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OPA4991: Reduce errors

anson zhou
Intellectual 345 points
Part Number: OPA4991

Tool/software:

Hi Sir  :  

We are using OPA4991 instrument amplifier for small signal amplification, but we are currently facing a problem. The differential input itself introduces a signal of about 6mv, which has a significant impact on our measurement signal (ranging from 0-100mv). What I can think of now is to use differential input and sacrifice the sampling range to bias the input signal to 100-200mv, reducing interference; The second issue is that the output signal range is -5V to 5V, and the input of the ADC pin at the back can only be -2.5V-2.5V, or 0-3.3V. Using a 3.3V voltage regulator will cause the signal output to attenuate significantly from 2.7V onwards. Is there any other compatible method ?

Is there a better design concept that can provide ？

  • 0
    TI__Guru* 77521 points

    Hi Anson, 

    anson zhou said:
    Is there a better design concept that can provide ？

    I can help you to solve the issue, but OPA4991 is not the best op amp for constructing a discrete instrumentation amplifier due to its poor slew rate in small signal. 

    anson zhou said:
    The differential input itself introduces a signal of about 6mv, which has a significant impact on our measurement signal (ranging from 0-100mv). What I can think of now is to use differential input and sacrifice the sampling range to bias the input signal to 100-200mv, reducing interference;

    First, I need to understand the measurement application and issues. What are your cost consideration? You may use our instrumentation amplifier (IA) or build your own discrete instrumentation amplifier. I would highly recommend to use integrated IA for the application. 

    https://www.ti.com/amplifier-circuit/instrumentation/products.html

    I am going to describe what I read and please see if my understanding is correct. 

    The measured input has 100-200mV common mode voltage (Vcm) signal, and you want to sense up to 6mV differential signal on top of Vcm DC bias. 

    anson zhou said:
    The second issue is that the output signal range is -5V to 5V, and the input of the ADC pin at the back can only be -2.5V-2.5V, or 0-3.3V. Using a 3.3V voltage regulator will cause the signal output to attenuate significantly from 2.7V onwards. Is there any other compatible method ?

    That is not problem. You can reduce the gain of an IA and output the swing signal accordingly. You do not have to output +/-5Vdc in amplitude. You may use our INA823, INA826, INA333 or construct a discrete IA for the application. 

    BTW, we do not support our competitors' product. If you want to use our competitors' product, I am unable to assist you. Please let me know and I can find a solution to meet your cost requirements. 

    Best,

    Raymond