# Should we use wheatstone bridges nowadays?

For measuring strain gauges, the Wheatstone bridge is the historical choice.

In a quarter Wheatstone brigde ($$\350 \Omega \$$ typically (?)), with a high input impedance amplifier, and some bridge power voltage $$\V\$$, the ratio between the bridge output voltage w.r.t. any resistor is 1/4, which is the same ratio in a voltage divider between the output voltage w.r.t. any resistor, compared with a stable voltage reference of $$\V/2\$$.

A voltage divider with a voltage reference would have a voltage noise of about 35.16 nV/rtHz (from a voltage reference of 35 nV/rtHz plus two resistors), and the Wheatstone bridge a voltage noise of 4.86 nV/rtHz (from four resistors), which should be enough (?) for most ADC systems (i.e. for 24 bit, 0-5V range: 300nV resolution).

And a voltage reference ($$\0.02\%\$$ initial error, $$\2 \ ppm/C^\circ\$$, AD, ADR4525), would be more stable and precise (?) about $$\4\ ppm/C^\circ\$$, than a precision resistor ($$\0.005\%\$$ error, $$\2 \ ppm/C^\circ \$$, Vishay, MR100), about $$\8 \ ppm/C^\circ\$$.

Hence, why bother using Wheatstone bridges?

• Hello Brethlosze, An interesting question that you are posing, but I find it hard to find the actual question to answer. "Hence, why bother using Wheatstone Bridges?". There are many alternatives to a Wheatstone bridge, but I'm not sure I quite understand which alternatives you are considering in this question, are you comparing it to one voltage divider with two resistors rather than the four used in a Wheatstone bridge?.. I think it would help more people understand the question if you added some diagrams, or made it a bit more wordy ;) – Vinzent Sep 21 '19 at 6:51
• I knew you would like a diagram. Granted. – Brethlosze Sep 21 '19 at 7:16
• Information: Wheatstone bridges can be driven from a current source to produce a more linear output with resistance change in quarter bridge applications compared to voltage excitation. They are not going away soon! Note that the ADR4525 does not have a tempco of 0.02 ppm per degC but typically 2 ppm per degC. – Andy aka Sep 21 '19 at 8:07
• @Andyaka Great note. But if i drive a bridge with a current source, also could made the same in the voltage divider case. The Voltage Regulation is noisier, but more stable? – Brethlosze Sep 24 '19 at 3:06

What you are missing is that the common purpose of using a Wheatstone bridge is to balance two sensors. Ie. when we use a Wheatstone bridge we usually have two variable resistances or current sources that we want to balance like this;

simulate this circuit – Schematic created using CircuitLab

You are right in concluding that if we only have one sensor to measure and not two that we want to balance then the Wheatstone bridge is not the ideal way to do it.

The problem with using a voltage reference version is that the circuit depends on the difference between two accurate voltage sources, V and Vref. In the Wheatstone bridge, there is just one voltage source, and there is some error cancelation.

Accurate voltage references are noisy and difficult to filter without losing accuracy. They have good long-term stability for voltage, but many have poor short-term stability.

The Wheatstone bridge and its variations are still widely used in the design of measurement equipment. There are many real-world measurement errors that are not just resistor noise, and a bridge circuit often helps.

• Note that the changes in resistance for most strain gauge applications (within the elastic region) are small. If the strain is 1000 microstrain -- large for most metals -- (so 1e3 / 1e6 = 1e-3, AKA 0.001) and the gauge factor is ~2, then the change in resistance is R_nominal * 0.002! So you need all the help and common-mode rejection you can get. Silicon gauges have much higher gauge factors, but they're expensive, tricky to mount, and quite temperature sensitive. See also Kelvin connection (6-wires), if you have long wiring runs, esp. around electrically noisy equipment. – Catalyst Sep 22 '19 at 22:32
• I am still in the doubt. One of the answer (now deleted, ?) said that a voltage reference was in concept just a form of a bridge arm, with an additional sophistication. Note that yes, the V voltage source should be regulated in both schemes. – Brethlosze Sep 24 '19 at 3:03