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the PSD output which allows the remaining DC components to be amplified to reach 10 V full scale.
This gain is distributed between AC gain before the PSD and DC gain following the PSD. The total gain is the product of
the AC gain and the DC gain. Suppose the total gain is a constant. If the AC gain increases and the DC gain decreases,
the input noise is easy to overload after AC gain. Thus, the dynamic reserve and the DC drifts decrease. In contrast, if the
AC gain decreases and the DC gain increases, the dynamic reserve increases. In this case, the output stability will
decrease and the accuracy of measurement will be lower.
The noise frequencies and amplitudes affect the accuracy of the DC output signal. Noise at the reference frequency with
large amplitude becomes part of the DC signal after the PSD. This enlarges the output error of the lock-in amplifier.
The dynamic reserve is related to noise frequency. The dynamic reserve is 0dB at reference frequency and increases
when the noise frequency moves away from the reference frequency. It reaches a maximum value when the frequency is
far enough. The dynamic reserve near the reference frequency is important to noise tolerance of the instrument.
Providing more low pass filter stages can improve the performance of the filters and then increase the dynamic reserve
close to the reference frequency. The dynamic reserve far from the reference frequency is generally high but has little
influence.
The dynamic reserve of SE1022D is greater than 120dB. High dynamic reserve will increase output noise and drift.
When the dynamic reserve is high, output noise will be increased due to the A/D converter. There is background noise at
any signal. When the signal is amplified by PSD, the output signal will contain noises. If the noise is very high, it will
result in large output noise. Otherwise, if the external noise is very low, the output is mainly affected by the noise of
SE1022D. Reducing dynamic reserve and DC gain can decrease the error. Therefore, low dynamic reserve should be
chosen firstly in actual application.
In fact, the minimum reserve changes with the sensitivity (gain) of the instrument. At high gains, the minimum dynamic
reserve increases with the increase of the sensitivity. In analog lock-in amplifiers, low dynamic reserve means low output
error and drift. In SE1022D, high dynamic reserve increases output noise, but not increases output error and drift.
However, if the gain of analog amplifier is high enough, the amplified intrinsic noise will be greater than the noise
generated by the A/D converter. In this case, increasing the analog gain cannot decrease the output noise. At high
sensitivity, decreasing the gain will increase the dynamic reserve.
1.8 Signal Input Amplifier and Filters
A lock-in amplifier can measure signals as small as a few nanovolts. The gain of the low noise signal amplifier should be
large enough so that the output signal can be digitized by the A/D converter without degrading the signal to noise ratio
(SNR). The analog gain of the SE1022D ranges from roughly 7 to 1000 times. Higher gains do not improve the SNR.
The overall gain (AC plus DC) is determined by the sensitivity and the distribution of the gain is set by the dynamic
reserve.
Input Noise
