Routing an External Sample Clock to a PXI-407x DMM

Updated Sep 4, 2018

Reported In

Hardware

  • PXI-4070
  • PXI-407X
  • PXI-4071

Issue Details

Can I route an external sample clock to a PXI-4070/4071/4072 DMM?

Solution


The PXI-407x DMMs cannot import an external sample clock to use in place of their onboard 28.8 MHz oscillator.

However, you can use an external sample clock as a trigger signal for a PXI-407x DMM.

There are two methods you can use to accomplish this objective:
  1. Configure the Trigger source to come from a PXI Trigger Line or from the AUX connector and use Sample Trigger as IMMEDIATE. 
    Notice that in the example program linked below, the PXI-407xs are configured for multi-point acquisitions that will start once they receive the Trigger from the external pin in the AUX connector. The Sample Trigger is left as default, and the default value used is IMMEDIATE. This means that once the Trigger is received, the PXI-407xs will take the measurements specified by "# of Data Points" as fast as they can.

  2. Configure both the Trigger and Sample Trigger sources to come from the same PXI Trigger Line or from the AUX connector.
    To ensure that they acquire at the same time, you could configure both the Trigger and the Sample Trigger to be received from the same source (for example, PXI Trigger Line) and configure all your PXI-407xs for multi-point or continuous acquisition. This way, all PXI-407xs will start at the same time and will take measurements at the same time.

When using the above methods, you must verify that your trigger source (which will correspond to your sampling clock) is not faster than the measurement period of your DMM. If a trigger arrives during the switching time, it will be queued, but the measurement will only start after the switching time period is over. If a trigger arrives during any of the other parts of the measurement cycle, the trigger will be ignored (not queued).

The measurement cycle of the PXI-407x has different phases. For documentation on each phase, refer to the section Devices Overview»NI 407x»DMM Measurements»NI 407x DMM Measurement Cycle of the NI Digital Multimeters Help. You can benchmark how long your measurement will take with the LabVIEW function niDMM Get Measurement Period.vi. The two VIs attached below are shipping example programs showing how to benchmark the measurement cycle. The only modification made to the shipping examples was adding the niDMM Get Measurement Period.vi (see the Attachments section below for the VIs).

One important thing to notice in these VIs is that both measurements have the same configuration, but the niDMM Get Measurement Period.vi does not return the same value. This is because the first measurement of any acquisition always has a longer switching time than subsequent measurements. When you configure the PXI-407x for a single point acquisition (see Modified Example - Measure DC Volts.vi), the output of the niDMM Get Measurement Period.vi will include the switching time. When you configure the PXI-407x for multi-point acquisition, the output of the niDMM Get Measurement Period.vi will optimize the switching time to its minimum value. This means that for multi-point acquisition, niDMM Get Measurement Period.vi returns the measurement period of sample number 2 and on. The first sample will have a measurement period as shown in the Modified Example - Measure DC Volts.vi.

The workaround is to initiate the PXI-407xs first so they all configure the measurement and perform the internal switching necessary. After they are ready to measure, the PXI-407xs will enter the "Wait for Trigger" state. Therefore, after initializing the PXI-407xs, make your program wait for a few ms, and then start sending your clock signal as the input for the Trigger and Sample Trigger. After this, the measurement period of the PXI-407xs will take as long as shown by the niDMM Get Measurement Period.vi in the Modified Example - Acq & Graph Multiple Samples.

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