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INA128 Input voltage issue

Other Parts Discussed in Thread: ADS1015, OPA211

Hello all,


I've designed a circuit to measure an EMG signal (attached file).  The circuit, by itself, works like I'd expect; however, for the end design, I need to connect it to an ADC (I'm using ADS1015, controlled by a PIC18F4520 microcontroller); the Vref and the Vout go directly to the A1 and A0 pins, respectively, of the ADS1015.  The voltages measured are correct, but, whenever I connect the circuit to the microcontroller/ADC circuit, I see a voltage between the Vin + and - pins (~0.25 V) and between each of the input pins and the reference pin (~1.8 V).  When I run the circuit alone, these voltages don't show up.  I tried putting in an optoisolator between the two circuits, but it didn't get rid of these voltages.   Does anyone have an idea how to fix this?  Thanks so much!

  • Hello Margaret,

    Please take a look at #6 in the following FAQ page on biopotential measurements.

    e2e.ti.com/.../design_notes

    I have a couple observations about your circuit. Here they are:

    1) There appears to be no input protection resistors or filtering. Please refer to the aforementioned FAQ. There is an excellent presentation called "Design Considerations for ECG Systems" that may be of use (starting on slide 21). I understand that you're working with EMG, but some of the concepts are the same.

    2) I recommend placing as much gain in the first stage as possible and using a precision op amp for the second stage. Here is a link that discusses the concept.

    e2e.ti.com/.../put-gain-up-front-waxing-philosophical

    For the second stage I recommend evaluating the OPA211 or using the ADS's internal PGA.

    3) Concerning the 250mV that appears when you connect the micro, it is difficult for me to understand where this is coming from. For example, if the 'extra' voltage appears after the signal is digitized, perhaps it's a coding issue. Alternatively, if the 'extra' voltage is actually measured at the inputs, perhaps there is a grounding issue.