Stanford Research Systems SR645 Data Sheet

· Two independent filter channels

· 115 dB/octave rolloff

· 1 Hz to 100 kHz frequency range

· 0.1 dB passband ripple

· 80 dB stopband attenuation

· 6 nV/

√√

Hz input noise

· Up to 80 dB of gain

· GPIB and RS-232 interfaces

· SR600 series... $3295

(U.S. list)

Stanford Research Systems phone: [phone]

www.thinkSRS.com

The SR600 series consists of three dual-channel instruments.
Each instrument channel has a low-noise preamplifier, a
precision high-pass or low-pass filter section and an output
amplifier. The filters are 8-pole, 6-zero elliptic-type with
115 dB per octave rolloff, 0.1 dB of passband ripple and 80 dB
stopband attenuation. Cutoff frequencies can be set between
1 Hz and 100 kHz making the SR600 series ideal for
applications including anti-aliasing, audio analysis and
general frequency-domain signal conditioning.

Model



Configuration

SR640



Dual low pass

SR645



Dual high pass

SR650



High pass/low pass (can be cascaded for band pass)

Inputs and Preamplifiers

Each filter channel has a floating, differential input with 1 MΩ
input impedance and 6 nV/√Hz of input noise. The analog
ground for each filter channel is available at a rear-panel BNC
connector to provide complete flexibility in grounding. The
filter grounds may be tied to the instrument's chassis ground
or to any other point. Both AC and DC input coupling can be
selected, and an invert key allows the phase of the signal to be
shifted by 180° with a single key press. Up to 60 dB of
preamplifier gain can be specified in 10 dB increments. A
simple offset adjust screw lets you null DC components in
your signal.

SR600 Series Programmable Filters

Programmable Filters

SR640, SR645, and SR650  High and low pass elliptical filters (2-channel)

www.thinkSRS.com

Filter Characteristics

Filter characteristics for the SR600 series are shown below.
The exact filter transfer function is given in the specifications
section. (The graph shows the low pass response. The high­pass response is found by inverting the curve around f=1.) The
large amount of excess phase, typical of elliptical filters,
makes the SR600 series filters optimum in frequency-domain
applications. Note that the 0.1 dB passband ripple
specification for the SR640 series filters is a measured
specification, not just the theoretical passband ripple of the
transfer function. Other filter characteristics include an
attenuation slope of −115 dB/octave and a stopband
attenuation of greater than 80 dB. Worst-case phase match
between channels is better than ±0.75° from DC to 100 kHz
and improves to ±0.25° for cutoff frequencies below 10 kHz.

Post-Filter Gain and Bypassing

After the filter, another amplifier section provides up to 20 dB
additional gain. The standard output drives loads of greater
than 300 Ω at 10 Vpp, while the optional high-power output
drives 10 Vpp into a 50 Ω load. Each filter may be
independently bypassed from the front panel or from the
computer interface. When bypassed, both the prefilter and
postfilter gains remain active, and the resulting amplifier has
a bandwidth of 450 kHz.

Programmable

A key feature of the SR640 series is its programmability. Both
GPIB and RS-232 interfaces are standard, and all instrument
settings can be queried and changed via the interfaces. Cutoff
frequencies can be changed in under 100 ms, making the
SR640 series ideal for applications involving frequency-agile
filters. Careful attention has been paid to minimizing noise
resulting from the presence of the microprocessor and GPIB
and RS-232 interfaces. Both the interface electronics and the
instrument's microprocessor control circuitry are optically
isolated from the filter sections in order to reduce digital­analog crosstalk. The CMOS microprocessor "sleeps" while
the interface and front panel are inactive in order to reduce the
amount of digital noise in the instrument.

Stored Settings

Up to nine complete instrument configurations, including
amplifier gains and cutoff frequencies for both channels, can
be saved in non-volatile memory. Settings can be stored or
recalled from the front panel or through the computer
interfaces.

SR600 Series Programmable Filters

Stanford Research Systems phone: [phone]

www.thinkSRS.com

SR640 gain response

SR640 phase response

0 dB

Gain

-80 dB

0.1 1.0 10
Frequency (f/fc)

˚

100

˚

200

Overall Phase

˚

300

˚

400

0.1 0.2 0.4 0.6 0.8 1.0
Frequency (f/fc)

20

40

60

80

100

˚

˚

˚

˚

˚

Phase Distortion

Filter

Frequency range 1 Hz to 100 kHz (3-digit resolution)
Type 8-pole, 6-zero elliptic
Rolloff 115 dB/octave
Passband ripple 0.1 dB peak to peak (typ.)
Stopband attenuation 80 dB (typ.)

Transfer Function (Bi-Quad Cascade)

Stage 1

ω

p

0.634752

Q 0.543939

ω

z

n/a

Stage 2

ω

p

0.805995

Q 0.950701

ω

z

1.675935

Stage 3

ω

p

0.985073

Q 2.095314

ω

z

1.935978

Stage 4

ω

p

1.076768

Q 7.375143

ω

z

2.883197

Input

Impedance 1 MΩ // 15 pF
Configuration Single-ended (A or B) or

differential (A−B)

CMRR >85 dB at 100 Hz

(input gain >20 dB)

Coupling AC or DC
Input noise 6 nV/√Hz at 1 kHz (with 60 dB

input gain)
Gain 0, 10, 20, 30, 40, 50, 60 dB ± 0.2 dB
Max. input signal 10 Vpp (overload)

5 Vpp (no distortion)

SR600 Series Specifications

Stanford Research Systems phone: [phone]

www.thinkSRS.com

Output

Impedance <1 Ω
Full-scale output signal 10 Vpp into >300 Ω
Option 01 10 Vpp into 50 Ω
DC offset Adjustable to 0 V
Gain 0, 10, 20 dB ± 0.2 dB
Harmonic distortion < −80 dB below full scale at 100 Hz

(<1 Vpp output) < −50 dB below full scale at 1 kHz

Spurious components < −80 dB below full scale

(with input source <50 Ω)

Channel crosstalk < −110 dB below full scale

(with input source <50 Ω)

Phase match between ±0.75° (DC to f

c

, typ.)

channels

General

Interfaces GPIB and RS-232 standard. All

instrument functions can be
controlled and read through the
interfaces.

Stored settings Nine complete dual-channel instrument

configurations may be stored in
non-volatile memory.

Power 45 W, 100/120/220/240 VAC,

50/60 Hz

Dimensions 15.7" × 3.0" × 14.0" (WHD)
Weight 12 lbs.
Warranty One year parts and labor on defects

in materials and workmanship

Ordering Information

SR640 Two-channel low pass filter with $3295

RS-232, GPIB and rack mount kit

SR645 Two-channel high pass filter with $3295

RS-232, GPIB and rack mount kit

SR650 One high, one low pass filter with $3295

RS-232, GPIB and rack mount kit

Option 01 High current output $150

SR640 rear panel