Digital Multimeter Measurement Cycle

Aperçu

This tutorial describes the various stages that a PXI DMM takes when performing a single reading as well as considerations for achieving the best measurement performance.

For more information return to the Digital Multimeters.

Contents

Overview

The PXI Digital Multimeters measurement cycle is made up of several measurement phases: switch time, settle time, signal measurement phase, AutoZero, 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. These values can also be independently controlled.



The internal switch time is required to configure the analog circuitry of the PXI Digital Multimeter for the next measurement. There is also default settle times preceding both ADC calibration and AutoZero. All of these times are not user programmable and are optimized in the design.

During resistance (ohms) measurements, the signal is measured with an applied current (amps) source. When offset compensated ohms is enabled, there is a settle phase and current (amps) source OFF measurement phase in place of the AutoZero 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.

When autorange is selected, the autorange measurement phase occurs before the AutoZero and signal measurement phases. Each autorange measurement uses the same aperture as the signal measurement. Once the correct range is identified, the AutoZero and signal measurements use the selected range.

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

Settle Time

Prior to every measurement phase, a settle time exists. The following table lists the default settle times for high-performance PXI Digital Multimeters.

Function
Settle Time
DC V (100 mV - 10 V) 1 ms
DC V (100 V, 300 V) 2 ms
DCV AutoRange 2 ms
Resistance 10 kΩ 1 ms
Resistance 10 kΩ 5 ms
Resistance 100 kΩ 25 ms
Resistance 1 MΩ 100 ms
Resistance 10 MΩ 250 ms
Resistance AutoRange 50 ms
AC V DC coupled 3 µs
AC V AC coupled 1 s
Frequency/Period 500 ms
DC I 100 µs
AC I 3 µs
Diode 10 ms


You can modify these values to be longer or shorter. The following figure shows a digital multimeter connected to three devices under test (DUTs) through a multiplexing switch:



Whenever the external switch changes channels, the digital multimeter input is connected to a different voltage or resistance (ohms). Each time this process happens, the digital multimeter, interacting with the source, behaves as an RC circuit with a settle time that corresponds to:

ST = kRC

Where:

R = equivalent resistance (ohms) of the source
C = the equivalent input capacitance seen by the digital multimeter (CDMM + CCX + Input C), where CCX = CC1 or CC2 or CC3 (cable capacitances) depending on value selected
k = value related to accuracy required, as shown in the following table; increases as required accuracy increases
ST = settle time required
Note  NI does not generally recommend scanning currents directly. The optimum method for scanning currents is to embed shunt resistances in the test system, then scan voltage.
k
Error
Required Resolution
(for full range step)
3 4.98% -
4 1.83% -
5 0.674% -
6 0.248% -
7 0.091% -
8 0.033% 3½ digits
9 0.012% -
10 0.0045% 4½ digits
11 0.0017% -
12 0.00061% 5½ digits
13 0.00023% -
14 0.000083% 6½ digits
15 0.000031% -
16 0.000011% 7 digits


Modifying the settle time from the default may be important when you are measuring or scanning resistances (ohms) with values above 10 kΩ, when C >500 pF, and when speed is of the utmost importance. The digital multimeter has an input capacitance of 120 pF, but cable capacitance can be significantly more. Cable material dielectric absorption complicates the settle time issue.

When switching to the next measurement, consider the following example. Assume a 10 V step occurs on the 10 V range with a required resolution of 6½ digits. Referring to the previous table for 6½ digit settling, k = 14 is required. Consider next a 10 mV step on the 10 V range. The step, being 0.1% of range (10 mV/10 V = 0.1%), can only be observed to 0.1% beyond that (assuming a 6½ digit resolution). In this example, the maximum time required from the previous table corresponds to k = 7, which yields 0.091%. If the difference between channels is known to never exceed 10 mV, then you can save considerable settle time. If you cannot predict what the values are from channel to channel, consider the accuracy required in the measurement. Then assume a step size equal to the full range to identify k.

The analysis in the previous example is for fixed range operation. You can select autoranging for the measurement, but additional delays occur as the digital multimeter seeks the appropriate range for the measurement. NI does not recommend AutoRange for optimum speed.

Aperture Time

 

Function
Default Aperture
DC 6½ digits 100 ms
DC 5½ digits 500 µs
DC 4½ digits 20 µs
DC AutoRange 500 µs
AC Max (DC or 4/(minFreq))
AC AutoRange Max (500 µs or 4/(minFreq))
Frequency 2/(minFreq)
Period 2 x (the maximum period)


By default, the aperture time for a measurement is chosen by the driver based on the configured measurement and resolution. These values are chosen to ensure accuracy for 6½ digit measurements while not sacrificing performance at lower resolutions. For AC, excluding AutoRange, the aperture is expressed as Max (DC or 4/minFreq). MinFreq, or minimum frequency, is a user programmable parameter with a default value of 20 Hz. Therefore, the aperture default is 200 ms.

