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