Is there an affordable way to balance a bridge without using expensive resistors?

Question

I have a full and balanced bridge circuit with four through-hole \$ 350 \,\Omega \$ resistors and an instrumentation amplifier AD8293G80 that has a gain of 80. Both the bridge and the amplifier are powered by an \$ 2.8 \,V \$ LDO with the amplifier referenced at \$ 1.4\,V \$ by a voltage divider.

(1% resistors) I was expecting \$ 1.4\,V \pm \text{some milli-Volts} \$ at the amplifier output due to resistor mismatches and tolerances even the bridge was "balanced", but I got values somewhere in between \$ 2.51V-2.79V \$. The amplifier output also drifted overtime until it hit the limit of the \$ 2.8V \$ which was my supplied voltage. I then realized that the my resistors were only 1% in accuracy. In the worst case, I could have an input voltage of \$ 28 \,mV \$ amplified to \$ 28 \,mV * 80 = 2.24 \,V \$ due to mismatch. Playing out my circuit on a cheap breadboard also didn't help...

(Trimmer) I then tried replacing one of the bridge resistor with a Bourns trimmer (\$ 500 \,\Omega \$), thinking that I could "tune" my way out and balance my bridge as close to "zero" as I wanted it to be. It sort of worked but only for some seconds, until the output drifted again. Also whenever I pulled the trimmer out of the breadboard or applied any pressure to its leads, the trimmer resistance could change up to a few Ohms!

(0.01% resistors) I was shocked by the price tag (~15-25 USD each!). One of these "precision" resistor costs just as much as a bathroom weigh scale ... and I have to buy four of these?

If those 20 dollar weigh scales manage to balance a bridge circuit, perhaps there are alternative ways to complete the bridge rather than using expensive resistors that cost more than the scale itself. Does anyone know of a more affordable way to balance a bridge?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

This AD8293G80 InAmp is unsuitable for applications where the input voltage is typically greater than 1.1 volts on a 2.8 volt supply. You are generating about 1.4 volts on each half of the bridge and this is beyond the device's capabilities: -

Graphically there is this picture that I have highlighted in red and yellow: -

I'm not saying that there aren't other potential problems but this one sticks out and is a definite show-stopper. The good news is that the reference input can handle up to 2 volts.

Answer 2

What you're trying to do is fairly expensive. I don't see a point of using an LDO specifically for the bridge. Bridge circuits with A/D conversion almost always are ratiometric. That is: the A/D reference voltage is a scaled version of the excitation voltage. For typical 3.3V microcontrollers, the excitation and A/D reference can be one and the same voltage. It doesn't matter what the exact value is, because the digital output of the ADC is relative to the excitation voltage, and the absolute value of said voltage does not appear in the A/D's output (!).

Also, instrumentation amplifiers are more expensive, power hungry and noisy than semi-decent op-amps. So you'd probably want a circuit where the in-amp isn't needed.

I'd use a circuit where the excitation voltage is adjusted to keep one side of the bridge at a particular fixed voltage. Then the other side of the bridge can be amplified with a single-ended op-amp. The op-amps you'd want should be low or "zero" drift, rail-to-rail output types. Their common mode will be around half supply voltage.

The bridge can be built out of whatever resistors you got around as long as they are wirewound, metal foil or thin film.

^{simulate this circuit – Schematic created using CircuitLab}