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.