3 – 4
Performance Details
The factor, 2
√
2
/π
=
0
.
9003, would cause the SR124 to read about
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
Actual Gain
=
10 V
V
FS
×
π
2
√
2
(3.2)
Continuing the example from above, for a sensitivity of 200
µ
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.
3.1.3
DC gain
The DC gain is programmed by the SR124 to make up the di
ff
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
µ
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
×
10
3
; for
Normal
reserve, the total AC gain is
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:
Reserve
DC Gain
Low Noise
22.21
Normal
222.1
High Res.
2221
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:
Low Noise : DC Gain is 222.1 for sensitivitys 100 nV, 200 nV, and 500 nV.
Normal : DC gain is 22.21 for sensitivities 100 mV, 200 mV, and 500 mV.
High Res. : DC gain is 222.1 for sensitivities 10 mV, 20 mV, 50 mV; and DC
gain is 22.21 for sensitivities 100 mV, 200 mV, and 500 mV.
3.2
Dynamic reserve and overloads
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
SR124
Analog Lock-In Amplifier
