Stanford Research Systems SR850 Data Sheet

· 1 mHz to 102.4 kHz frequency range

· >100 dB dynamic reserve

· 0.001 degree phase resolution

· Time constants from 10

µµ

s to 30 ks

(up to 24 dB/oct rolloff)

· Auto gain, phase, reserve and offset

· Data logging (up to 65k samples)

· Smoothing, curve fitting & statistics

· GPIB, RS-232 and 3.5” disk drive

· SR850 ... $7500

(U.S. list)

Stanford Research Systems phone: [phone]

www.thinkSRS.com

The SR850 is a digital lock-in amplifier based on an innovative
DSP (Digital Signal Processing) architecture. The SR850
boasts a number of significant performance advantages over
traditional lock-in amplifiershigher dynamic reserve, lower
drift, lower distortion and dramatically higher phase resolution.
In addition, the CRT display and 65,536 point memory make
it possible to display and process data in a variety of formats
unavailable with conventional lock-ins.

Digital Precision

At the input of the SR850 is a precision 18-bit A/D converter
which digitizes the input signal at 256 kHz. The A/D converter,
together with a high-speed DSP chip, replace the analog
demodulator (mixer), low pass filters and DC amplifiers
found in conventional lock-ins. Instead of using analog
components, the SR850 is implemented by a series of precise
mathematical calculations which eliminate the drift, offset,
non-linearity and aging inherent in analog components. The
same DSP chip digitally synthesizes the reference oscillator
providing a source with less than −80 dBc distortion, 100 mHz
frequency resolution and 2 mV of amplitude resolution.

Digital Flexibility

The SR850 has a 7" CRT display which supports a large
selection of display options. Data can be viewed numerically
or graphically in bar graph, polar plot and strip chart formats.
With 65,536 points of memory and data acquisition rates up to

SR850 DSP Lock-In Amplifier

Digital Lock-In Amplifiers

SR850  DSP lock-in amplifier with graphical display

www.thinkSRS.com

512 Hz, you are able to see exactly how your data changes in
timenot just what the current output value is. After the data
has been acquired, the SR850 offers a variety of data reduction
options such as Savitsky-Golay smoothing, curve-fitting and
statistical analysis. Abuilt-in 3.5" disk drive, along with standard
RS-232 and GPIB interfaces, makes it easy to transfer data to
your computer.

Input Channel

The SR850 has a differential input 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
10

6

and 108V/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 of the
SR850. This filter is used by conventional lock-ins to increase
dynamic reserve. Unfortunately, band pass filters also introduce
noise, amplitude and phase error, and drift. The DSP based
design of the SR850 has such inherently large dynamic
reserve that no tracking band-pass filter is needed.

Reference Channel

The reference source for the SR850 can be an externally
applied sine or square wave, or its own digitally synthesized
reference source. Because the internal reference source is
synthesized from the same digital signal that is used to multiply
the input, there is virtually no reference phase noise when
using the internal reference. The internal reference can operate
at a fixed frequency or can be swept linearly or logarithmically
over the entire operating range of 1 mHz to 102.4 kHz.
Harmonic detection can be performed at any integer harmonic
of the reference frequency, not just the first few harmonics.

The DSP approach also offers considerable advantages when
working with an external reference. The time to acquire an
external reference is only 2 cycles + 5 ms (or 40 ms, whichever
is greater)about ten times faster than conventional lock-ins.

Because the SR850 uses a digital phase-shifting technique
rather than analog phase-shifters, the reference phase can be
adjusted with one millidegree resolution. In addition, the X
and Y outputs are orthogonal to within one millidegree.

Outputs and Time Constants

The output time constants on the SR850 are implemented
digitally. Low pass filter rolloffs of 6, 12, 18 and 24 dB/octave
are available, with time constants ranging from 10 µs to 30 ks.
Below 200 Hz, the SR850 can perform synchronous filtering.
Synchronous filters notch out multiples of the reference
frequency, an especially useful feature at low frequencies
where the proximity of the 2f component requires a long time
constant for effective filtering. The SR850 makes working at
low frequencies a far less time consuming task.

High 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
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.

The SR850 has the highest dynamic reserve (>100 dB) of any
lock-in available. In conventional lock-in amplifiers, dynamic
reserve is increased at the expense of stability. Because of the
digital nature of the filtering and gain process in the SR850,
the ultra-high dynamic reserve is obtained without any sacrifice
in stability or accuracy. In addition, the SR850's high dynamic
reserve is obtained without the use of analog band-pass filters,
eliminating the noise and error that such filters introduce.

Traces and Displays

Data acquired by the SR850 is stored in up to four user­defined traces. Each trace can be configured as (A×B)/C,
where A, B and C are selected from X, Y, R, θ, noise, frequency
or any of the four rear-panel auxiliary inputs. Common
operations, such as ratioing, can be performed in real time by
defining an appropriate trace. Trace values can be displayed as

SR850 DSP Lock-In Amplifier

Stanford Research Systems phone: [phone]

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Graphical, numerical and bar-graph display

Large numeric readout with bar graph

a bar graph with an associated large numerical display, or as a
strip chart showing the trace values as a function of time.
Additionally, you can display polar plots showing the phasor
formed by the in-phase and quadrature components of the signal.
All displays can be easily scaled from the front panel or the

computer interface, and an auto-scale feature is available to
quickly optimize the display. The screen can be configured as
a single large display, or as two horizontally split displays.

