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Topic: Strain gauge circuit question (Read 168 times) previous topic - next topic

liudr

I found this design schematic in a specsheet (LMP2232A)

http://www.mouser.com/ProductDetail/Texas-Instruments/LMP2232AMAE-NOPB/?qs=sGAEpiMZZMutXGli8Ay4kCls1U27HsKUEUotRe%252bq1aI%3d

Page 1. There is an op-amp right after the voltage regulator. I suppose it is a follower but why is it needed?

I assume the other three op-amps are amplifying the difference and shift to unipolar voltage range, right?

Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

liudr

OK, I thought the LM4140A was a voltage regulator. It turns out to be a precision voltage reference. So this makes sense to have a follower to stabilize the output.

Also, the rest of the op-amps are two followers and an inverse amplifier. The single supply chops off the negative half of the voltage but that should be OK since a strain gauge only needs to read one way (weighing plate is not used to sense upward force).
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

Randy_HT

Hi,

Most likely you have moved on but you might consider using a "instrumentation" amplifier IC in place of the three amplifier circuit.  The instrumentation amplifier has closely matched internal resistors for better performance over time and temperatures.  You would then set the gain using only one precision resistor.  That resistor should be a 0.1% with 25ppm/C as a min.  Those cost are still affordable. 

Also, if interested, I'm working on a higher precision analog to digital converter (DAC) for Arduino.  It's a small shield with a 22bit DAC.  The shield has a trimmable voltage reference which would allow you to connect your strain circuit and trim the DAC output to a Zero value.  The DAC also has a built in trap filter for 60 Hz noise found in most labs. 

Cheers

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