
GAIN = 1 +
200
400
Rf1 + Rf2
R
SP8026 Differential APC Sensitivity
EXT
600
+ 62
versus Gain Set Resistor
SP8024, 25, 26 200V/µs Integrated APC Amplifier
800
Resistance (Ω)
1000
4
1200
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 1kΩ to ground. How-
ever, this resistor can be adjusted in value to
accommodate the characteristic impedance of
the signal trace.
THEORY OF OPERATION : Continued
1400
Sensitivity
R
R
1600
GAIN
EXT
1800
© Copyright 2003 Sipex Corporation
2000
2200
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