Analog Output Signal Clipping when Reading with Low Z Setting on Oscilloscope

Updated Jun 15, 2018

Reported In

Hardware

  • USB-6356

Driver

  • NI-DAQmx

Issue Details

I am using an NI-6356 to generate sinusoidal waves and I monitor the output using an oscilloscope with 1 Mega Ohm (High Z setting) and 50 Ohm (Low Z setting) impedance. With a 1 V output from my NI device, I experience signal clipping as shown below. Do you have some insight on this issue?
 

Solution

You are likely pulling too much current from the NI device. For instance, the NI-6356 Device Specifications states that the device has an output current drive of ± 5 mA. If we are outputting 1 V from the device with the low Z setting (50 Ohms), using the following equation:
 
Voltage = Current * Resistance
 
we would be pulling 20 mA from the device. To resolve this, use the High Z setting on the oscilloscope, which at 1 Mega Ohm will result in a current draw of 0.001 mA, which is well within the device specifications. 

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