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SIM921AC Resistance Bridge

SIM921AC Resistance Bridge
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Small Instrumentation Modules SIM921 — AC resistance bridge · Accurate millikelvin thermometry · microvolt/picoamp excitation · Measurements from 1 mΩ to 100 MΩ · Adjustable excitation (2 Hz to 60 Hz) · Linearized analog output · Resistance, temperature and phase SIM921 AC Resistance Bridge The SIM921 AC Resistance Bridge is a precision, low-noise instrument designed for cryogenic thermometry applications. With excitation power below 100 aW, thermistors and other resistive samples can be measured at temperatures below 50 mK with negligible self-heating errors. Measuring Resistance The SIM921 measures resistance using a four-wire circuit, eliminating the direct effect of lead resistance on the result. Thermal EMFs and amplifier offset drifts are avoided by using an AC excitation current source. The excitation frequency can be adjusted from 2 Hz to 60 Hz, either from the front panel or over the computer interface. This flexibility allows the user to operate at a synchronous sub-harmonic of the power line frequency (such as 15 Hz/12.5 Hz) or at some incommensurate frequency, depending on requirements. Multiple SIM921s can be operated at different frequencies in the same experimental set-up without risking in-band crosstalk. Also, for very high impedance measurements, the Stanford Research Systems variable frequency makes it possible to probe any capacitive effects in the resistance result. Excitations are sinusoidal, eliminating the high-frequency harmonic content associated with square wave excitations. The actual determination of resistance is achieved ratiometrically, passing the selected excitation current through both an internal, high-stability reference resistor as well as the user’s device under measurement. An internal auto-cal is available to calibrate the two arms of the ratio readout for greater accuracy. Excitation Two excitation modes, constant current and constant voltage, are available with the SIM921. Most low-temperature thermometry applications use negative temperature-coefficient resistors. The constant voltage mode has the benefit of decreasing the power dissipated in the thermometer as the temperature drops. In this mode, the SIM921 servos the applied AC excitation to maintain the selected voltage across the user’s resistor.

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Constant current mode is appropriate when measuring small resistances, such as characterizing superconducting transitions. In constant current mode, the internal reference resistor is used as the input to the servo, giving a constant current equal to the selected voltage divided by half the resistance range (for instance, 100 µV on the 20 kΩ range gives 10 nA rms excitation current). Phase Sensitive Detection A pair of dual-phase, sinusoidal AC demodulators in the SIM921 provide excellent signal-to-noise ratio in the most difficult test conditions. Further, dual-phase demodulation enables...

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Measurement Measurement type 4 wire AC bridge Number of inputs 1 Resistance range 1 mΩ to 100 MΩ Time constant 0.3 s to 300 s Reading rate Two updates per second Demodulator resolution 32-bit Resolution see table Accuracy (% reading + % range) 2 Ω to 200 kΩ ±(0.05 % + 0.05 %) at an excitation of ≥30 µV and ≥3 nA 200 mΩ to 2 MΩ ±(0.15 % + 0.15 %) at an excitation of ≥300 pA Stability With auto-cal (±0.001 % of reading)/°C Without auto-cal (±0.02 % of reading)/°C Max. lead resistance 100 Ω + 25 % range Input impedance >10 GΩ (typ.) Source Type Sinusoid, constant current or voltage Source...

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