Convenient Auto Measurements

Common measurement parameters are available as single-key
"auto" functions. The gain, phase, dynamic reserve and display
scaling can all be set with a single key press. For many
measurements, the instrument can be completely configured
simply by using the auto functions.

Auxiliary A/Ds and D/As

Four rear-panel A/D inputs allow you to measure external
signals with millivolt resolution. The measured values can be
incorporated into one of the SR850's trace definitions, or can
be displayed on the front panel, or read via the computer
interface. Four D/A outputs can provide either fixed output
voltages or a voltage level which scans synchronously with
the SR850's frequency scans. Both the A/D inputs and the D/A
outputs have a ±10 V range.

Analysis Features

The SR850's performance doesn't stop once data has been
acquireda full set of data processing features is also included.
Multiple-range Savitsky-Golay smoothing can be applied to
any of the trace arrays, and statistical information (mean,
variance, sum) can be calculated for a selected trace region. A
curve fitting routine calculates best fits to lines and exponential
and Gaussian curves for any portion of your data. And a trace
"calculator" lets you perform a variety of simple arithmetic
and trigonometric operations on trace data.

Interfaces and Hardcopies

The SR850 comes standard with RS-232 and GPIB interfaces.
All instrument functions can be queried and controlled via the
interfaces. For convenient debugging, characters received and
sent via the interfaces can be viewed on the front panel.
Several hardcopy options are available on the SR850. Screens
can be dumped to a dot-matrix or LaserJet compatible printer
through the standard Centronics printer interface. Displays
can also be plotted on any HP-GL compatible plotter via GPIB
or RS-232.

SR850 DSP Lock-In Amplifier

Stanford Research Systems phone: [phone]

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Polar plot display

Ordering Information

SR850 DSP dual phase lock-in $7500

amplifier (w/ rack mount)
O850H Carrying handle kit $100
SR550 Voltage preamplifier $595

(100 MΩ, 3.6 nV/√Hz)
SR552 Voltage preamplifier $595

(100 kΩ, 1.4 nV/√Hz)
SR554 Transformer preamplifier $995

(0.091 nV/√Hz)
SR540 Optical chopper $1095

SR850 rear panel

Signal Channel

Voltage inputs Single-ended or differential
Sensitivity 2 nV to 1 V
Current input 10

6

or 108V/A

Input impedance

Voltage input 10 MΩ + 25 pF, AC or DC coupled
Current input 1 kΩ to virtual ground

Gain accuracy ±1 % (±0.2 % typ.)
Noise (typ.) 6 nV/√Hz at 1 kHz

0.13 pA/√Hz at 1 kHz (10

6

V/A)

0.013 pA/√Hz at 100 Hz (10

8

V/A)
Line filters 50/60 Hz and 100/120 Hz (Q=5)
CMRR 100 dB at 10 kHz, decreasing by

6 dB/oct above 10 kHz

Dynamic reserve >100 dB (without prefilters)

Reference Channel

Frequency range 0.001 Hz to 102.4 kHz
Reference input TTL or sine (400 mVpp min.)
Input impedance 1 MΩ, 25 pF
Phase resolution 0.001°
Absolute phase error <1°
Relative phase error <0.001°
Orthogonality 90° ± 0.001°
Phase noise

Int. reference <0.0001° rms at 1 kHz
Ext. reference 0.005° rms at 1 kHz, 100 ms,

12 dB/oct

Phase drift <0.01°/°C below 10 kHz,

<0.1°/°C, 10 kHz to 100 kHz
Harmonic detection 2F, 3F, ... nF to 102.4 kHz
Acquisition time (2 cycles + 5 ms) or 40 ms,

whichever is greater

Demodulator

Stability

Digital outputs no drift
Analog outputs <5 ppm/°C for all dynamic reserves

Harmonic rejection −90 dB
Offset/Expand ±100 % offset, expand up to 256×
Time constants 10 µs to 30 ks

(6, 12, 18, 24 dB/oct rolloff)

Synchronous filtering available

below 200 Hz.

Internal Oscillator

Range 1 mHz to 102.4 kHz
Accuracy 25 ppm + 30 µHz
Resolution 0.01 % or 0.1 mHz

(whichever is greater)
Distortion −80 dBc (f <10 kHz)

−70 dBc (f >10 kHz) at 1 Vrms

Amplitude 0.004 to 5 Vrms into 10 kΩ

(2 mV resolution)
Output impedance 50 Ω
Amplitude accuracy 1 %
Amplitude stability 50 ppm/°C

Outputs Sine and TTL (both can be phase-

locked to an external reference)

