Stanford Research Systems SIM918, Operating And Service Manual

Page: 16 / 68

1 – 4 Getting Started
If the maximum bias voltage is exceeded, or the chosen gain setting
causes the output voltage to exceed its maximum, the appropriate
overload LED turns on. If the module cannot lock to an external
reference clock signal, an LED indicates an unlocked state. If armed,
the module also generates a status signal to alert the user of the
overload or unlocked condition.
1.1.6 Block diagram
The output of the main amplifier (transimpedance stage) is refer-
enced to the bias voltage. A di
ff
erence amplifier subtracts the bias
voltage, so the output of the instrument is directly proportional to
the input current
i
in
and the gain R
F
:
V
out
=
( V
bias
− i
in
× R
F
) − V
bias
=
− i
in
× R
F
.
(1
.
1)
A block diagram of the preamplifier is shown below in Figure 1.1.
LPF
0.5 Hz
(±1)
R
F
INPUT
OUTPUT
BIAS
+
+
+
−
−
−
Main
amp
Difference
amp
Zeroing
amp
offset
adj
Figure 1.1: The SIM918 block diagram.
SIM918 Precision Current Preamplifier