Chapter 3, TECHNICAL DESCRIPTION
3-5
AC Gain (dB) INPUT LIMIT (mV)
0 2000
6 1000
12 500
18 250
24 125
30 62
36 31
42 16
48 8
54 4
60 2
66 1
72 0.5
78 0.25
84 0.125
90 0.062
Table 3-1, Input Limit vs. AC Gain
It is a basic property of the digital signal processing (DSP) lock-in amplifier that the
best demodulator performance is obtained by presenting as large a signal as possible
to the main analog-to-digital converter (ADC). Therefore, in principle, the AC Gain
value should be made as large as possible without causing the signal channel
amplifier or converter to overload. This constraint is not too critical however and the
use of a value one or two steps below the optimum value makes little difference.
Note that as the AC Gain value is changed, the demodulator gain (described later in
section 3.3.14) is also adjusted in order to maintain the selected full-scale sensitivity.
The full-scale sensitivity is set by a combination of AC Gain and demodulator gain.
Since the demodulator gain is entirely digital, changes in full-scale sensitivity which
do not change the AC Gain do not cause any of the errors which might arise from a
change in the AC Gain.
The user is prevented from setting an illegal AC Gain value, i.e. one that would
result in overload on a full-scale input signal. Similarly, if the user selects a full-scale
sensitivity that causes the present AC Gain value to be illegal, the AC Gain will
change to the nearest legal value.
In practice, this system is very easy to operate. However, the user may prefer to make
use of the AUTOMATIC AC Gain feature that gives very good results in most cases.
When this is active the AC Gain is automatically controlled by the instrument, which
determines the optimum setting based on the full-scale sensitivity currently being
used.
At any given setting, the ratio
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