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Topic: Strain gauge circuit question (Read 740 times) previous topic - next topic


I found this design schematic in a specsheet (LMP2232A)


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


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



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. 



Look at the HX711-frontend for your needs.

// Per.


Apr 10, 2016, 04:02 pm Last Edit: Apr 11, 2016, 04:47 am by TerryVA
I would sugest that you use a circuit similar to the one shown with the ISL26102.

There are a few similar parts from various manufacturers.

It encompases the intrumentation amp, Variable gain amplifier and high resolution ADC converter with programable filter. The best part is these devices work ratiometrically so that the quality of the voltage at the top of strain bridge is not important and the +5V rail of the Arduino would be just fine.



Here is a quad HX711 board for the Mega2560  SPI port for Ethernet (I use the Mudbus lib for ModbusTCP) and I like the Digole I2c for a touch screen.

I have revised it but it uses the orange chip on Ebay.  and the Hanrun W5100 for ethernet.

It also has a pot for the 3/5v rs485 BOB



Here are the evil twins built out;

one with jacks; 3.5mm stereo plugs;
the other with terminal blocks; these are tiny;

the digole display plugs into either of the headers on the right either spi or i2c;

these are set for 5v operation on a Mega.  but they could be set for 3.3 on a due;

I think I will delete the fourth hx711 and add the ADS1115 (sitting on the display);

the RS485 is either 3/5v OK....it sits on the UNDER SIDE of the shield.  it just misses the reset button;

I have the reset pulled up to just near the reset pin but I didn't bother to put the switch on the board;

THe 10kNTC sits under the third HX711 and was supposed to be a smaller one;

THe HANRUN Enet comes out the angle header at the front for now;  Modbus or S& if I don't use the local display.....I guess I could do both if I use  i2c on the display....

I will move the connector 0.05 forward to be OK with a vertical connector, but for now I am targeting a hammond 1590 deep enclosure.......


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