Aperture Time

Aperture Time

Aperture time is the period during which the ADC is reading the input signal. Resolution, measurement speed, and frequency rejection are functions of the aperture time. The larger the aperture time, the better the resolution. Select short aperture times for faster measurement speed. You can specify aperture time either in seconds or in power line cycles (PLCs).

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

Function	Default Aperture
	NI 4065	NI 4070	NI 4071	NI 4072
DC 7½ digits	—	—	100 ms with Number of Averages = 4	—
DC 6½ digits	200 ms	100 ms	100 ms	100 ms
DC 5½ digits	3.33 ms	500 µs	500 µs1	500 µs
DC 4½ digits	333 µs	20 µs	50 µs	20 µs
DC Auto Range	3.33 ms	500 µs	500 µs	500 µs
AC	AC 6½ digits — 1 sec AC 5½ digits — Max (100 msec or (10/(minFreq))) AC 4½ digits — Max (10 msec or (5/(minFreq)))	Max (DC or 4/(minFreq))	Max (DC or 4/(minFreq))	Max (DC or 4/(minFreq))
AC Auto Range	Max (100 ms or 10/(minFreq))	Max (500 µs or 4/(minFreq))	Max (500 µs or 4/(minFreq))	Max (500 µs or 4/(minFreq))
Frequency	—	2/(minFreq)	2/(minFreq)	2/(minFreq)
Period	—	2 x (the maximum period)	2 x (the maximum period)	2 x (the maximum period)
Capacitance and Inductance	—	—	—	Inductance (only) at 10 kHz = 13.65 ms Inductance and capacitance at 1 kHz = 125.2 ms Inductance and capacitance at 91 Hz = 1.081 s
1For the NI 4071, on the 5.5 DCV 100V range, use 1.6 ms.

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 Auto Range, the aperture is expressed as Max (DC or 4/minFreq) for the NI 4070/4071/4072 and 1 sec for the NI 4065. MinFreq, or minimum frequency, is a user-programmable parameter with a default value of 20 Hz.

The same applies for other user selectable values of minFreq.

For example, on the NI 4070/4071/4072, 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 in this example.

For example, on the NI 4065, if a minFreq of 100 Hz is selected, then 5/minFreq = 5/100 Hz = 50 ms. In this case, 50 ms would be used as the AC aperture for 4.5 digits. For 5.5 digits, 10/minFreq = 10/100 Hz = 100 ms would be used in this example. For resolutions >6 digits, 1 sec would be used in this example.

For settling time defaults, refer to Settling Time.

Number of Averages for DC Measurements

This function is specific for high-resolution DC measurements.

Note  When taking averages of inductance or capacitance measurements, refer to Number of LC Measurements to Average.

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

To configure an averaged measurement, set the Number of Averages property. 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. Auto Zero must be enabled when the Number of Averages property is greater than one. The DMM will take ten 50 ms measurements each with Auto Zero and return a single measurement.

Note  (NI 4071 only) NI-DMM breaks up large aperture times (greater than 6 PLCs) in DC modes (DC voltage, DC current, 2-wire resistance, 4-wire resistance, and diode modes) into blocks of 6 PLCs and averages the resulting measurements. For example, if you request an aperture time of 30 PLCs, NI-DMM averages five readings of 6 PLCs. If you do not want to subdivide a long aperture time, you can override this behavior by explicitly setting Number of Averages to 1. Specifying a value >1 for Number of Averages causes NI-DMM to behave exactly as requested without subdividing the aperture time.