Atec SR810, SR830 User Manual

Digital Lock-In Amplifiers

SR810 and SR830 — DSP lock-in amplifiers

SR830 DSP Lock-In Amplifier

· 1 mHz to 102.4 kHz frequency range

· >100 dB dynamic reserve

· 5 ppm/°C stability

· 0.01 degree phase resolution

· Time constants from 10 µs to 30 ks
(up to 24 dB/oct rolloff)

·

Auto-gain, -phase, -reserve and -offset

· Synthesized reference source

· GPIB and RS-232 interfaces

· SR810 ... $3850 (U.S. list)

· SR830 ... $4750 (U.S. list)

SR810 & SR830 DSP Lock-In Amplifiers

The SR810 and SR830 DSP Lock-In Amplifiers provide high
performance at a reasonable cost. The SR830 simultaneously
displays the magnitude and phase of a signal, while the SR810
displays the magnitude only. Both instruments use digital
signal processing (DSP) to replace the demodulators, output
filters, and amplifiers found in conventional lock-ins. The
SR810 and SR830 provide uncompromised performance with

an operating range of 1 mHz to 102 kHz and 100 dB of drift-

free dynamic reserve.

Input Channel

The SR810 and SR830 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 inputs 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 lock­ins to increase dynamic reserve. Unfortunately, band pass
filters also introduce noise, amplitude and phase error, and
drift. The DSP design of these lock-ins has such inherently
large dynamic reserve that no band pass filter is needed.

Extended Dynamic Reserve

The dynamic reserve of a lock-in amplifier, at a given full­scale input voltage, is the ratio (in dB) of the largest interfering

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SR810 and SR830 DSP Lock-In Amplifiers

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 and SR830 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)
and is free of the errors associated with analog instruments.

Digital Filtering

The digital signal processor also handles the task of output
filtering, allowing time constants from 10 µs to 30,000 s with
a choice of 6, 12, 18 and 24 dB/oct rolloff. 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.

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°.

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.

Useful Features

Auto-functions allow parameters that are frequently adjusted
to automatically be set by the instrument. Gain, phase, offset
and dynamic reserve are quickly optimized with a single
key press. The offset and expand features are useful when
examining small fluctuations in a measurement. The input

signal is quickly nulled with the auto-offset function, and
resolution is increased by expanding around the relative value

by up to 100×. Harmonic detection isn’t limited to 2F — any
harmonic (2F, 3F, ... nF) up to 102 kHz can be measured.

Analog Inputs and Outputs

Both instruments have a user-defined output for measuring X,
R, X-noise, Aux 1, Aux 2, or the ratio of the input signal to an
external voltage. The SR830 has a second, user-defined output

that measures Y, θ, Y-noise, Aux 3, Aux 4 or ratio. The SR810

and SR830 both have X and Y analog outputs (rear panel)
that are updated at 256 kHz.
ADCs) are provided for general purpose use — like normalizing
the input to source intensity fluctuations. Four programmable
outputs (16-bit DACs) provide voltages from –10.5 V to +10.5 V
and are settable via the front panel or computer interfaces.

Internal Memory

The SR810 has an 8,000 point memory buffer for recording
the time history of a measurement at rates up to 512 samples/s.
The SR830 has two, 16k point buffers to simultaneously
record two measurements. Data is transferred from the buffers
using the computer interfaces. A trigger input is also provided
to externally synchronize data recording.

Easy Operation

The SR810 and SR830 are simple to use. All functions are
set from the front-panel keypad, and a spin knob is provided
to quickly adjust parameters. Up to nine different instrument
configurations can be stored in non-volatile RAM for fast and
easy instrument setup. Standard RS-232 and GPIB (IEEE-

488.2) interfaces allow communication with computers.

Four auxiliary inputs (16-bit

Ordering Information

SR830 DSP dual phase lock-in $4750
amplifier (w/ rack mount)
SR810 DSP single phase lock-in $3850
amplifier (w/ rack mount)
SR550 Voltage preamplifier $750

(100 MΩ, 3.6 nV/√Hz)

SR552 Voltage preamplifier $750

(100 kΩ, 1.4 nV/√Hz)

SR554 Transformer preamplifier $1200

(0.091 nV/√Hz)

SR555 Current preamplifier $1095
SR556 Current preamplifier $1095
SR540 Optical chopper $1195

SR810 DSP Single Phase Lock-In Amplifier

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