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MLIDA, Rev A
3.1.1
Input Channel
The SR810 Lock-
In Amplifiers have differential inputs with 6 nV/√Hz input noise. The input
impedance is 10 MΩ, and minimum full-scale input voltage sensitivity is 2 nV. The input can also
be configured for current measurements with selectable current gains of 106 and 108 V/A. A line
filter (50 Hz or 60 Hz) and a 2× line filter (100 Hz or 120 Hz) are provided to eliminate line related
interference. However, unlike conventional lock-in amplifiers, no tracking band-pass filter is
needed at the input. This filter is used by conventional lock-ins to increase dynamic reserve.
Unfortunately, band pass filters also introduce noise, amplitude and phase error, as well as drift.
The DSP based design of these lock-ins has such inherently large dynamic reserve that no
tracking band-pass filter is necessary.
3.1.2
Extended Dynamic Reserve
The dynamic reserve of a lock-in amplifier at a given full-scale input voltage is the ratio (in dB) of
the largest interfering signal to the full-scale input voltage. The largest interfering signal is
defined as the amplitude of the largest signal at any frequency that can be applied to the input
before the lock-in cannot measure a signal with its specified accuracy.
Conventional lock-in amplifiers use an analog demodulator to mix an input signal with a reference
signal. Dynamic reserve is limited to about 60 dB and these instruments suffer from poor
stability, output drift, and excessive gain and phase error. Demodulation in the SR810 Lock-In
Amplifier is accomplished by sampling the input signal with a high-precision A/D converter, and
multiplying the digitized input by a synthesized reference signal. This digital demodulation
technique results in more than 100 dB of true dynamic reserve (no prefiltering). It is free of the
errors associated with analog instruments.
3.1.3
Digital Filtering
The digital signal processor also handles the task of output filtering, allowing time constants from
10 µsec to 30,000 s, with a choice of 6, 12, 18 and 24 dB/oct roll-off. For low frequency
measurements (below 200 Hz), synchronous filters can be engaged to notch out multiples of the
reference frequency. Since the harmonics of the reference have been eliminated, (notably 2F),
effective output filtering can be achieved with much shorter time constants.
3.1.4
Digital Phase Shifting
Analog phase shifting circuits have also been replaced with a DSP calculation. Phase is
measured with 0.01° resolution, and the X and Y outputs are orthogonal to 0.001°.
3.1.5
Frequency Synthesizer
The built-in direct digital synthesis (DDS) source generates a very low distortion (-80 dBc)
reference signal. Single frequency sine waves can be generated from 1 MHz to 102 kHz with 4½
digits of resolution. Both frequency and amplitude can be set from the front panel or from a
computer. When using an external reference, the synthesized source is phase locked to the
reference signal.
