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Want to make an Electromyography amplifier circuit.

Discussion in 'General Electronics Discussion' started by Dhiman, Oct 21, 2017.

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  1. Dhiman

    Dhiman

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    Oct 21, 2017
    Hi

    I am trying to make an Electromyography amplifier circuit using the differential amplifier ic AD620 and the operational amplifiers TL072. I want to take my biceps voltage for this project. I am using a 5V DC supply from Arduino to power my amplifiers.

    I have uploaded the circuit diagram for my project, please check it and suggest me any changes that I need to consider. Please suggest me some way to test it. I am trying to measure my biceps voltage but there is no response in the multimeter.

    I have sent the amplified output to the A0 Pin of Arduino IMG_0810.JPG .
     
  2. Audioguru

    Audioguru

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    Sep 24, 2016
    Your schematic has a few serious problems. The 5V supply is too low for a TL072 and it needs an additional negative supply or it should be biased at half a higher supply voltage. Its datasheet shows that its inputs do not work if their voltage is within 4V from a supply voltage. Your inputs are at the negative supply voltage of 0V.

    The AD620 also needs an additional negative supply or be biased correctly at half its supply voltage.
     

    Attached Files:

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  3. Dhiman

    Dhiman

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    Oct 21, 2017
    Thank you for your reply. I have changed the bias voltage to +9V and -9V for TL072. Also in place of AD620, i am using TI INA106. Will this be a better solution to the problems?[/QUOTE]
     
  4. Dhiman

    Dhiman

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    Oct 21, 2017
    Hello, I have redesigned my EMG circuit using instrumentation amplifier, a differential amplifier, and filters.
    Components used:

    INA106 Differential amplifier
    TL072 differential amplifier
    First order high and low pass filter.
    low frequency cut off 16Hz and high-frequency cutoff 365Hz for EMG signal.

    Please check it and suggest any changes that I main circuit diagram.png need to consider.
     
  5. Alec_t

    Alec_t

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    Jul 7, 2015
    I would consider raising all the resistor values around the TL072 opamps by at least an order of magnitude, to reduce power consumption.
     
  6. Dhiman

    Dhiman

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    Oct 21, 2017
    I would try to increase the resistors values. Beside that is my circuit correct I am getting 4.4V output for 2mV input at within these frequency ranges.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    U3 and U4 combine for a DC gain of 55. Any DC offset error out of U2 will be amplified by this amount, which may be enough to cause a problem. Consider AC coupling between U3 and U4 to reduce this.

    You are using the input diff amp as a single-ended opamp. That's ok, I guess, but you are not getting any of the benefits of a true differential input stage. Search the web for EEG and EKG amplifier circuits to see how to make a better input stage.

    ak
     
    Last edited: Oct 22, 2017
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  8. Dhiman

    Dhiman

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    Oct 21, 2017
    I think I should place a 1uF capacitor (electrolytic) between U3 and U4. Will that work? Can you please suggest me better circuit for ina106 as a preamplifier.
     
  9. Dhiman

    Dhiman

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    Oct 21, 2017
    Thank you, sir,
    I have made some changes to my circuit.
    I have given two inputs one of 50uV and 60uV both of frequencies 100Hz (EMG signal from the muscles).
    At the output, I am getting 3.75V output. Which I could easily send to Arduino inputs for evaluation.
    I have used two 1st order filters.
    Now Please check it and suggest me necessary changes.
    PLease suggest me some input EMG values with frequency. So that I can test my circuit in the simulator. main circuit 2 supply 1.png
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    The datasheet for the TL072 and most other opamps shows that its maximum output current is 11mA. If you want the output to have a peak voltage of 5V then the current at high frequencies into the 5 ohms of R2 must be 5V/5 ohms= 1000mA which is 91 times higher that is possible. Why such a low resistor value and then the value of C2 must be very high??
    Use 4.7k ohms for R2 then use a non-polarized 5% film 0.1uF (100nF) capacitor for C2.

    The resistor values for R8, R4, R9, R10 and R3 are also much too low. You should read the datasheet of an opamp before randomly selecting resistor values.

