Stanford Research Systems SIM918, Operating And Service Manual

Page: 47 / 68

3.4 Commands 3 – 15
Read Microvoltmeter READ? m
Query instrument voltage m , in microvolts. READ? 1 queries the
voltage at the overall Output terminal, and READ?
3
, the voltage
at the output of the transimpedance stage. These diagnostics are
useful in manually adjusting the output and input o
ff
sets with the
commands OFST
1
and OFST
2
/
OFST
3
, respectively.
The command READ?
2
measures the control output of an internal
digital-to-analog converter. This output adds together with the out-
put of the autozero control loop to form the overall control output of
the autozero circuit, and is adjusted with
OFST
2
.
Disconnect all inputs and outputs to the SIM918 before issuing READ? .
Connect the center and shield terminals of the Bias BNC together externally,
e.g. with a grounding cap.
There will be a wait of several seconds for the
command to execute while internal switches are configured and the
voltages are sampled and averaged. Autozero is
ON while READ?
executes, and the reference clock is used during the measurement.
The results of
READ? will be unpredictable if the reference clock
is external and Unlocked . Remote commands are not processed
until READ? is complete.
READ? 1
Example:
-12
O
ff
set Trim OFST(?) m { , j }
Set (query) o
ff
set trim m { to j } . The trims are established by auto-
calibration. If needed, the o
ff
sets in the SIM918 (Section 2.4) may be
adjusted manually as follows:
Trim m Range of j Adjusts o
ff
set When By
1 -128
–
+126
Output BIAS
ON
3
.
9
µ
V
/
count
2 -32768 – +32767 Input CHOP OFF 0
.
45
µ
V
/
count
3 -128
–
+126
Input CHOP
ON
0
.
45
µ
V
/
count
A greater value of OFST 1 will make the output voltage more pos-
itive. A greater value of OFST
2
or OFST
3
will make the input
o
ff
set more positive, i.e. will make the input voltage exceed the bias
voltage by more microvolts.
OFST? 3
Example:
-13
READ? 3
14
OFST 3,-42
SIM918 Precision Current Preamplifier