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INA128: Bridge Amplifier, Noise Reduction, General Best Practices Questions

Justin Miller1
Prodigy 20 points
Part Number: INA128
Other Parts Discussed in Thread: LM317

Hello,

Let me preface my question with the fact that I am a circuit design novice, so please forgive me for any glaringly obvious mistakes or omissions.

I am working on building an amplifier circuit for a Wheatstone bridge arrangement sensor. My input signal range is from 100 Microvolts to about 1 volt. Since it seemed relevant I measured the input resistance and it varies from 140 ohms to 31k ohms depending on bridge output. My desired maximum output would be 10VDC. 

I am using an INA128 amp and here is a picture of the amplifier circuit. Currently I am powering it with a ±12VDC power supply, but will be getting a ±15VDC power supply which I believe is more ideal for this Amplifier? I am using a 5k ohm resistor yielding a gain of 11±.05. Generally the gain seems to be just a little off at very low input voltages. ±.05 or so. But i think this is acceptable for me in general. This circuit was mostly taken from the application notes.

Other important notes are i am only concerned with stability, this will be used to amplify a DC signal. There is some noise present in the input signal from the bridge that i would like to eliminate, but putting a small cap (.01microF) did not seem to do much. I also tried a 2.2microF and had slightly reduced noise. I have read but not tried referencing my inputs to ground with a small cap, maybe this would improve my noise issue?

Generally I am happy with the results I am seeing with the circuit shown, but I would like to get feedback on improvements from others. 

Also I have a question regarding why grounding the -input terminal makes this circuit work correctly, but if that is omitted, i do not get the expected results. I have a feeling it may have some thing to do with my input circuitry. As ab example of what i mean about it not working correctly, with the -in terminal grounded, if my input applied is 0.506V i get  5.554V out Gain=10.94, if i remove the reference to ground i get 3.06 V out unless i am remembering incorrectly and it was 3.06mV. Either way, i am not sure what is causing me to need to reference my input to ground.

Edit: I tried capacitively referencing my input to ground and that worked as well. I am just stumped as to why this is required when i do not see it on the application notes. Also are there any downsides to using all these caps?

Also the black/white wires in the upper area of the picture are where i am measuring the output. The input signal wiring is not shown, but is across terminals 2 and 3(14 and 15 on the breadboard) if that is not obvious.

I really just want to create the most linear and quietest amplification circuit that i can, any help is appreciated.

Thank you,

Justin

  • 0
    TI__Genius 10850 points

    Hi Justin, 

    Have you taken a look at the following application report: http://www.ti.com/lit/an/sloa034/sloa034.pdf

    This report covers three different methods of measuring a wheatstone bride - single op amp configured as a diff amp, three op amp instrumentation amp, and two op amp instrumentation amp. 

    In regards to stability, you may actually build this schematic in our SPICE simulator (http://www.ti.com/tool/TINA-TI) and accurately measure the phase margin associated with your design. We aim for phase margin of >45°. You can learn more about op amp stability in our TI Precision Labs training material covering stability here (section 10): https://training.ti.com/ti-precision-labs-op-amps 

    Run through this and feel free to share your TINA schematic with me for further analysis.

    -Tamara

  • 0
    Guru 140200 points
    Hi Justin,

    take care, the TMPM04212 is a AC/DC switcher. Such a switcher can generate lots of noise and EMI at its output! Datasheet of TMPM04212 says that the output noise is 1% of Vout which is 120mVpp in your case. This is huge! You should think about some filtering. You could also add some voltage regulation (LM7809/LM7909 e.g.). This would also decrease the switcher noise.

    Equally how you manage it, the INA128 and the bridge need a stable, clean and ripple-free supply voltage with little noise.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 20 points
    Kai,
    I appreciate the response and have looked at the output waveform from the power supply on an oscilloscope, and did see that it did indeed have some noise, about 80mV p-p max. I guess I did not realize this was significant. Thank you for mentioning this. How quiet must the power supply be, and if it would not be too much trouble could you point me to a resource on the filtering? Because I have tried various capacitors across the power supply, and they did not seem to have too much effect in lowering the noise, i got it down to perhaps 30-40mV P-P, and then decided it was not worth it or that i was just picking up stray noise in my oscilloscope leads. Also i do have some LM317 voltage regulators on hand that i could try.

    Thank you,

    Justin
  • 0 in reply to Justin Miller1
    Guru 140200 points
    Hi Justin,

    why not using simple batteries for the beginning?

    Kai
  • 0
    TI__Genius 10850 points
    Hi Justin,

    We haven't heard back from you in a few days. I hope we were able to resolve your question. Closing this thread now.

    -Tamara