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1.2 Front-Panel Operation
1.2.3 Input
1.2.3.1 Input shield
1.2.4 Bias
1.2.4.1 Bias overload
bias overload limits
SIM918 Precision Current Preamplifier
The [INPUT Open] button opens and closes a relay in the path of the
input current. A green LED indicates a disconnected input terminal.
The input capacitance of the SIM918 is at its lowest with input open,
and is specified in the table on Page viii.
Successive presses of the [INPUT Shield] button connect the outer
shell of the Input BNC to the rear-panel Shield Program Voltage
terminal, a bu ered copy of the bias voltage, and to signal ground.
The state of the input shield connection is indicated by one of three
LEDs: the yellow INPUT Shield Prog, the yellow INPUT Shield Bias,
or the green INPUT Shield GND.
To float the shield of the Input connector, leave the Shield Pro-
gram Voltage BNC open and select INPUT Shield Prog.
The [BIAS GND] button toggles the source of the bias between
the voltage at the center terminal of the Bias BNC and the signal
ground of the instrument. If the bias source is set to ground, the
green BIAS GND light is on.
With Bias grounded, the di erence amplifier (Figure 1.1) is switched
out and the output of the instrument is taken directly from the trans-
impedance stage. With this configuration, there is no common-mode
error and the output-o set error is reduced.
When Bias is connected to a user voltage, the voltage is bu ered
internally before being distributed to other parts of the preamplifier.
The o set error of the bias bu er is included in the input o set
accuracy specifications in the table on Page viii.
An overload condition is recognized and the BIAS OVLD
activated if the absolute value of the voltage applied to the Bias
input exceeds certain limits. These limits are typically 5 0 V and
are between
5 2 V
V
min
The overload LED stays on for a minimum of 50 ms; after this time it
turns o if the overload condition has ceased.
4 9 V
4 9 V
V
max
1 – 7
LED is
5 2 V
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