Sweeps Linear and log

Inputs and Outputs

Interfaces IEEE-488.2, RS-232 and Centronics

interfaces standard. All instrument
functions can be controlled and read

though the interfaces.
X, Y outputs ±10 V, updated at 256 ksamples/s
CH1 output ±10 V output of X, R or Trace 1 to 4
CH2 output ±10 V output of Y, θ or Trace 1 to 4
Aux. A/D inputs 4 BNC inputs, 1 mV res., ±10 V
Aux. D/A outputs 4 BNC outputs, 1 mV resolution,

±10 V (fixed or swept amplitude)
Sine out Internal oscillator analog output
TTL out Internal oscillator TTL output
Trigger In TTL signal starts internal oscillator

sweep or triggers instrument

data taking (rates to 512 Hz).
Remote pre-amp Provides power to the optional

SR550, SR552 and SR554 preamps

Displays

Screen format Single or dual display
Displayed quantities Each display shows one trace. Traces

are defined as A×B/C or A×B/C

2

where A, B, C are selected from X,

Y, R, θ, X-noise, Y-noise, R-noise,

Aux 1 to 4 or frequency.
Display types Large numeric readout, bar graph,

polar plot and strip chart
Data buffer 64k data points. Buffer is configured

as a single trace with 64k points,

two traces with 32k points each, or

four traces with 16k points each.
Sample rate 0.0625 to 512 Hz, external to 512 Hz

Analysis Functions

Smoothing 5, 9, 17, 21, 25 pt. (Savitsky-Golay)
Curve fitting Linear, exponential or Gaussian
Calculator Arithmetic, trigonometric and

logarithmic calculations
Statistics Mean and standard deviation

General

Hardcopy Screen dumps to dot-matrix or

LaserJet printers. Plots to HP-GL

plotters (RS-232 or GPIB).
Disk drive 3.5" MS-DOS compatible format,

1.44 Mbyte. Storage of data and

instrument setups (binary or ASCII).

Screens can be saved as PCX files.
Power 60 W, 100/120/220/240 VAC,

50/60 Hz
Dimensions 17" × 6.25" × 19.5" (WHD)
Weight 40 lbs.
Warranty One year parts and labor

SR850 Specifications

Stanford Research Systems phone: [phone]

www.thinkSRS.com

DSP lock-in amplifiers differ considerably from their analog
counterparts as a glance at the block diagram below will
quickly reveal. Although the front-end of both types of lock-ins
contain a low-noise AC amplifier, and 60 Hz and 120 Hz line
filters are provided in both cases, the similarity ends there. In
the SR850 the signal is filtered with a 100 kHz, 9

th

order,
elliptical anti-aliasing filter. This filtering is crucial to ensure
that the signal can be digitized by the 256 kHz, 18-bit A/D
converter with no aliasing.

The A/D converter passes the digitized signal to the DSP chip.
The DSP chip synthesizes a 24-bit digital reference sine wave
at the reference frequency. In the internal reference case, the
reference is synthesized from a high-accuracy crystal
oscillator. In the external reference case, a phase-locked loop
locked to the external reference serves as a source for the
reference signal. The reference is multiplied by the signal in
the DSP chip, which is capable of performing 16 million
24-bit × 24-bit multiplies and additions each second. After
multiplication, up to four stages of digital low-pass filtering
are applied to generate time constants from 10 µs to 30 ks,
with filter rolloffs of 6, 12, 18, and 24 dB/octave.

SR850 DSP Lock-In Amplifier

Stanford Research Systems phone: [phone]

www.thinkSRS.com

About DSP Lock-In Amplifiers

The resulting X and Y (in-phase and quadrature) signals are
used to digitally calculate the values of R and θ. The results
are sent to the main system processor for display on the CRT,
and passed through an 18-bit digital to analog converter to
generate the front-panel outputs.

The same digital reference that was used to multiply the signal
is converted by another 18-bit D/A converter and is used as the
SR850's internal oscillator. Thus, the internal oscillator output
is actually the same signal as the reference, and there is
virtually no reference phase noise when using it. Phase
shifting the reference is also simple in this model, since only
digital calculations are involved.

It is this digital architecture which makes possible the
performance advantages found in the SR850. Comparing the
diagram below with a block diagram of a conventional lock-in
shows that many of the most troublesome components, noisy
input prefilters, nonlinear demodulators, inaccurate analog
filters, and drift-prone, high-gain DC amplifiers, have all been
removed, along with their performance penalties. The
resulting instrument comes closest to implementing the
theoretical model of lock-in amplification.

DSP Lock-In Amplifier Block Diagram

Voltage

Current

Low Noise

Differential Amp
A

B

I

60 Hz

Notch Filter

120 Hz

Notch Filter

100 kHz

256 kHz 18 bit

Anti-Alias

Analog to Digital

Filter

Converter

A to D

Reference In
Sine or TTL

Crystal Osc

RS-232 Interface

GPIB Interface

Printer Interface

Aux Inputs

Phase

Locked

Loop

Clock

Generator

256 kHz 18 bit

Digital to Analog

Converter

D to A

Main System

Processor

Digital
Signal

Processor

256 kHz 18 bit

Digital to Analog

Converter

D to A

CRT Display

Front Panel

Disk Drive

Aux Outputs

X Output
Y Output
R Output
Ø Output

Sine Output