Stanford Research Systems SIM918, Руководство по эксплуатации и обслуживанию, Страница: 29 / 68

2.3 Output 2 – 5
To reduce output noise of the SIM918, the Bias input is limited to a
bandwidth specified in the table on Page viii. Beyond this frequency,
the transimpedance stage cannot follow variations in the bias voltage,
but the output di
ff
erence amplifier (discussed in the next section)
does. Hence the common-mode rejection of the instrument is greatly
reduced at frequencies above DC.
The bias sensing circuitry is always active, and will signal BIAS
OVLD when the applied input exceeds the voltage limits in Sec-
tion 1.2.4.1, even if Bias is set to GND .
2.3 Output
When Bias is switched to GND , the output of the instrument is taken
directly from the transimpedance stage; otherwise, from a di
ff
erence
amplifier (Figure 1.1) that subtracts the bias voltage from the output
of the transimpedance stage. Both outputs have equal drive capacity.
The output impedance of the SIM918 Precision Current Preamplifier
is 100
Ω
. The preamplifier can drive load impedances from ∞ to 0
Ω
for the full ± 10 V range of output voltage. When driving a 50
Ω
load,
the gain will be one third of that displayed on the front panel.
The output signal is filtered by a passive LRC, with f
− 3 dB
=
25 kHz.
The filter eliminates broad-spectrum noise, while adding a negligi-
ble amount of overshoot in the step response. The
R in the filter
contributes to the output resistance.
The output di
ff
erence amplifier, when engaged (Bias not at [GND]),
introduces an o
ff
set error that can be greater than the maximum input
o
ff
set error of the preamplifier. The error is reduced by autocalibra-
tion (Section 2.6). The output o
ff
set can also be trimmed from the
remote interface by using the command OFST 1 .
2.4 Autozero Trim
The autozero control loop is fully analog.
6
Its settling time, to
within the maximum input o
ff
set voltage specification in the table on
Page viii, is 40 s. Two adjustments can be made to loop parameters
via the remote command
OFST . The first one to consider, OFST
3
,
adjusts the zero point of the loop itself, i.e. the voltage to which the
control loop drives the input o
ff
set when autozero is ON .
The command OFST
2
sets the code in a digital-to-analog converter,
the output of which adds together with the output of the loop to
form the overall control output of the autozero circuit. When au-
tozero is engaged, the control loop will compensate for changes made
6
With discrete time steps.
SIM918 Precision Current Preamplifier