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Troubleshooting Steps to Improve Scope Accuracy



Primary Software: Driver Software>>NI-SCOPE
Primary Software Version: 3.0
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)

Problem:
I am aware of the DC and AC accuracy specifications listed for my NI High-Speed Digitizer. However, my voltage measurements are not as accurate as I would like them to be. What can I do to fix this?

Solution:
Despite the device specifications, you may still obtain inaccurate voltage measurements if you do not configure your channel characteristics properly. This knowledge base will address several commonly overlooked issues that will decrease accuracy. These factors are the channel input impedance, maximum input frequency, and vertical range.

Channel Input Impedance

If your channel impedance is not properly matched, you may see voltage readings that are significantly incorrect. For example, if your signal source has an impedance of 50 ohms, and your scope is configured for an input impedance of 1 mega ohm (default), you'll see a signal that is effectively twice as large as the actual signal.

Channel impedance is configured using the niScope Configure Channel Characteristics.vi. Common valid values are 50, 75, and 1mega ohm.



Maximum Input Frequency

The maximum input frequency property specifies the bandwidth of the channel at which the input circuitry attenuates the signal by 3 dB. If left unconfigured, you may read a voltage value that is slightly larger (in the 10's of mV) than your expected signal. This property is also configured using the niScope Configure Channel Characteristics.vi.

Vertical Range
Lastly, DC resolution is largely affected by the configured vertical voltage range. The larger the input range (generally defaults to largest input range), the less resolution you'll have to measure your signal. Generally, it is good practice to limit your vertical range to the smallest range that still encompasses the entire range of your input signal.

Vertical range is configured using the niScope Configure Vertical.vi. Please refer to your specifications sheet for the valid input ranges for your Scope device.


Related Links:
KnowledgeBase 3XAFDNKQ: AC Amplitude Accuracy of the NI 5922 Digitizer
KnowledgeBase 3WEMERKQ: AC Amplitude Accuracy of the NI 5122 Digitizer
Product Manuals: NI High-Speed Digitizers Help
Specifications Explained: NI Oscilloscopes and Digitizers

Attachments:


ScopeConfigChan.JPG - ScopeConfigChan.JPGScopeConfigVertical.JPG - ScopeConfigVertical.JPGScopeConfigChan.JPG - ScopeConfigChan.JPG


Report Date: 10/11/2006
Last Updated: 10/04/2017
Document ID: 42AJ3IJQ

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