SP8025 SIPEX [Sipex Corporation], SP8025 Datasheet - Page 3

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

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.
Rev. 6/02/03
PIN NUMBER
1
2
3
4
5
6
7
8
NAME
R
R
V
V
GAIN
R
GND
V
GAIN1
GAIN2
OUT
OUT
COM
CC
+
-
SP8024, 25, 26 200V/µs Integrated APC Amplifier
FUNCTION
Supply Voltage
Gain Select
Common connection point for R
Power Ground
Gain Adjust 2
Output Voltage -
Output Voltage +
Gain Adjust 1
3
Variable Gain Amplifier
This stage is used to vary the gain of the system.
It provides selection for two different gain set-
ting resistors, R
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
The gain of this balanced amplifier topology is
given by:
where R
in feedback.
The nominal gain is defined as 6.25.
0 or Open
Select
Pin 2
Gain
1
GAIN
Rexternal,
R
R
Pin 3 to:
G1
G2
GAIN = 1 +
is external and Rf1 = Rf2 = 850Ω
- Pin 5
- Pin 6
GAIN1
THEORY OF OPERATION
Factor (x)
and R
Gain
6.25
6.25
© Copyright 2003 Sipex Corporation
GAIN
Rf1 + Rf2
PIN DESCRIPTION
R
GAIN2
1 and R
EXT
SP8024 SP8025 SP8026
R
1
1
G1
, at pins 5 and
Sensitivity
= R
(mV/µW)
GAIN
G2
2
2
= 262Ω
2
3
3