Dc Gain; Dynamic Reserve And Overloads - Stanford Research Systems SR124 Operation And Service Manual

3 – 4

3.1.3 DC gain

Low Noise : DC Gain is 222.1 for sensitivitys 100 nV, 200 nV, and 500 nV.
High Res. : DC gain is 222.1 for sensitivities 10 mV, 20 mV, 50 mV; and DC

3.2 Dynamic reserve and overloads

The factor, 2 2
10% too low if the circuitry were tuned to provide a total gain as
given by Equation (3.1). Instead, the actual total gain is set about
11% greater, to correct for this factor.
The correct formula for the actual overall gain is
Continuing the example from above, for a sensitivity of 200 V, the
actual overall gain, given by Equation (3.2), is 55,536.
A natural consequence of this calibration is that, when the input
waveform is a square wave, the output reads approximately 11%
greater than the actual RMS value of the (square wave) input.
The DC gain is programmed by the SR124 to make up the di erence
between the total AC gain and the required Overall Gain from Equa-
tion (3.2). Continuing the example above, with full-scale sensitivity
of 200 V, we can see from Table 3.1 three possible AC gain config-
urations, based on the dynamic reserve setting. At Low Noise, the
total AC gain is 2 5
250; while at High Res., we see the total AC gain is 25. Since in each
case, the overall Actual Gain must be 55,536, we find the DC gain is:
These three DC Gains are used for most of the sensitivity settings of
the SR124. The exceptions are at the extreme values of sensitivity,
and are as follows:
Normal : DC gain is 22.21 for sensitivities 100 mV, 200 mV, and 500 mV.
gain is 22.21 for sensitivities 100 mV, 200 mV, and 500 mV.
The SR124 provides significant flexibilty to recover small signals in
the presence of noise and other interference: besides overall sensi-
tivity control, the input filter and dynamic reserve settings can both
0 9003, would cause the SR124 to read about
10 V
Actual Gain
V
FS
3
10 ; for Normal reserve, the total AC gain is
Reserve

DC Gain

Low Noise 22.21
Normal 222.1
High Res. 2221
SR124 Analog Lock-In Amplifier
Performance Details
(3.2)
2 2