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Original question:

Microvolt Signal Amplification

MicroVolt signal Amplification

Shashank Kumat
Prodigy 30 points
Other Parts Discussed in Thread: INA333, OPA333, TINA-TI

Hello,

My question is in regards to the following post https://e2e.ti.com/support/amplifiers/f/14/t/293906

1. In here Mr. Mock suggested to filter out the signal before passing it to the Instrumentation Amplifier. The cutoff frequency mentioned is 101Hz however with the R value of 7.5KOhm and C value of 10nF, I calculated the cutoff frequency to be 2122.06Hz. Am I doing something wrong?

2. What is the use of R1 and R2 in your circuit diagram, are those the bridge resistor, did you use a quarter bridge configuration or simply voltage divider circuit?

3. What the use of OPA333 at pin5 for INA333, can I just connect pin5 of INA333 to ground directly?

Looking forward towards your reply, I am a novice as far as electronics is concerned. I am trying to implement a quarter bridge configuration using a 5KOhm strain gage and the peak to peak output voltage from the bridge is 100microvolts.

  • 0
    TI__Genius 10850 points

    Hi Shashank,

    Thank you for your post. I can help you.

    1. Looking into the input of the amp, the 10nF and 100nF capacitors are in parallel: 110nF. Frequency = 1/(2*pi*R*C) = 1/(2*pi*7.5k*110n) = 193Hz

    2. R1 & R2 is your half-bridge. What kind of bridge sensor do you have? Is it a classic 4-resistor bridge? Half-bridge? What is the resistance of each leg? What is the sensitivity?

    3. Since you are using single supply on INA (0 to 5V), the common practice is to make the reference pin (pin 5) mid-supply. The OPA333 acts as a reference buffer. The buffer is required because you cannot connect a high-impedance source to this pin. 

    To learn more about these pins and the function of the IA, take a look at our training videos here: https://training.ti.com/ti-precision-labs-instrumentation-amplifiers?context=1139747-1139745-1140241

    We will be releasing more content soon.

  • 0 in reply to Tamara M Alani
    Prodigy 30 points

    1. As far as the cutoff frequency is concerned that is (193Hz) what I got too instead of 110Hz.

    2. I am using a classic 4-resistor bridge, with the quarter bridge configuration. The resistance of each leg is 5.1KOhm.

    3. Since I will be expecting change in voltage just in one direction, I might not require to shift the reference to mid supply. 

    The video link was quiet helpful. Looking forward towards more content. 

    Just on another note, I implemented the cutoff frequency method to my input signal, and it helped me tremendously to improve the noise due to EMI.

  • 0 in reply to Shashank Kumat
    Guru 140200 points

    Hi Shashank,

    why not running a TINA-TI simulation? TINA-TI is free and can be downloaded here:

    It plays a role where exactly you measure the frequency response:

    shashank_mock_ina333.TSC

    Kai

  • 0 in reply to kai klaas69
    Prodigy 30 points

    Hello Kai,

    Thank you for the advise,

    I have a question though, how could one identify a cutoff frequency if the input signal for my circuit is DC. For instance, why did we select the cut off frequency as 100 Hz, why not 10 or 20 or ...

    Is there any method to select the cut off frequency?

  • 0 in reply to Shashank Kumat
    Guru 140200 points

    Hi Shashank,

    there's no special need for a corner frequency of 100Hz. Michael Mock could have chosen any other corner frequency.

    The prime task of input low pass filtering is to suppress common mode EMI and differential mode EMI. If this EMI exceeds the bandwidth of OPAmp, unwanted demodulation of EMI can occur at the unlinearities of input stage of OPAmp. This can result in eratic offset voltages, noise and intermodulation products appearing at the output of OPAmp which cannot be filtered out anymore when falling into the signal frequency range.

    But there's still another issue: The filtering components must exactly match, otherwise common mode noise can be converted into a differential signal which can unwantedly fall into the signal frequency range. This the more, the closer the corner frequency comes to the upper limit of signal frequency range.

    Because of this, "less is more" when it comes to common mode filtering :-) In any case the components providing the low pass filtering should be matched very carefully. Take low drift resistors and caps (0.1...1% metal film resistors and 1...5% NP0 caps).

    Kai