Determining an Appropriate Pull-Up Resistor for the Digital Inputs of the R Series Devices

Updated Feb 14, 2019

Reported In

Hardware

  • PXIe-7822
  • PCIe-7822

Issue Details

I want to interface with an open collector or open drain device. Since the PXIe R Series does not provide an internal pull-up I wish to create my own. What is an appropriate value for the pull-up?

Solution

Before selecting a Pull-Up Resistor a couple of factors have to be considered:
  • The impedance of the Digital Input, 50kΩ.
  • The voltage pull up value and the configured logic level (3.3 V, 2.5 V, 1.8 V, 1.5 V, and 1.2 V).
  • The corresponding minimum logic high level for the configured logic level.
With the factors known the voltage divider equation may be solved to find the pull-up resistance. 

Voltage Divider Equation:

Solving Voltage Divider Equation for RPU:

Where RPU is the pull-up resistor, VIH is the minimum digital input high voltage for the configured logic level, RIN is the Digital Input Impedance, and VPU is the pull up voltage. 

For example a maximum of 32.5kΩ RPU is needed when configuring the PXIe-7820R for a 3.3V logic level. The RIN is 50kΩ, the VIH is 2V for 3.3V configuration, and the VPU is selected to be 3.3V. 
 

Additional Information

Note: The RPU chosen should be less than the value calculated to provide tolerance. A user may pick any pull-up voltage (VPU) desired but must not exceed the maximum Digital Input voltage provided in the NI 782xR, PXIe-784xR, PXIe-785xR, or USB R Series Specifications.

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