Bias And Ground; Grounds; Bias - Stanford Research Systems SIM918 Operating And Service Manual

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2.2 Bias and Ground

2.2.1 Grounds

2.2.2 Bias

stage amplifies its input voltage noise by the factor
where R
is the source resistance. This noise adds in quadrature
source
with the current noise of the stage.To prevent the voltage noise term
from dominating the overall output noise, set the current gain to
The output of the SIM918 is referenced to ground. To maintain
the DC accuracy of the instrument, there are two separate ground
references. Power Ground (Pin 8 of the SIM interface connector)
provides a current return path for digital control signals, power-
intensive analog amplifiers, and the power supplies. Signal Ground
(Pin 1 of the interface connector) serves as the reference point for
analog voltages. The outer shell of the Output BNC connector is tied
to Signal Ground. The output current of the preamplifier returns
to the power supply through Signal Ground. When INPUT Shield
GND is selected, the shell of the Input BNC is also tied to Sig-
nal Ground.
The outer shells of the rear-panel Shield Program Voltage and
Ref Clock Sync BNCs are connected to chassis ground, Pin 9 of the
DB–15 SIM interface connector. The separate power, signal, and
chassis grounds are not directly connected within the preamplifier.
When operating in the SIM900 Mainframe, the three grounds are
tied together inside the mainframe, and through the mainframe
to the Earth. The signal and power grounds are connected inside
the SIM918 through back-to-back Schottky diodes, so they cannot be
more than 0 35 V apart.
The bias potential is received by an ultralow-o set voltage bu er. It
is this bu ered voltage that appears on the shield of the Input BNC
when INPUT Shield Bias is selected.
The shield of the Bias connector is not used as a reference for the
bias voltage. When Bias Shield GND is selected, the shield is at
Signal Ground. The limits on the bias voltage in Section 1.2.4.1
are relative to this ground. A voltage exceeding these limits by
more than 1 V will be clamped , through diodes, to 5 5 V relative to
Power Ground. The 10 M
and Signal Ground.
5
At frequencies low enough that the resistive components of the two impedances
are dominant.
Description of Operation
5
R R
F source
input resistor is connected between Bias
SIM918 Precision Current Preamplifier
of (1 R R )
F source