Programmable Filters
SR640, SR645, and SR650 — High-pass and low-pass elliptical filters (2-channel)

SR600 Series Programmable Filters· 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

The SR600 series consists of three dual-channel instruments.
Each instrument channel has a low-noise preamplifier, aprecision high-pass or low-pass filter section, and an output
amplifier. The filters are 8-pole, 6-zero elliptic-type with115 dB per octave rolloff, 0.1 dB of passband ripple, and80 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, andgeneral frequency-domain signal conditioning.
Model — Configuration· Up to 80 dB of gain

SR640 — dual low-pass
SR645 — dual high-pass
SR650 — high-pass/low-pass (can be cascaded for band-pass)· GPIB and RS-232 interfaces

Inputs and Preamplifiers· SR600 series... $3595 (U.S. list)

Stanford Research Systems

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. Thefilter grounds may be tied to the instrument’s chassis groundor to any other point. Both AC and DC input coupling canbe 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 ofpreamplifier gain can be specified in 10 dB increments. Asimple offset adjust screw lets you null DC components inyour signal.

phone: (408)744-9040
www.thinkSRS.com

SR600 Series Programmable Filters

Filter Characteristics

Programming

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 ellipticalfilters, makes the SR600 series filters optimum in frequencydomain 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 ofthe transfer function. Other filter characteristics include anattenuation slope of 115 dB/octave and a stopband attenuation ofgreater 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.

A key feature of the SR600 series is its programmability. BothGPIB 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 theSR600 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 opticallyisolated 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 theamount of digital noise in the instrument.
Stored Settings

Post-Filter Gain and Bypassing
After the filter, another amplifier section provides up to20 dB additional gain. The standard output drives loads ofgreater 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 thecomputer interface. When bypassed, both the prefilter andpostfilter gains remain active, and the resulting amplifier hasa bandwidth of 450 kHz.

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

20 °
100 °

40 °
60 °

200 °

80 °
100 °

Phase Distortion

Gain

Overall Phase

0 dB

300 °

-80 dB

400 °
0.1

1.0
Frequency (f/fc)

SR640 gain response

Stanford Research Systems

100.1 0.2

0.4

0.6

0.8

1.0

Frequency (f/fc)

SR640 phase response

phone: (408)744-9040
www.thinkSRS.com

SR600 Series Specifications

FilterOutput

Frequency range
TypeRolloff
Passband rippleStopband attenuation

1 Hz to 100 kHz (3-digit resolution)
8-pole, 6-zero elliptic
115 dB/octave
0.1 dB peak to peak (typ.)80 dB (typ.)Transfer Function (Bi-Quad Cascade)Stage 1ωp
Qωz
Stage 2
ωp
Qωz
Stage 3
ωp
Qωz
Stage 4
ωpQ
ωz0.634752
0.543939
n/a0.805995
0.950701
1.675935
0.985073
2.095314
1.9359781.076768
7.375143
2.883197

Input
Impedance
Configuration
CMRR
Coupling
Input noise
Gain
Max. input signal

1 MΩ // 15 pF
Single-ended (A or B) ordifferential (A – B)
>85 dB at 100 Hz(input gain >20 dB)
AC or DC
6 nV/√Hz at 1 kHz (with 60 dBinput gain)
0, 10, 20, 30, 40, 50, 60 dB ± 0.2 dB
10 Vpp (overload)
5 Vpp (no distortion)

Impedance<1 Ω
Full-scale output signal 10 Vpp into >300 ΩOption 01
10 Vpp into 50 Ω
DC offset
Adjustable to 0 VGain
0, 10, 20 dB ± 0.5 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±0.75° (DC to fc, typ.)GeneralInterfaces

Stored settings

PowerDimensions
WeightWarranty

GPIB and RS-232 standard. All
instrument functions can becontrolled and read throughthe interfaces.
Nine complete dual-channel instrument
configurations may be stored in
non-volatile memory.
45 W, 100/120/220/240 VAC,
50/60 Hz15.7" × 3.0" × 14.0" (WHD)
12 lbs.
One year parts and labor on defects
in materials and workmanshipOrdering InformationSR640
SR645
SR650
SR640 rear panel

Stanford Research Systems

Option 01

Two-channel low pass filter with
RS-232, GPIB and rack mount kit
Two-channel high pass filter with
RS-232, GPIB and rack mount kit
One high, one low pass filter with
RS-232, GPIB and rack mount kit
High current output

phone: (408)744-9040
www.thinkSRS.com