Stanford Research Systems SR865A, Operation Manual

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Chapter 2 Basics 51
SR865A DSP Lock-in Amplifier
Anti-aliasing Filter
Between the input amplifier and A/D converter there is an anti-aliasing filter. This filter
is required by the signal digitization process. According to the Nyquist criterion, signals
must be sampled at a frequency at least twice the highest signal frequency. In this case,
the highest signal frequency is 4 MHz and the sampling frequency is 10 MHz so things
are OK. However, no signals above 5 MHz can be allowed to reach the A/D converter.
These signals would violate the Nyquist criterion and be undersampled. The result of this
undersampling is
“aliasing”: these higher frequency signals appear as lower frequencies
in the digital data stream. Thus a signal at 9 MHz would appear as 1 MHz in the digital
data stream and be detectable by the digital PSD. This would be a problem.
To avoid this aliasing, the analog signal is filtered to remove any signals above 4 MHz.
This filter has a flat pass band from dc to 4.5 MHz so as not to affect measurements in the
operating range of the lock-in. The filter rolls off rapidly from 5 MHz to 6 MHz.
Amplitude variations and phase shifts due to this filter are calibrated at the factory and do
not affect measurements. This filter is transparent to the user.
Input Impedance
The input impedance of the SR865A is 10 MΩ. If a higher input impedance is desired,
then a preamplifier such as the SR550 or SR551 must be used. The SR550 has an input
impedance of 100
MΩ and is ac coupled from 1 Hz to 100 kHz, while the SR551 has an
input impedance of > 1 TΩ and is dc coupled with 1 MHz bandwidth .
Input Connections
In order to achieve the best accuracy for a given measurement, care must be taken to
minimize the various noise sources that can be found in the laboratory. With intrinsic
noise (Johnson noise, 1/f noise or input noise), the experiment or detector must be
designed with these noise sources in mind. These noise sources are present regardless of
the input connections. The effect of noise sources in the laboratory (such as motors,
signal generators, etc.) and the problem of differential grounds between the detector and
the lock-in can be minimized by careful input connections.
There are two basic methods for connecting a voltage signal to the lock-in. The single-
ended connection is more convenient, while the differential connection eliminates
spurious pick-up more effectively.
Single-Ended Voltage Connection (A)
In the first method, the lock-in uses the A input in a single-ended mode. The lock-in
detects the signal as the voltage between the center and outer conductors of the A input
only. The lock-in does not force the shield of the A cable to ground, rather it is internally
connected to the lock-in's ground via a resistor. The value of this resistor is selected by
the user. Float uses 10 kΩ and Ground uses 10Ω. This avoids ground loop problems
between the experiment and the lock-in due to differing ground potentials. The lock-in
lets the shield
‘
quasi-float
’
in order to sense the experiment ground. However, noise
pickup on the shield will appear as noise to the lock-in. This is bad since the input
amplifier cannot reject this noise. Common mode noise, which appears on both the center
and shield, is rejected by the common-mode rejection of the lock-in input, but noise on
only the shield is not rejected at all.