PXIe-5122 Not Reading Attenuated Signal

Updated Mar 12, 2018

Reported In

Hardware

  • PXIe-5122
  • PXI Oscilloscope
  • Oscilloscope Probe
  • Oscilloscope Device
  • PXI Digitizer

Driver

  • NI-SCOPE

Issue Details

I'm reading an attenuated signal through a probe or probe-like device connected to my NI Scope/Digitizer. My probe attenuation is 10:1 and my signal into the scope channel is 300mV. I set the vertical range to 300mV, but the waveform I fetch clips the signal above 2V.

Solution

The vertical range of the scope is set too low to capture the attenuated signal. The vertical range needs to be set to the maximum expected signal from the niScope fetch/read (i.e. 3V in the above example), not the maximum of the true signal coming into the scope channel (i.e. 0.3V, or 300mV). The niScope driver automatically takes care of the attenuation scaling.

Additional Information

To configure the onboard ADC to capture the best measurements, the niScope driver takes the vertical range and divides it by the probe attenuation. The resulting value is then coerced to the nearest ADC range that the device can do, and will set the ADC range to that value. In the above example, setting the vertical range to 300mV and a 10:1 probe attenuation will actually set the ADC range to 0.2V since 300mV divided by 10 is lower than 0.2V, which is the lowest ADC range on the PXIe-5122. Since a 0.2V ADC range can not capture a 0.3V (or 300mV) signal, clipping will occur.

Here is another example:
  • Signal before attenuation: 5V
  • Signal after attenuation (coming into the scope channel): 500mV,
  • Probe attenuation = 10:1
  • Vertical range = 10V
10V vertical range / 10:1 probe attenuation = 1V ADC range. For the PXIe-5122, this is a valid ADC range (see Table 1 in the PXIe-5122 Specification Manual), so no coercion will occur. A 1V ADC range can handle a 0.5V (or 500mV) input signal without the ADC becoming saturated, so no clipping will occur. A niScope fetch/read will return a 5V sample (0.5V × 10:1 = 5V).

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