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3.4 Attenuators and DC bias constraints
3.4 Attenuators and DC bias constraints
3.5 Automatic functions
3.5.1 Auto-phase
SR124 Analog Lock-In Amplifier
The SR124 Reference Output includes several resistive output at-
tenuators to provide low excitations for sensitive experiments. The
architecture imposes constraints on the magnitude of DC bias avail-
able, depending on the current reference amplitude setting. This
constraint is because the DC bias signal is passed through the same
resistive attenuator network as the AC reference signal.
Table 3.4 shows the four attenuator ranges, set by the reference output
amplitude. Note that any time one of the attenuators is switched in
or out (when the amplitude is commanded across one of the bound-
eries listed in the table), the actual output waveform is zeroed for
approximately 20 ms. Also note that, for large values of DC bias
and amplitude, the actual output voltage will saturate outside of the
range 14.5 V DC.
Table 3.4: Available DC Bias values by Amplitude setting
AC Amplitude
(RMS)
10 mV – 10 V
100 V – 9.99 mV
1 V – 99.9 V
100 nV – 990 nV
The various automatic functions of the SR124 all involve the instru-
ment performing some internal measurement and then adjusting a
parameter as a result. This section discusses the constraints and
required time for the functions.
The automatic phase cycle will adjust the phase setting of the SR124
to maximize the detected signal in Lock-in mode. Three steps occur
in an auto-phase cycle:
1. The phase shift is internally set to 0 , and the instrument then
pauses for 8 output Time Constant periods (for settling). At the
end of the settling time, the voltage at the output is internally
measured.
2. The phse shift is then internally set to 90 , and the instrument
again waits 8 Time Constants for settling. At the end of this
settling time, the output voltage is again measured.
DC Bias
DC Bias
Range
Resolution
10 V
1 mV
100 mV
10 V
1 mV
100 nV
100 V
10 nV
3 – 9
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