Bridge Amp Question (Not PIC Specific, a bit long)

[Gizz]

Hi all.

Sorry for the non-PIC q, but I bet theres someone out there who knows the answer. (I will pay in beer...)

I have a straing guage, standard 4 resistor bridge, and I want to interface it to a PIC16LF877A running on 3.3V. The strain guage is 5kohm nominal and should have 2mA through it, which gives 10V accross it. I only have +12V unregulated or 3.3V from a switch mode. Strain guage differential output is 30mV FSD.

My initial thort was to use a quad op-amp running from the unregulated 12V supply, maybe a low power device with a zener to help a bit of regulation.... run the SG off a 10V zener and a series resistor to give 1 or 2mA. Run each output of the SG to a voltage following non-inverting opamp (negative input tied to output) and run each of these outputs (now low impedance) to the + and - input of another opamp. The output goes through a resistor divider and a protection 3V zener to the ADC on the PIC.

QUESTION: How do I get the zero offset and gain to be a non-tweak setting? These units will be inaccessible so can't be calibrated, really I'd like the output to be 0V when there is 0V differential and (say) 2V when there is FSD (30mA). How do I get around not having a -ve supply line for the opamp? Help!

Rgds
Gizz

[Guest]

A bridge with 5kohm resistors is pretty good. Some notes:
1. Don't worry about 10V supply. You can drive the bridge even from 5VDC when the sensitivity decreases (of course) to 15mV FSD.
2. Check out an electronic handbook for "difference amplifier". If you already ready for use a quad opamp then your original concept will work: 2 opamp as unity gain amp and a 3rd as a differential amplifier (1.out - R1 - opamp+ - R2 to gnd;2.out - R1' - opamp- R2' to opamp output; R2/R1 = A what is @10V 2000mV/30mV = 66.6666)[/img]

[SherpaDoug]

If you use an instrumentation amp with a lot of gain and a limited voltage supply watch out for the Input Common Mode spec. The classic three op amp instrumentation amp requires a lot of voltage swing at the output of the first stage. What you want is the two op amp type of instrumentation amp like a LT1101 or an INA122.
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[Gizz]

INA122 it is. Superb. Thanks to you both! :o)