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INA126 Single Sided Supply

Lance Henderson
Prodigy 30 points

Hi there,

I am currently trying to design a whetstone bridge circuit. The only amplifier I have available is the IN126 instrumentation amplifier, and I have a 0-5V single powered power supply (USB voltage output).

The input from the strain bridge is 0 < x < 3mV.

I have wired up the IN126 as a single sided supply, and connected Vref to GND. However,  Vo to GND is stuck at 2.6 volts (even when the inputs IN+ and IN- are tied together).

The same thing happens even when the strain bridge is connected. I think it has something to do the the CMRR and the fact that the lower op amp inside the instrumentation amplifier is limited by

Vo2 = 1.25*Vin- + (Vin+ - Vin-)10k/Rg

 Which in my case gives:

Vo2 = 4.55V at one limit and (maximum strain)

Vo2 = 4.75V at the other limit (minimum strain)

I am not too sure what this means. However, when I look at the CMRR garph in the data sheet it tells me that the CMRR can be at 2.3V and I can still not saturate the A2 amplifier so it should be working correctly.

I have attached a schematic showing the setup I have. Any help would be much appreciated!

Regards,

Lance

  • 0
    TI__Genius 14825 points

    Lance,

    Thank you for your question.  I'm looking into it presently and will get back to you as soon as possible.  I'm trying a TINA model of your application circuit first just to see what it shows for a transfer characteristic from 2.305V to 2.311V on the inputs.

    Jason Bridgmon

  • 0
    TI__Intellectual 920 points

    Lance,

    With an input common mode voltage (CMV) around 2.3V, you should not be violating the devices input CMV range. With Vref set to ground (0V), I am concerned about your output voltage when your strain gauge "x" input is zero. With Vref at 0V, your output would be non-linear and saturated to your negative rail. One way to solve the non-linear output at zero differential voltage to the inputs is to set Vref equal to 1V. This should let your output voltage operate linearly with your differential input voltage 0 to 3mV from the strain gauge. With a gain of 538.3 and a maximum differential voltage of 6mV, your output should then range from approximately 1V to 4.2298V for an input "x" from 0 to 3mV from the bridge.

    The formula you referenced in the datasheet is different then what you have stated. The datasheet formula is listed below:

    Vo2 = 1.25*Vin- + (Vin+ - Vin-)10k/Rg

    According to my understanding of your circuit, your Vin- should be between 2.308 V (strain min) and 2.305 V (strain max). Your Vin+ should be between 2.308 V (strain min) and 2.311 V (strain max) which results in your Vo2 ranging from 2.88125 V (strain min) 2.48125 V (strain max).

    Please confirm that your test circuit matches your drawn schematic and please let me know what your measured output voltage is with Vref set to 1V. Thank you.