How Can I Reduce My PXI(e) DMM Delay?

Updated Jan 17, 2020

Issue Details

  • When acquiring a measurement from my DMM card, I am noticing a delay in response. How can I troubleshoot this?
  • What factors are contributing to my DMM delay?
  • Why is there a delay between my DMM receiving a trigger and acquiring samples?

Solution

The delay in a PXI(e) card can be influenced by a number of factors. The four main areas to check are:
  • Cable impedance
  • Settling time
  • Auto-Zero
  • Sample rate
This article will address how each factor can affect your DMM delay.


Cable Impedance

The performance of your DMM cards will be affected by the cables that are being used in your PXI system. National Instruments recommends using short cables with low dielectric absorption and minimal capacitance.

When using high frequency signals, the source and load impedances should be matched to the characteristic impedance of the transmitting cable.
For more information on impedance matching see Impedance and Impedance Matching.
 

Settling Time

Prior to every measurement phase, your DMM will have a Settling Time. The Settling Time is required for a measurement system to stabilise to a specified accuracy limit.
You can modify the Settling Time to equal zero in order to try and reduce your DMM delay.

This is an advanced configuration property that can be accessed using a Property Node in LabVIEW, as shown:

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it your computer, and grad the file onto your LabVIEW diagram.

Refer to DMM Measurement Defaults for the default settling times for your DMM card.
A further explanation of the settling time property can be found at Settling Time.


Auto-Zero

Auto-zero is a DMM property that prevents any offset voltages from affecting the measurement accuracy.
This property can be configured to "OFF" to further reduce the delay in your DMM response.

To configure Auto-Zero, use the niDMM Configure Auto Zero VI or access the property using a Property Node, as shown:

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

A further explanation of this property can be found at Auto Zero.


Sampling Rate

Increasing the DMM Sampling Rate directly affects the timing resolution. This can be especially useful if you have multiple DMMs that need to be synchronised. The timing resolution is equal to 1/sampling rate. 

For example, at a sampling rate of 1.8MS/s, you can expect a maximum delay of approximately 560ns between each DMM. 

The Sampling Rate can be specified using the niDMM Configure Waveform Acquisition VI, or by using a Property Node as shown:

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.

A further explanation of the Sampling Rate can be found at Sample Rate.

Additional Information

Adjusting the Auto-Zero and Settling Time of your DMM may reduce the accuracy of your measurement. 
See niDMM Properties for a full list of properties that can be configured.

If the steps in this article do not minimise your DMM delay as expected, please contact National Instruments for further assistance.