Stanford Research Systems SIM914, Operation And Service Manual

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2.4 Offset Calibration 2 – 3
the AD8009. The offset will need to be adjusted after the HF com-
pensation is adjusted.
1. Split the pulse output from from the DG535 (set to 1 V ampli-
tude) with a coax tee. Take one cable from the tee to channel 1
of the oscilloscope (set to 50
Ω
input termination) and the other
to the top channel of the SIM914 via a 20 dB coaxial attenuator.
2. Adjust P102 (Channel 1 “HF COMP” pot) to match the output
rise time and overshoot to the input rise time and overshoot.
3. Repeat for Channel 2, adjusting the pulse response with P202.
2.4 Offset Calibration
The output offset is affected by the HF compensation and so the off-
set should be nulled after the HF compensation is adjusted. The
offset may also be affected by the amplifier gain adjustment if there
is a large input offset voltage.
1. Leave the inputs unconnected.
2. Connect the output (without a 50 Ω load) to the DMM on the
millivolt DC range.
3. Adjust P101 (Channel 1 “OFFSET” pot) to null the output volt-
age.
4. Verify that the output voltage shifts down by less than 2.5 mV
when a 50 Ω terminator is placed on the input. (The voltage
shift
∆ V
out
= 10
×
i
b
×
∆ R
s
, where
i
b
is the input bias current,
and ∆ R
s
is the change in input source impedance, here 25 Ω .
This confirms that the input bias current i
b
< 10 µ A .)
5. Repeat the procedure to null the output of Channel 2 by ad-
justing P201.
2.5 Gain Calibration
The overall gain of the amplifier is 5
×
when driving a 50 Ω load
and 10
×
when driving a high impedance load. The input source to
the amplifier is typically a current source (such as the output from a
photomultiplier tube) and so the magnitude of the input resistance is
included in the gain calibration by measureing the transimpedance.
(Calibration is done with a current source as an input while measur-
ing the voltage at the output.)
A DMM used in the 4-wire resistance mode is convenient for per-
forming the calibration. Typically a DMM will measure small resis-
tances by measuring the voltage across the resistor while passing a
SIM914 350 MHz Preamplifier