Can The USB-6001 Digital Output Generate 3.3 Volts Across a 50 Ohm Load?

Updated Aug 14, 2019

Reported In

Hardware

  • USB-6001

Issue Details

  • I am trying to generate a 3.3 V digital output signal using my USB-6001. When I measure the signal with an oscilloscope that is set to 50 Ω, I read a value under 1 V. Why is this happening?

Solution

This happens because a USB-6001 digital output has a maximum source/sink current limit of ±4 mA, according to the user manual on page 22. A 4mA signal cannot generate a 3.3 V signal across a 50 Ω load as dictated by Ohm's Law (V = IR). 

In order to read the desired voltage measurement, switch the setting on your oscilloscope to a large impedance --typically 1 MΩ. After this step, you should read a signal between 3.3 V and 5 V when measuring an active digital output on your USB-6001.

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