The same applies for other user selectable values of minFreq. For example, if a minFreq of 1 kHz is selected then 4/minFreq = 4/1 kHz = 4 ms. In this case 4 ms would be used as the AC aperture for resolutions <6 digits. For resolutions >6 digits, 100 ms would be used. For settle time defaults, refer to the previous section on Settle Time .

Aperture time is the period during which the ADC is reading the input signal. This time is a function of resolution. The larger the aperture time, the better the resolution. Select short aperture times for faster measurement speed.

The table above lists the default conditions for aperture along with other conditions related to the measurement cycle. These defaults optimize the digital multimeter performance to its specified accuracy.

Averaging
If your application requires a long aperture time (>100 ms) it is recommended that AutoZero is enabled. The offset present may actually drift during the measurement, so that the stored AutoZero value is invalid by the time the measurement completes. To compensate for this drift, several shorter measurements can be taken with a new AutoZero offset applied to each measurement. These measurements can then be averaged together by the digital multimeter so that a single value is returned.

To configure an averaged measurement, set the Number of Averages attribute. For example, if you desire a 500 ms measurement aperture, you can set the aperture to 50 ms and set Number of Averages to 10. AutoZero must be enabled when the Number of Averages attribute is greater than one. The digital multimeter will take ten 50 ms measurements each with AutoZero and return a single measurement.

ADC Calibration

ADC Calibration is a feature exclusive to high-performance PXI Digital Multimeters that allows you to appropriately trade off measurement speed for long-term accuracy. The NI 4070 Digital Multimeter ADC is designed for precision, linearity, and stability. By doing routine calibration of the ADC back to a single well-controlled component, you can ensure absolute accuracy of the conversion.

In DCV and resistance (ohms) at 6½ digit resolutions, NI recommends using ADC calibration for the greatest accuracy. In ACV and current (amps), or at resolutions of 4½–5½ digits, ADC calibration is not required for satisfactory performance. When ADC calibration is enabled, every measurement cycle includes an additional phase for acquiring the value of the high-precision reference. This phase yields the most exacting precision because any ADC gain drift is normalized to the input signal, and the ADC gain drift is removed in the resulting mathematical calculation on every measurement.

Traditional methods disperse these measurements over time—compromising performance, speed, and deterministic timing. For optimum drift performance, NI recommends using ADC calibration as part of each measurement cycle.

Your application may demand speed over accuracy. In these instances, you can disable ADC calibration.
The following figure represents the process of the ADC calibration cycle. During this cycle, the input is disconnected from the ADC and the precision DC reference is measured. This measurement consists of an ADC calibration AutoZero (REF LO) and an ADC calibration HI (REF HI). The normalized value of the reference voltage is calculated from Vref = REF HI - REF LO.



With ADC calibration disabled, the measurement speed increases by a factor of up to two and you need to add a temperature coefficient error of 3 ppm/ºC to the appropriate range specification. Although the error is small, you should still consider the effects. Long-term drift typically degrades by 50 ppm/3 month. If you need to turn off ADC calibration, as you might when you want optimum speed in 6½ digit resolution, you can recover to specified accuracy by running periodic self-calibration operations.

Note  NI recommends that you run self-calibration before taking a 6½ digit measurement with ADC calibration OFF.

The measurement effect of ADC calibration decreases as your selected resolution decreases.

AutoZero

AutoZero is a method used to compensate for internal PXI Digital Multimeter offsets.

When AutoZero is enabled, the internal digital multimeter input is connected to its input LO and measured. The subsequent input signal is measured, and the AutoZero value subtracted from it. Thus, any offsets in the measurement path or ADC are subtracted from the signal, correcting for the offsets. You can disable AutoZero before initiating or reading a measurement; it remains disabled until you enable it again.

When you disable AutoZero, the digital multimeter restores the AutoZero value from the previous calibration, either self-calibration or external calibration.

If you find the offset present is unacceptable, you can either subtract it algorithmically in a later process, such as by shorting the inputs and recording it, or you can run self-calibration, which calculates and stores new AutoZero offsets. To maintain stable offset performance in high resolution modes (for example, 6½ digit resolution) you should enable AutoZero.

You can also enable AutoZero . AutoZero once performs an AutoZero and stores the value for use in subsequent measurements, as long as AutoZero once remains selected. If you disable AutoZero once, the digital multimeter restores the AutoZero offset from the previous calibration.

NI recommends AutoZero once only for multiple measurements taken on the same range. If you enable AutoZero once for an AutoRange measurement, AutoZero on is used to ensure that the correct AutoZero value is applied to each range.