Verifying Ac Performance; Input Bias Current; Transfer Characteristic; Step Response - Stanford Research Systems SIM983 Operation And Service Manual

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4.1.3 Input bias current

4.2 Verifying AC Performance

4.2.1 Transfer characteristic

4.2.2 Step response

A simple test of the input current can be done by connecting the
input of the SIM983 to the input of a voltmeter that has a microvolt
range, such as the SIM970. The current will flow through a parallel
combination of the 1 M input resistance of the SIM983 and the input
resistance of the voltmeter, which is typically 10 M
and is that or greater in other voltmeters. Divide the voltmeter
reading by the resistance (e.g. 0.9 M ) to obtain the current. A current
that exceeds the specification in the table on Page vi indicates a
damaged front end. The module should then be returned to Stanford
Research Systems for repair.
Most information about the AC behavior of the SIM983 Scaling Am-
plifier can be deduced by observing the response of the instrument
to a square wave at the input. The equipment required for the test is
a function generator with at most 25 ns square-wave rise time, such
as the Stanford Research Systems' DS345, and an oscilloscope with
at least 100 MHz bandwidth. An FFT spectrum analyzer, such as
the Stanford Research Systems' SR785, is needed to measure total
harmonic distortion and noise.
It is possible to measure the small-signal bandwidth of the amplifier
by applying a 100 mV peak-peak sine wave to its input, and increas-
ing the frequency of the applied signal until the output amplitude
reduces to 3 dB, i.e. 1
small-signal bandwidth can also be measured from the rise time
of the instrument's response to a small-input step. For example,
in Figure 4.1 measure the rise time of the output from 10% to 90%,
i.e. 400 mV to 400 mV:
so the small-signal bandwidth
f
3 dB
which is consistent
GBP 3 0 MHz in the specification table on Page vi.
Figures 4.1–4.6 illustrate the typical responses of a SIM983 to steps
in the input voltage. Figure 4.2 is for G
Performance Verification
of its low-frequency value. However, the
2
t 111 ns
rise
0 35
(G 1) 3 1 Mhz
t
rise
with the bandwidth
4 00 a value near the
SIM983 Scaling Amplifier
in the SIM970
expected from