Visualizing the Impact of Multiplex-ADC in DAQ Applications

This article explains how to visualize the impact of using a multiplexed Analog-to-Digital Converter (ADC) in a Data Acquisition (DAQ) application. Understanding these impacts can help optimize your DAQ system for better performance and accuracy.

Before You Begin

To use this tutorial, you must have the following software and hardware installed on your system:

• LabVIEW Base, Full, or Professional Development System 2008 or later.
• NI-DAQmx™ 8.1 or later. Refer to the NI Driver and Development Software Compatibility, NI-DAQmx and LabVIEW Compatibility and NI-DAQ Readme file for a list of devices supported by NI-DAQmx.
• A DAQ device that is supported by NI-DAQmx. Refer to the NI-DAQ Readme file for a list of devices supported by NI-DAQmx or NI Hardware and Software Operating System Compatibility for the compatible driver for the device according to the operating system used.

Refer to the DAQ Quick Start Guide and the device documentation for information about installing and configuring the software and hardware.

Setting Up Hardware

You will need to use a multiplex DAQ device and a waveform generator. For this example, we will be using PXIe-6363 and Keysight 33500B Series Waveform Generator. Connect the hardware according to the diagram below using the same length of cables.

Configuring Task

In this tutorial, we utilize the Voltage - Continuous Input VI from the shipping examples. Configure the channel, timing and waveform settings according to the picture below:

Result

1. As shown in the picture above. it is expected that ai0 is sampled first, resulting in a lower amplitude as the sine wave generator is just beginning to rise.
2. ai1, which is sampled later than ai0, will has a higher amplitude.

Disclaimer: The X-axis reading is inaccurate due to manual reading and interpolation of the graph.

3. From the above graph, the time difference between channels for the same amplitude is approximately 10µs at a 20k/s sampling rate (50µs). This value aligns with the Convert Clock Rate, which is around 90k/s when configured with a 20k/s sampling rate.

4. Despite ai7 being 6 channels ahead of ai1, the shift is only one channel away from ai1 instead of 6 channels. This is expected because the channels were set up consecutively as ai0, ai1, and ai7, where the multiplexing occurs between these 3 channels (the channels currently use for acquisition) and not the entire range from ai0 to ai7.
5. Despite the multiplexing, t0 for all 3 channels is the same. This is an expected behavior because t0 is acquired when DAQmx Read.vi is called from the OS, not when the data is being sampled in the ADC.