Stanford Research Systems SIM983, Operation And Service Manual

Page: 22 / 74

2 – 2 Description of Operation
2.1 Signal Connections and Grounding
2.1.1 Output drive
The output impedance of the SIM983 Scaling Amplifier is 50
Ω
.
The amplifier can drive load impedances from ∞ to 50
Ω
for the
full ± 10 V range of output voltage. When driving a 50
Ω
load, the
gain will be half of that displayed on the front panel.
The rear-panel output connector is wired in parallel with the front-
panel output, and shares some of the output impedance (Figure 1.1).
The output stage is not designed to drive two 50
Ω
loads simultane-
ously.
2.1.2 Grounds
Both the input and the output of the SIM983 are referenced to ground.
To maintain the DC accuracy of the instrument, there are two sepa-
rate ground references. Ground 1 (Pin 1 of the SIM interface connec-
tor) provides a return path for digital control signals and the power
supply currents, while Ground 2 (Pin 8 of the interface connector)
serves as the reference point for analog voltages. The outer casings
of the input and the output front-panel BNC connectors are tied to
Ground 2. The output current of the amplifier returns to the power
supply through Ground 2.
The outer casing of the rear-panel output BNC is connected to chas-
sis ground, Pin 9 of the DB–15 SIM interface connector. The sepa-
rate power, analog, and chassis grounds are
not directly connected
within the amplifier. When operating in the SIM900 Mainframe, the
three grounds are tied together inside the mainframe, and through
the mainframe to the Earth. Grounds 1 and 2 are connected inside
the SIM983 through back-to-back Schottky diodes, so they cannot be
more than
∼
± 0
.
35 V apart.
2.2 Autocalibration
To ensure DC o
ff
set accuracy, the amplifier must be self-calibrated
within the 24 hours preceding a measurement. A valid autocali-
bration must take place at (23
±
5)
◦
C with the module warmed up
for at least 2 hours at (23 ± 5)
◦
C
.
If the module is being used in-
side the SIM900 Mainframe, the autocalibration must also be inside
the mainframe. Otherwise, perform the autocalibration with the
same connection to an independent supply as you use for the opera-
tion. The autocalibration is only accurate if the output has stabilized
within
± 15 mV of zero for at least 2 minutes immediately preced-
ing the calibration. However, the gain and the o
ff
set need not be at
SIM983 Scaling Amplifier