What does the instrument reference establish?
NI describes high source impedance as a possible cause of a drop in the voltage measured by an analog-to-digital converter. Its troubleshooting guidance also identifies other possible causes of unexpected readings. A source-loading explanation is therefore a question to investigate, not an automatic diagnosis.
Commissioning Docs is independent and is not affiliated with NI. This guide organizes evidence; it does not specify a connection, select an input impedance or authorize work on live equipment. Use the actual device documentation and approved test procedure.
Sources: NI: NI: Troubleshooting Unexpected Analog Measurements, updated 1 April 2025
Which details make two readings comparable?
Keep the measurement chain attached to the result. The same nominal voltage range is not sufficient evidence that two input arrangements interact with the source in the same way.
| Record | Evidence | Review purpose |
|---|---|---|
| Source | Actual output and equipment reference | Which circuit was observed? |
| Source impedance | Applicable approved specification | Could loading matter? |
| Acquisition | Instrument, input mode and channel | Same measurement arrangement? |
| Connection | Approved lead, probe and wiring record | What was actually connected? |
| Conditions | Operating state and observation time | Was the source comparable? |
| Disposition | Technical review reference | What explains the difference? |
What should happen when a second instrument reads differently?
Illustrative example. A fictional test folder has two voltage readings from the same labeled output. The first used one acquisition input and the second used another, but the comparison sheet records only the two numbers. It calls the difference equipment drift without recording either input configuration.
The document reviewer retains both original readings and requests the two measurement-chain records. Loading, operating changes and other causes remain unresolved until the competent technical reviewer assesses the evidence. No correction factor or acceptable deviation can be calculated from the two numbers alone.
How should the evidence pack close the issue?
Link the technical conclusion to the original test records and any approved repeat test. If input settings were changed, preserve the before and after configuration and the authorization rather than replacing the earlier record with a cleaner-looking value.
Do not alter termination, grounding, probes or wiring to try this checklist. The approved procedure and competent personnel govern the test. Engineering sign-off is performed only by the registered engineer on the job.
Common questions
What else should the bid team know?
Does a changed reading prove the source changed?
No. The measurement chain and operating conditions also need review.
Does a calibration certificate rule out loading?
No. It does not establish the interaction of every source and input arrangement.
Can this checklist select an input impedance?
No. The responsible technical reviewer must use the actual equipment and procedure.