Configuring Measurement Timing

Configuring Measurement Timing

If the resolution-based NI 4070/4071/4072 measurement times or NI 4065 measurement times selected by the driver are not acceptable for your application, you can set the aperture and/or settling times for the measurement directly using properties.

You can specify the aperture time in either seconds or powerline cycles. Set the Aperture Time Units property to the appropriate setting. If you choose powerline cycles as the unit, specify the Powerline Frequency using either niDMM Configure Powerline Frequency or the Powerline Frequency property. Set the Aperture Time property to the desired value. If the requested aperture time is less than the minimum required value, the driver uses the minimum value instead of the requested value.

Notes  The NI 4050 is not supported. You must set the Aperture Time Units property before setting the Aperture Time property.

To configure the settling time for a measurement or a waveform acquisition, set the Settle Time property. The settling time is specified in seconds. If the requested settling time is less than the minimum required value, the driver uses the minimum value instead of the requested value.

When the aperture time is explicitly set, the absolute resolution value is ignored. The effective resolution is determined by the chosen aperture. To reset the aperture or settling time, set these properties to Auto Selection (-1). Calling niDMM Config Measurement resets the aperture time to Auto. This allows the driver to program the hardware using the requested resolution.

When you set the aperture time directly, you may also want to set the Auto Zero, ADC calibration, and DC noise rejection values. These properties default to Auto Selection (-1). When the aperture is set, the driver disables Auto Zero and ADC Calibration and uses the Normal DC Noise Rejection setting. If you desire different values, set these properties to the desired setting. Reset the values to Auto Selection when you want the driver to choose the setting for these properties.

Note  Aperture cannot be set directly for a waveform acquisition. Refer to Sample Rate for more information.

To select the measurement time:

• LabVIEW
  1. Set the Aperture Time Units property in seconds or powerline cycles.
  2. Use the Aperture Time property.
• CVI/C++/Visual Basic
  1. Use niDMM_SetAttributeViInt32 to set the desired value for the Aperture Time Units attribute.
  2. Use the niDMM_SetAttributeViReal64 function to set the desired value for the Aperture Time attribute.

Averaging

During single or multipoint measurements with long aperture times, several shorter measurements can be taken with a new Auto Zero offset applied to each measurement to compensate for drift. To specify the number of averages in a measurement:

• LabVIEW
  1. Enable Auto Zero.
  2. Set the Number Of Averages property.
• CVI/C++/Visual Basic
  1. Enable Auto Zero.
  2. Use the niDMM_SetAttributeViReal64 function to set the desired value for the NIDMM_ATTR_NUMBER_OF_AVERAGES attribute.

Note  When averaging resistance measurements, offset compensated ohms must be disabled.