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350ohm strain gauge measurement questions

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cmcraeslo:
Hello all,

I'm designing a 350ohm strain gauge measurement (2 half bridges)

This is what I have so far:
- I have noisy environment and it's not important to output the absolute value of the measurement, so I was thinking of ratiometric conversion, rather than absolute
- Planning to use INA3333 (https://www.ti.com/lit/gpn/INA333)
- Supply voltage for the ADC's REF voltage will be 3.3V (standard LDO, filtered)
- I'm planning to split the 3.3V supply to 1.65V and feed that to a INA's VREF AND E+ (split will be done using a opamp so low output impedance into the REF pin). Plan is to get 1.65V +/- 1.5V into the ADC.
- According to Figure 35 of the INA3333, they show "proper common mode voltage" resistor of 150 ohm (R1) connected in series to ground. Am I right to assume that I need to do 350/2 resistor here? I've seen some designs using 1.27k ohms here. Not sure what this resistor is for?
- I will not be able to calibrate it on power up, only when installed in the product for the first time, so some temperature compensation will be needed. Was thinking of adding a NTC to the uC board and just do error compensations/measurements/calibrations vs temperature once I have the circuit running. NTC will be in a place that will 100% correlate to the temperature of the strain gauges and wires. Am I over simplifying things here?

Would love your comments before I start.

Thanks

Kleinstein:
Are the two half bridges coupled to form a full bridge, or are they separate to measure 2 independent loads ?
For 2 independent measurements one would need a reference divider of some sort (e.g. 2 precision resistors or alternatively use a high resolution (e.g. 24 bit) ADC with no extra amplification and use a digital reference level).

The resistor at the OPA333 is there to set the gain.

cmcraeslo:
It will form a full bridge.

@Kleinstein See attached schematics for resistor I had in mind.

Kleinstein:
The resistor R1 is there to reduce the current through the bridge and maybe adjust the common mode voltage. Usually the point half way should be OK.

It depends on the thermal properties if the bridge can use a full 3.3 V. It may still run a little to warm and thus may need the added resistor. If the ADC supports differential reference inputs one could use the reference from only the bridge and this way reduce the effect of R1 on the scale factor.

The ADC should probably run in differential mode, with one input to the virtual ground.

cmcraeslo:
I plan to supply the bridge (E+) with 1.65V and E- to ground.

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