Electronics > Projects, Designs, and Technical Stuff
Load cell (Wheatstone bridge) excitation voltage hurdles
gogoman:
Hello David, I appreciate your help, few more questions;
"ratiometric measurement is absolutely an improvement over an absolute measurement technique"
Can you suggest several usage cases where and absolute measurement technique has the advantage?
"In practice this means that using the same reference for excitation and the analog-to-digital converter places much less of a demand on the performance of the referenc"
Is there a usage case where using a different excitation and ADC reference is an advantage?
thanks
Kleinstein:
There is hardly a real advantage of a separate reference. However than can be cases when the disadvantages of a separate reference may not be so bad and could be tolerated, e.g. to use an ADC that has an ADC internal reference only. So would low demands it may be simpler, especially if one does not have access to the ADC ref.
gogoman:
--- Quote from: Kleinstein on August 09, 2020, 04:15:47 pm ---There is hardly a real advantage of a separate reference. However than can be cases when the disadvantages of a separate reference may not be so bad and could be tolerated, e.g. to use an ADC that has an ADC internal reference only. So would low demands it may be simpler, especially if one does not have access to the ADC ref.
--- End quote ---
It's advantages to have the two references from the identical reference source, but there are devices available which allow two separate references. Having limited
experience resulted in me asking :-// why...
If the excitation and ADC reference differ, then the common mode volt applied to the ADC is not centered in the ADC Span, resulting a asymmetrical
ADC limit around the common mode voltage, is this statement correct?
Kleinstein:
The question of a 2 nd reference has nothing to do with the measurement range. if the used range is in the center mainly depends on the symmetry of the bridge.
With a 2 nd reference one has an additional source of gain drift, as the ratio of the 2 references can change (e.g. with time or temperature). In the radiometric mode, with only 1 reference the drift of the reference does not have an effect and one can thus even use a very simple one, like a regulator.
The drift of the 2 refs relative to each other essentially only effects the gain, not the zero stability. Here zero is where the 2 nd ref. (ADC) has no direct effect, e.g. the ADC reading zero. This may be different from the mechanical zero.
David Hess:
--- Quote from: gogoman on August 09, 2020, 03:51:49 pm ---Can you suggest several usage cases where and absolute measurement technique has the advantage?
--- End quote ---
Not all circuit topologies and applications allow for the reference to be shared between the excitation and analog-to-digital converter. The transducer may have its own internal excitation like an integrated hall device. Galvanic isolation would mean crossing the isolation barrier twice with the reference and signal. Not all instrumentation amplifier topologies support ratiometric measurements.
--- Quote ---Is there a usage case where using a different excitation and ADC reference is an advantage?
--- End quote ---
None occur to me; it removes a major source of error.
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