Voltage or Current Excitation for Wheatstone Bridge - Page 1
Forum section: electronics / projects

author: yashrk
author_rank: Frequent Contributor
timestamp: June 24, 2021, 07:21:23 am
content: Hey guys,I am working with a pressure sensor with a strain gauge arranged in a Wheatstone Bridge. I have a two-parter question.1. Why does the sensor performs better with voltage excitation vs current excitation, for a temperature test over -40°C to 80°C?With this question, I want to get some understanding of our findings.We are getting approximately1.166 %error/°Cwhen we are using0.5mAcurrent source across the sensor. There is a 3V voltage drop across the sensor.Whereas if we use say2.5Vvoltage excitation we get0.233 %error/°C. Of course, the absolute error of the current/voltage source doesn't account for the difference in errorIntuitively I would think a higher voltage across the sensor will yield better stable readings, irrespective of how it is generated (voltage/current excited). We also tested the sensor with a higher voltage of4Vbut there were diminishing returns, we just got0.2166 %error/°C.2. The Second question I have is about the configuration shown in the image belowHere the main advantage is that this configuration doesn't need a stable reference source. What I want to know is apart from not needing a precision voltage reference is there any added advantage for such a configuration?

author: mikerj
author_rank: Super Contributor
timestamp: June 24, 2021, 10:43:07 am
content: If you drive a Wheatstone bridge with a constant current then a change in impedance on one side of the bridge will change the current flowing in the other side of the bridge (and therefore the voltage in the middle of the divider).

author: yashrk
author_rank: Frequent Contributor
timestamp: June 24, 2021, 11:17:33 am
content: Quote from: mikerj on June 24, 2021, 10:43:07 amIsn't that what we want a voltage difference between two points. And I am not saying it doesn't work on current excitation it's just less temperature stable with the current.

author: bob91343
author_rank: Super Contributor
timestamp: June 24, 2021, 06:04:47 pm
content: Perhaps a careful analysis is in order. Figure the Thevenin equivalent of the bridge arms and so on.

author: mikerj
author_rank: Super Contributor
timestamp: June 24, 2021, 06:14:05 pm
content: Quote from: yashrk on June 24, 2021, 11:17:33 amIf the resistance on one side of your bridge has not changed, why would you want it's voltage to change?

author: fcb
author_rank: Super Contributor
timestamp: June 24, 2021, 06:45:40 pm
content: Driving a bridge with a CC (constant current) will result in a change in gain (non-linearity) UNLESS it is perfectly balanced (i.e. both sides keep an identical resistance regardless of strain/pressure) - unlikely.In your 2ndpart - the results will be better if the ADC reference voltage and the bridge drive voltage are the same, or at least ratiometrically linked - having the bridge driven by a 5V regulator and then the ADC reference being perhaps a precision 4.096V will lead to changes in the gain (non-linearity) if either deviates - in that case you either want the reference to be the 5V rail or perhaps the drive the bridge from the 4.096V.The higher voltage leads to more dissapation in the strain-gauges and you will probably see greater thermal effects.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 24, 2021, 07:56:42 pm
content: The output voltage of the bridge depends on the ratio of the resistors when you feed it from a voltage source, but also on the absolute value of the resistors if you feed it from a current source. With current source feed you basically get an extra error term from the temperature coefficient of the resistors.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 24, 2021, 09:36:25 pm
content: You're measuring an output voltage differential, and therefore must also excite the thing with a voltage.It's as simple as that.A part for putting elements in a bridge configuration is for temperature compensation for the elements.Let's say that strain gauges can deviate 2% with temperature. If you excite the bridge with a current, then that would directly result in a 2% error when you measure the output differential voltage.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 24, 2021, 11:16:12 pm
content: The answer to your question 2 is that the bridge becomesratiometric. (Italicized because this is a good google search term. Hint).In a proper ratiometric bridge the error from the voltage reference drops out, since it goes both to the bridge and the ADC.This can be combined with bridge reversal, e.g.https://www.analog.com/media/en/technical-documentation/application-notes/an96fa.pdfand alsohttps://www.analog.com/en/analog-dialogue/articles/transducer-sensor-excitation-and-measurement-techniques.htmlRandall.P.S. post mod'ed to add links.

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2021, 09:16:08 pm
content: Quote from: Wolfram on June 24, 2021, 07:56:42 pmResistive bridges usually have very tight absolute tolerances however there are exceptions like semiconductor bridges which a pressure sensor may very well be.Quote from: yashrk on June 24, 2021, 07:21:23 amMaybe it is a semiconductor bridge? Externally they operate like resistive bridges but with higher sensitivity and lower accuracy.QuoteThat is generally true but only up to the point where self heating effects manifest. Sometimes pulsed high voltage excitation is used to get the advantage of high voltage without self heating.QuoteRatiometric operation removes both excitation drift *and* excitation noise.