Characterisation, Measurement & Analysis
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SRS SR650

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  • SRS SR650
  • SRS SR650

SRS SR650 Dual Channel Programmable High & Low Pass Filter

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Product Code: 14913
Manufacturer: Stanford Research Systems

  • Two independent filter channels

  • 115 dB/octave rolloff

  • 1 Hz to 100 kHz cutoff frequency

  • 0.1 dB passband ripple

  • 80 dB stopband attenuation

  • 4 nV/vHz input noise

  • Up to 80 dB of gain (x10,000)

  • GPIB and RS-232 interfaces

  • Sharp Filter Cutoff

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 of 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 frequency-domain signal conditioning.

SR640 - dual low-pass

SR645 - dual high-pass

SR650 - high-pass/low-pass (can be cascaded for Band -pass)

The Stanford Research Systems SR650 is a dual-channel programmable filter with one low pass and one high pass filter. The instrument channel has a low-noise preamplifier, a precision high pass and low pass filter section, and an output amplifier. Applications include anti-aliasing, audio analysis, and general frequency-domain signal conditioning.

Each filter has an 8-pole, 6-zero elliptic frequency response that provides extremely sharp rolloff. The filters roll off at 115 dB per octave, with only 0.1 dB of passband ripple and 80 dB of stopband attenuation. Cut off frequencies may be set between 1 Hz and 100 kHz with three digits of resolution.

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

Amplifiers
Each channel contains a low-noise differential preamplifier with an adjustable gain of 0 to 60 dB. Preamplifier noise is less than 4 nV/vHz (at 1 kHz), while common mode rejection is greater than 90 dB at 1 kHz. An additional amplifier stage provides up to 20 dB of postfilter gain. The filter section can be bypassed, allowing the SR640 series filters to operate as flexible, low-noise amplifiers with a bandwidth of approximately 750 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 digital-analog crosstalk. The CMOS microprocessor "sleeps" while the interface and front panel are inactive in order to reduce the amount of digital noise in the box.

Inputs and Preamplifiers
Each filter channel has a floating, fully differential input with 1 MO input impedance and 4 nV/vHz of input noise (at 1 kHz). 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 keypress. 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.

Filter Characteristics
Filter characteristics for the SR640 series filters are shown below. The exact filter transfer function is given in the specifications section. (The graph shows the lowpass 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 SR640 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 >80 dB. Worst case phase match between channels is better than ±0.75° from DC to the cutoff frequency. The phase match improves to ±0.5° for cutoff frequencies between 10 and 50 kHz, and to ±0.25° for cutoff frequencies below 10 kHz.

Post Filter Gain and By-Passing
After the filter, another amplifier section provides up to 20 dB additional gain. The standard output drives loads of >300 O at 10 Vpp, while the optional high power output drives 10 Vpp into a 50 O 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.

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