SIM921 AC Resistance Bridge0 pages

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 ... $2495 (U.S. list)

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

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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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