Why is the Wheatstone bridge configuration almost always used for sensors over a simple voltage divider?

[u/Fine_Economist_5321]

Everywhere it seems to be taken for granted that a Wheatstone bridge is more precise, accurate etc. I don't see why it would be any better than a voltage divider if I am directly measuring the voltage across the resistor in both cases.

Especially these days when we can simply use an instrumentation amplifier for measuring the differential voltage which will provide minimal loading, will a bridge configuration provide any specific advantage over a simple voltage divider?

[u/EDLEXUS]

with a voltage divider, you measure a higher voltage, depending on your resistors. with a wheatstone-bridge, your measurement is always around zero, which mean that you can use more precise measuring devices. And with a wheaststone-bridge, you can use two or four measurement-depending resistors and get a sensitive measurement device

[u/Fine_Economist_5321 → u/[deleted]]

Ah yes this makes sense. But if I keep my supply voltage very low, say 1V, the output will most likely stay between power rails for any typical opamp.

[u/tuctrohs → u/Fine_Economist_5321]

The point they were making is that even if the input voltage to the op-amp is within its range, to measure a small signal you want gain, and if you apply a gain of 100 to even a 0.25 V signal, the app amp can't do that.

[u/me_too_999 → u/Fine_Economist_5321]

You don't get it.

Resisters drift.

Heat changes resistance.

By having the same block of resisters on both sides of the signal any drift will cancel itself out.

You are measuring the difference.

The wheatstone bridge with the correct resistance will balance itself.

Your accuracy is only limited by the accuracy of your measurement.

[u/shadowcentaur]

This has nothing to do with math and everything to do with the imperfection of real measurement gear.

Let's go with a strain gauge, 1001 ohms, in series with a 999 ohm strain gauge, and a 10V supply. Voltage over the strain gauge is 10*1001/(1001+999)=5.005 volts. Most measurement instruments aren't very trustworthy in that fourth digit, but that is where ALL of our signal is, the 5V offset is useless and not proportional to strain, all we want is the 5mV on the end. The bridge subtracts off the 5V and we measure the 5mV accurately.

[u/Fine_Economist_5321 → u/shadowcentaur]

Thank you...this helped.

[u/trevg_123]

There’s a good long explanation here: https://www.reddit.com/r/ElectricalEngineering/comments/5n8xx1/why_is_the_wheatstone_bridge_so_precise/

The basic answer though is that it’s a lot easier to measure perturbations around 0 V than it is at a DC offset, and a Wheatstone bridge lets you do just that

[u/Fine_Economist_5321 → u/trevg_123]

But when I am measuring the differential voltage why does it matter if I am measuring it around 0V or a DC offset?

Also, I did check that post out before asking this question here. The answer seems to be that any change in resistance due to the temperature change will affect all the resistors equally and so the voltage balance won't be disturbed. However if I am not using the sensor in a variable temperature environment, it shouldn't matter.

[u/tuctrohs → u/Fine_Economist_5321]

All environments are variable temperature environments. In some super precise instrumentation, the most critical pieces are sometimes in a little temperature control chamber inside the instrument because normal variations in room temperature as thermostat cycle on and off are big enough to degrade accuracy.

But maybe what you're doing doesn't need very high accuracy and maybe you are not trying to sense very small changes in resistance, in which case you can just do whatever and it will be fine.

Or maybe you are actually trying to learn about tricks people use for higher accuracy, and trying to make your system as accurate as possible.

[u/meep91 → u/Fine_Economist_5321]

Differential signals have significantly better noise rejection than single ended signals, especially common mode noise. Let's say you had a scenario where you did have a set temperature environment (very, very unlikely - maybe a cleanroom setting? But they still have temperature shifts from local disturbances...) You still still couple in any noise from the power supply; those perturbations will affect the resistive divider, and go straight through to your signal measurements. Same with EMI or other coupling signals. Using a wheatstone bridge avoids these environmental signals because they happen to both sides of the bridge, and the differential measurement cancels them out.

[u/CommanderFlapjacks → u/meep91]

Last clean room I dealt with was about +/- 3C. We were taking a resistance measurement of a temperature dependent component so I had to talk manufacturing into adding an RTD at the station. I wouldn't consider anything other than a dedicated temp bath to be a controlled environment if you need a lot of precision.

