DMM Measurement Cycle

NI Digital Multimeters Help (NI-DMM 18.1)

Edition Date: March 2018

Part Number: 370384V-01

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The DMM measurement cycle is made up of several measurement phases: switch time, settling time, signal measurement phase, Auto Zero, and ADC calibration. Refer to the figure below for relative timing of these phases. The length of the signal measurement phase is set by the aperture time. Generally the settle and aperture times are selected by the device driver based on the specified resolution. Settling time and aperture can be set using the niDMM Property Node. Refer to Configuring Measurement Timing for more information.

The internal switch time is required to configure the analog circuitry of the device for the next measurement, and default settling times precede both ADC calibration and Auto Zero. All of these times are not user programmable.

During resistance measurements, the signal is measured with an applied current source. When Offset Compensated Ohms is enabled, a settle phase and current source OFF measurement phase occurs in place of the Auto Zero phase depicted above.

The AC measurement cycle contains the same phases, but the minimum signal measurement time is based on the minimum frequency required of the measurement. For example, for a minimum frequency of 50 Hz, an aperture of 4x the period of the 50 Hz signal is required, or in this example, 80 ms.

For capacitance and inductance measurements, the measurement cycle consists of applying a current source and measuring the response of its harmonics. Because the harmonics are a set frequency, the aperture time is fixed. See DMM Measurement Defaults and Capacitance/Inductance for more information.

When Auto Range is selected, the Auto Range measurement phase occurs before the Auto Zero and signal measurement phases. After the correct range is identified, the Auto Zero and signal measurements use the selected range.

Note  When using Auto Range, keep in mind that longer apertures significantly slow down Auto Ranging because multiple measurements are made to find the best range possible. For best measurement speed, manually select the desired range.


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