    You increased the supply voltages so that the TL072 will work. But now if its output swing is large it will destroy the input of the Arduino. You must use a voltage divider to reduce the opamp output voltages.

    You changed the input amplifier from an AD620 that has a very high input impedance to a differential amplifier that has a very low input impedance that cannot be used as a medical device input amplifier, because you did not read its datasheet! You have its voltage gain set to 110,000 times so it will amplify its input offset voltage so much that its output will be saturated.

    I agree with the others that coupling capacitors should be used between amplifier stages so that the input offset voltage of 10mV max for U2 is not amplified. The DC voltage gains of your latest circuit are U2= 2, U3= 2, U4= 151 and U5= 6 so the output will try to be +36.2V or -36.2V which is very badly saturated.

    1st-order filters are so simple and have such a gradual slope that they are nearly useless.
     
  11. Dhiman

    Dhiman

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    Oct 21, 2017
    Which instrumentation amplifier should i use for EMG?
     
  12. AnalogKid

    AnalogKid

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    Jun 10, 2015
    The #9 schematic is worse than the #4 schematic. There now is a DC gain of 9,966. This means that a signal of only 1 mV at the output of U2 will saturate the output of U5. Also, as mentioned above, all of the gain setting resistor values should be increased by 10x. In this way the opamp output stages do not have to supply so much current, which makes for more linear operation.

    ak
     
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  13. Dhiman

    Dhiman

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    Oct 21, 2017
    I am new to these type of circuits. This is the first time i am working with op-amps and differential amplifiers. So, Please help me.

    In EMG circuit do i have to remove all the dc voltage by using a capacitor? If so than why?
     
  14. Alec_t

    Alec_t

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    Jul 7, 2015
    If DC is present it gets amplified along with the AC of interest. With any significant gain the DC will drive the output hard against either power rail. That prevents the AC giving any useful output.
     
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  15. Audioguru

    Audioguru

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    Sep 24, 2016
    Look at the datasheet of the AD620 instrumentation amplifier and all other instrumentation amplifier ICs. They usually show an ECG circuit that is almost the same as what you want. The INA106 differential amplifier datasheet shows two opamps added to it to make it an instrumentation amplifier, didn't you see its datasheet?
     
  16. Audioguru

    Audioguru

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    Sep 24, 2016
    Why didn't you learn about them in college or university?
     
  17. Dhiman

    Dhiman

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    Oct 21, 2017
    I have used two-second order filter one HIgh Pass and the other one is Low Pass. Please have a look at this circuit and suggest me some changes.
    I Will replace INA106 with AD620 keeping filters and the noninverting amplifier as it is.
    main circuit 2nd order.jpg
     
  18. Dhiman

    Dhiman

    20
    0
    Oct 21, 2017
    Hello Sir,
    I was first using the AD620 but I cam into the problem when i saw the ECG circuit in their datasheet. Between Pin 1 and 8 they have used a circuit that i am not able to understand for EMG circuits.
    If you can tell me what is it than i can use it. Or can i directly attach a resistor between pins 1 and 8 to get the required gain? ad620 ECG.png
     
  19. Audioguru

    Audioguru

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    Sep 24, 2016
    Your INA106 still has a gain of 110 times, is that what you want? Times the 148 times gain of the output opamp= 16,280 times full of noise and interference.

    The datasheet for the INA106 shows the two opamps added to its inputs to make it an instrumentation amplifier with a high input impedance and its gain set with the same resistor that is between pin 1 and pin 8 on an AD620 instrumentation amplifier.
    The ECG circuit also adds together the common-mode AC and DC interference picked up by both inputs with the two 24.9k resistors then inverts the signal in an opamp and feeds it to the leg of the patient (instead of a signal ground) so it cancels the interference. The interference is AC mains hum and DC muscle movements that you do not want signals from. The datasheets explain all this.

    If you do not want to cancel interference then simply use a single resistor between pin 1 and pin 8, then connect the patient to the shields of the cables.
     
  20. Dhiman

    Dhiman

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    Oct 21, 2017
    I will we be using AD620 with a single resistor attached to pin1 and pin8 in my final circuit. But for the sake of simulation, i am using INA106 to check my filter.
     
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