Input Impedance - Stanford Research Systems SR810 Manual

Using either of these filters precludes making
measurements in the vicinity of the notch
frequencies. These filters have a finite range of
attenuation, generally 10 Hz or so. Thus, if the
lock-in is making measurements at 70 Hz, do not
use the 60 Hz notch filter! The signal will be
attenuated and the measurement will be in error.
When measuring phase shifts, these filters can
affect phase measurements up to an octave away.
Anti-aliasing filter
After all of the signal filtering and amplification,
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 100 kHz and the sampling frequency
is 256 kHz so things are ok. However, no signals
above 128 kHz 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 to make these higher frequency
signals appear as lower frequencies in the digital
data stream. Thus a signal at 175 kHz would
appear below 100 kHz in the digital data stream
and be detectable by the digital PSD. This would
be a problem.
To avoid this undersampling, the analog signal is
filtered to remove any signals above 154 kHz
(when sampling at 256 kHz, signals above
154 kHz will appear below 102 kHz). This filter has
a flat pass band from DC to 102 kHz so as not to
affect measurements in the operating range of the
lock-in. The filter rolls off from 102 kHz to 154 kHz
and achieves an attenuation above 154 kHz of at
least 100 dB. Amplitude variations and phase
shifts due to this filter are calibrated out at the
factory and do not affect measurements. This filter
is transparent to the user.

Input Impedance

The input impedance of the SR810 is 10 MΩ. If a
higher input impedance is desired, then the
SR550 remote preamplifier must be used. The
SR550 has an input impedance of 100 MΩ and is
AC coupled from 1 Hz to 100 kHz.
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SR810 Basics