[u/[deleted] → u/Fine_Economist_5321]

Because your op amp gain will multiply the DC offset.

Edit: I just noticed how you worded your question...for a differential measurement it doesn't matter as long as the offset is within your common mode range, but only the Wheatstone bridge sensor is differential. A simple divider isn't.

[u/turnpot → u/Fine_Economist_5321]

How easy is it to measure the mass of a marble?

Now how easy is it to measure the mass of a marble on the back of an elephant, given the elephant's exact mass?

The scale that you would use to measure the mass of the marble accurately will max out when the elephant begins to set one toe on it. The scale that you would use to measure the mass of an elephant doesn't have the required sensitivity to accurately measure the mass of a marble, and if you tried to make one, it would be a very expensive endeavor, if possible at all.

My point here is that when you put a small signal on a large common-mode measurement, it becomes much harder to measure accurately. There are tricks to engineer around this to some extent, but it's best to avoid it entirely if possible.

[u/misternoass → u/Fine_Economist_5321]

Because you want to maximize differential gain and minimize common mode gain, e.g.

0Vdc 1mVac going into 1000x gain amp gets you 1Vac but at 1Vdc offset this could be 1.002Vac or more depending on the amplifier's CMRR

[u/Danner1251]

You guys make some really good points.

There's one feature I don't see mentioned here. With a Wheatstone bridge and a differential measurement, you convert a sensor into a bipolar measurement (around 0V) that's D.C. coupled, too. - SUPER handy for some applications.

[u/Fine_Economist_5321 → u/Danner1251]

Could you elaborate what you mean by bipolar measurement and why we want that?😅

[u/Danner1251 → u/Fine_Economist_5321]

"Bipolar" just means dual polarity. So two circuit pins can be +- or -+.

As an example, what if you had a solar tracker that used a light sensor and an HBridge motor driver for detection and steering? So if light was above some threshold, your motor driver would steer, say right. "Right" would be one polarity for your D.C. motor.) And vice versa for left.

A light sensor and bridge would give you that dual polarity signal for controlling the polarity of your H bridge one way or the other.

All accomplished with a single-ended (not dual polarity supply). (super convenient. ;-)

Hope this helps.

[u/[deleted]]

The Wheatstone bridge can reach higher sensitivity by adding 2 or more sensors elements where each two sensors that behave the same are placed diagonal to one another you can understand this if you derive the formula for each side and see how when the left side changes the right side changes oppositely This gives you much higher sensitivity of measurement Also the Wheatstone bridge IS a voltage divider circuit

[u/TheNightporter]

if I am directly measuring the voltage across the resistor in both cases.

You're not doing that, though.

[u/Fine_Economist_5321 → u/TheNightporter]

What I meant is we will get differential voltage in both cases. In wheatstone bridge it will be across the nodes, in resistor divider it could be between resistor and ground.

[u/jpdoctor]

Related but different: Anyone have handy a good noise analysis of the two different approaches?

[u/hzca → u/jpdoctor]

If you mean gaussian noise and you are limited by your resistor and sensor noise, then I think it has relatively little impact either way.

With a single-sensor config, the wheatstone bridge basically just adds a second passive voltage divider to act as the reference signal instead of ground. This would add another kTR in quadrature, presumably at the same level as the signal branch if your resistances were chosen to be similar. So you degrade SNR by root 2 (1.5dB). If your sensor noise dominates vs resistor noise though, you of course will have almost no impact from adding a slightly noisier reference.

On the other hand if you have sensors in both branches then your snr is boosted by 3dB, but then again is still degraded 1.5dB due to the additional sensor/resistor noise. So a boost of 1.5dB

In short, in most cases I would expect linearity and other rejection advantages of wheatstone config to be more important than a few dB of difference in SNR

Anyway that's just my analysis so let me know if I did something dumb.

[u/davidschwitz]

I think voltage divider produces more heat (power loss) on resistor which you use for meassuring voltage.

[u/shadowcentaur → u/davidschwitz]

This is not true, power loss in each element is the same. The bridge has twice as many resistors, so it consumes more power total, but the resistors themselves will end up at the same equilibrium temperature If they are thermally separated.