Is Precision of Excitation Voltage Important for Load Cell Accuracy?

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Discussion Overview

The discussion centers on the importance of the precision of excitation voltage for the accuracy of load cells, specifically in the context of using a Wheatstone bridge configuration and an LT1167 instrumentation amplifier. Participants explore the implications of excitation voltage variations on output accuracy and the role of common mode rejection in this setup.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the necessity of precision in excitation voltage, suggesting that a standard ±5% linear regulator might suffice due to common mode rejection by the instrumentation amplifier.
  • Another participant argues that the accuracy of the load cell output is directly proportional to the excitation voltage, indicating that a 5% variation in excitation translates to a 5% variation in output, which could be problematic in certain applications.
  • There is a discussion about whether variations in excitation voltage would be considered common mode, with one participant asserting that only the difference between IN+ and IN- is amplified.
  • A participant raises questions about the output of the load cell under different excitation voltages, emphasizing the passive nature of the bridge and the importance of understanding the gain characteristics of the instrumentation amplifier.
  • Another participant acknowledges the complexity of the topic and expresses gratitude for the clarification provided by others.
  • Common mode rejection is discussed in the context of environmental noise and its effects on the inputs of the amplifier.

Areas of Agreement / Disagreement

Participants express differing views on the significance of excitation voltage precision, with some emphasizing its critical role in accuracy while others suggest that common mode rejection may mitigate its effects. The discussion remains unresolved regarding the best approach to excitation voltage in load cell applications.

Contextual Notes

Participants reference the specifications of the LT1167 instrumentation amplifier and the characteristics of the Wheatstone bridge, indicating that further details on these components may be necessary for a complete understanding of the implications of excitation voltage variations.

j777
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Hello,

I'm trying to decide how I will provide excitation voltage to the wheatstone bridge of a load cell. The output of the load cell will be amplified by an LT1167 In-Amp. How concerned should I be about the precision/accuracy of the excitation voltage? Since the In-Amp will reject common mode voltages (which should include any fluctuations caused by change in excitation voltage) would a standard +-5% linear regulator work without introducing and real error in the output?


Thanks
 
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Are you looking at bridge output as instrument output, or are you balancing the bridge and looking at the balance as instrument output?
 
Bridge output as instrument output.
 
Then, accuracy is a linear function of excitation voltage; output is proportional to excitation voltage. 5% in excitation is 5% in output. If you're going to use that as input to a control circuit and drive that to a "zero set-point," no sweat. If you're buying or selling bulk material by weight, you're screwed.
 
Wouldn't any variations in excitation voltage be common mode since only the difference between IN+ and IN- from the load cell is amplified?
 
What's the output of the cell under load at a zero excitation voltage? At 1 V? At 10? The bridge is a passive device. It's output is the difference between the voltages at mid-points on two voltage dividers. If by "common mode," you mean that the amp gain is a linear function of excitation voltage, you're going to have to post the specs on the 1167. "In-amp" ain't something I've run into, but I ain't been in the game lately. I've been talking on the assumption that "In" is short for "instrumentation amp," a souped up op-amp. Without some input giving it "information" on excitation voltage, it has no way of knowing what scale factor to apply to gain, given a variable bridge output for fixed load.
 
Thank you for making some sense of what I was saying. I wasn't thinking clearly but I understand what you are saying now. Your explanation was very helpful as I am definitely not an expert with this stuff.
 
Welcome. I failed to point out that the usual use of "common mode rejection" refers to "stray" inputs that appear simultaneously at both inputs to an op-amp --- inductive pick-up of power line noise, or other environmental phenomena.
 

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