Resistance
Optimizing low ohm measurement and speed
When measuring resistance of 100 Ω or less, cable, connectors, and Series 3700A switch cards can
have thermal offsets, which can result in additional reading uncertainties.
Autodelays for 100 Ω or less have been optimized for throughput and settling, resulting in the best
measurement. If thermal offsets cause additional uncertainty, adding a delay of 10 ms can improve
accuracies. Refer to the autodelay table for additional details. If the application requires additional
settling delay, send the following commands to the channel or slot of interest:
channel.setdelay("4004", 0.010)
Adds 10 ms of delay after closing channel 4 in slot 4.
channel.setdelay("slot4", 0.010)
Adds 10 ms of delay to all channels in slot 4.
Dry circuit ohms (DRY+)
Standard resistance measurements have open-circuit voltage levels from 6.4 V to 14.7 V, depending
on the selected range. Dry circuit ohms limits open-circuit voltage to between 20 mV and 27 mV. This
allows you to perform resistance measurements that require low open-circuit voltage, such as power
and low-glitch resistance measurements.
You can use dry circuit ohms for the 1 Ω, 10 Ω, 100 Ω, 1 kΩ, and 10 kΩ ranges (maximum resistance
of 2.4 kΩ) for the four-wire ohm function.
You can use offset-compensated ohms (OC+) used with dry circuit ohms to cancel the effect of
thermal EMFs. When dry circuit is enabled, offset compensation is automatically set to on.
Measuring contact resistance (oxide film build-up)
The ideal resistance between switch connectors or relay contacts is 0 Ω. However, an oxide film may
be present on the switch or relay contacts. This oxide film could add resistance on the order of
several hundred milliohms. Also, this oxide film changes the contact resistance over time and with
changes in the environmental conditions (such as temperature and humidity).
Typically, the four-wire ohm function of the Series 3700A or a standard DMM is used to measure low
resistance. However, if standard resistance measurements are performed, the relatively high open-
circuit voltage may puncture the oxide film, and render the test meaningless.
Dry circuit ohms limit voltage to 20 mV to minimize any physical and electrical changes in a measured
contact junction. This low open-circuit voltage will not puncture the film, and will therefore provide a
resistance measurement that includes the resistance of the oxide film.
Oxide films may also build up in connections on a semiconductor wafer. To accurately measure the
resistance introduced by the oxide film, dry circuit ohms should be used to prevent oxide film
puncture.
Measuring resistance of voltage-sensitive devices
Dry circuit ohms should be used for any device that could be damaged by high open circuit voltage. If
you are not sure the slightly degraded accuracy is a consideration, it is good practice to use dry circuit
ohms to measure low resistance.
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