Stanford Research Systems SIM918, Руководство по эксплуатации и обслуживанию

Страница: 14 / 68

1 – 2 Getting Started
1.1 Introduction to the Instrument
1.1.1 Current amplifiers and autozero
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
,
is accurately made equal to the user-provided
potential at the bias terminal, V
bias
,
or to ground. The absolute mag-
o
ff
set voltage
nitude of the resulting input o
ff
set voltage is nearly zero:
V
ofs
=
V
in
− V
bias
,
| V
ofs
|
<
10
µ
V
.
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-
virtual ground
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
ofs
every 2 seconds and
makes the adjustment necessary to keep the o
ff
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
ff
set to within its
specified limits.
The gain, or transimpedance, of the preamplifier can be set to
R
F
=
10
6
,
10
7
,
or 10
8
V
/
A, remotely and from the front panel. Along
with voltage accuracy, the SIM918 o
ff
ers a low input bias current
1
and a current noise that is close to the lower limit imposed by the
Johnson noise of the transimpedance.
1.1.2 Clocks
The autozero circuit switches between measuring the input o
ff
set
voltage, and the o
ff
set voltage of the zeroing amplifier itself, at one
half the frequency of an internal or external reference clock. The in-
reference clock
ternal clock signal (typically 1.0 pulse per second, pps , i.e. 1.0 Hz)
one pps
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
ff
erent meanings. The bias current is the input current present in the
instrument in the absence of a current from an external source.
SIM918 Precision Current Preamplifier