TL;DR My wheatstone bridge won't respond to small weight changes when using single supply.
I'm trying to wire up this circuit below using these 5kg load cells.
I am able to get the circuit working with a single supply to the INA125P but the output does not change (stays around 54mV) with small weights in the 10-100s of grams range and will then start working normally with larger weights.
I switched over to a dual supply which seems to work well and is extremely sensitive but I'd rather not need to add to the circuit for negative voltage nor have to offset the entire output voltage as to only output positive voltages to the ADC since it can't handle negative inputs.
Being as this is a prototype for a project I'd like to eventually make a small batch of, I'm amenable to using nearly any other configuration of electronics so long as cost stays fairly low. A different instrument amplifier that may be rail-to-rail, adding in an ICL7660 for negative voltage, or any other recommendation is welcome. That said, I'm extremely limited in physical footprint for the scope of this project and could not use a board like the HX711.
So my questions are:
1) Why am I not getting any response from the bridge with small loads when using single-supply and how can I fix it?
2) If it comes to needing to use a negative voltage, would an alternative rail-to-rail amplifier be a better selection bang-for-buck?
Best Answer
Your in-amp does not have rail to rail outputs. The minimum output voltage is (V-)+0.4 V:
With a single supply, if the weight measured is not enough to produce an output above 0.4 V, then the in-amp will not give a valid output.
See FIGURE 6 in the datasheet. By connecting IA_REF to the VREF2.5 pin, you can produce a differential output referred to 2.5 V, so that neither input needs to be near ground. You can then convert this signal with a differential ADC.