Slow Settling Time on Digital Output Devices

Updated Apr 4, 2019

Reported In

Hardware

  • C Series Digital Module
  • NI-9475
  • C Series Digital Output Device

Issue Details

I have a digital output device like the NI 9475.  I have the device hooked up to a high impedance device such as a MOSFET gate and when I go from high to low, it takes a long time to get back to the low state and looks like capacitive decay. How can I fix this? 

Solution

Digital output devices have a large input impedance and with a high impedance source device, there will be minimal current flow and therefore take a long time to settle.  Consider adding a pull down resistor to the output line to allow a path to ground.

Additional Information

Pull up and and pull down resistors are resistors used to drain charge from communication lines allowing the logic level to settle to their respective state.