What is Causing Delay When Exporting Trigger from PXIe-6570 to PXIe-4468?

Issue Details

I am using the set_signal(event_list) opcode to generate a trigger from the PXIe-6570 to the PXIe-4468. Both modules are configured to output a signal upon receiving the trigger. However, I am observing a delay of approximately 500 µs between the trigger signal from the PXIe-6570 and the response from the PXIe-4468.

For the PXIe-6570, I had set the opcode to generate a 50 kHz square waveform for few vectors and then export the trigger using set_signal(event_list). For the PXIe-4468, I had used a sample VI provided by LabVIEW, Voltage - Finite Output.vi to generate a 50 kHz sine waveform.

In the waveform, the yellow signal represents the PXIe-6570 output, while the blue signal shows the PXIe-4468 output. Ideally, both signals should start simultaneously. What is causing the delay to occur?

Solution

The delay relates to the settings for the PXIe-4468, specifically the sample rate. There is a specification called AO Filter Delay for the PXIe-4468. The AO Filter Delay in the PXIe-4468 refers to a brief delay that occurs after the device receives a trigger signal but before it begins outputting. This delay is caused by the Analog Output (AO) filter, which processes the signal before it is sent out. The length of this delay depends on the sample rate configured for the device. In essence, the filter introduces a small processing time, which results in a delay between the trigger and the actual output signal.

You can estimate the output delay using the following formula: Output Delay = (1 / Sample Rate) * Number of Samples.

For example, if the sample rate is set to 200,000 samples per second, and the delay is 106 samples, then:

Output Delay = (1 / 200,000) * 106 = 530 µs

This matches the delay observed in practice. The delay can be reduced by using a lower sample rate.