Introduction To The Instrument; Current Amplifiers And Autozero; Clocks - Stanford Research Systems SIM918 Operating And Service Manual

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1.1 Introduction to the Instrument

1.1.1 Current amplifiers and autozero
o set voltage
virtual ground

1.1.2 Clocks

reference clock
one pps
A current, or transimpedance, amplifier converts electric current into
a proportional output voltage. Unlike a simple resistor, the am-
plifier presents a low-impedance terminal to the input current. In
the SIM918 Precision Current Preamplifier, the electric potential of
the input terminal, V
in
potential at the bias terminal, V
nitude of the resulting input o set voltage is nearly zero:
V
ofs
In all transimpedance amplifiers, the input potential is kept near
that of the bias through the action of negative feedback. When the
bias voltage is at ground, the input terminal is often said to present
a virtual ground, or a virtual null. Without autozeroing, this vir-
tual ground drifts, in some cases by many millivolts. This error in
the electric potential of the input terminal may be unacceptable in
precision measurements.
In the SIM918, an autozero circuit measures V
makes the adjustment necessary to keep the o set voltage at zero.
The autozero feature can be engaged or inhibited remotely or from
the front panel, giving the user flexibility in sensitive applications.
With autozero inhibited, the preamplifier retains microvolt input
accuracy for many hours. When engaged, it takes the autozero only
a few cycles of a reference clock to restore the o set to within its
specified limits.
The gain, or transimpedance, of the preamplifier can be set to
6 7 8
R 10 10 or 10 V A, remotely and from the front panel. Along
F
with voltage accuracy, the SIM918 o ers a low input bias current
and a current noise that is close to the lower limit imposed by the
Johnson noise of the transimpedance.
The autozero circuit switches between measuring the input o set
voltage, and the o set voltage of the zeroing amplifier itself, at one
half the frequency of an internal or external reference clock. The in-
ternal clock signal (typically 1.0 pulse per second, pps, i.e. 1.0 Hz)
can be selected, remotely or from the front panel, to be output on
1
In the unfortunate but established terminology of electronics, the word bias con-
veys di erent meanings. The bias current is the input current present in the
instrument in the absence of a current from an external source.
is accurately made equal to the user-provided
or to ground. The absolute mag-
bias
V V V
in bias ofs
SIM918 Precision Current Preamplifier
Getting Started
10 V
every 2 seconds and
ofs
1