Digital Input and Output Impedance in M or X Series DAQ Specifications

Updated Aug 1, 2018

Reported In

Hardware

  • PCIe-6321
  • PXIe-6363

Issue Details

I am connecting my M Series (62xx) or X Series (63xx) multifuctions DAQ device to external circuitry and need to know the impedance of the Digital Input and Output (DIO) lines. I don't see a digital impedance specification for my device. What are the input and output impedances of my device's DIO lines?

Solution

The output impedance of an M or X Series DAQ device DIO port is dependent on the voltage level, current, and temperature. A series of plots are provided in the specifications to help estimate the output impedance of the port. Please refer to the PCIe-6321 DIO chart below as an example. 
On the chart, there are three curves representing typical output of the PCIe-6321 P0 lines at 0°C, 25°C, and 55°C, for Ioh versus Voh. At 25°C and a measured Voh of 4 V, the Ioh is about -30 mA. Using Ohm's Law, the output impedance would be 4 V/30 mA = 133 Ω. 

The input impedance can be estimated using the information about the Max Input High voltage and Max input high current. See the table below as an example. The actual input impedance will be dependent on the signal input
For the PCIe-6321 connected to a 5 V 250 µA signal, the input impedance would be 5 V/250 µA = 20 kΩ. Different signal voltage and current levels will change the input impedance. Each M and X Series device specifications will contain similar charts and tables for determining the input and output impedances of the DIO ports and lines under various conditions.

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