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Galvanic Isolation for an EMG circuit (ISO 124)

Ahmad Jazlan
Prodigy 80 points
Other Parts Discussed in Thread: ISO124, INA128, OPA2604, UAF42

Hello to all.

For a university course project I have constructed an EMG circuit which consists of an INA128 instrumentation amplifier (U1), a high pass filter with 50Hz cut off frequency (U2) and an anti aliasing low pass filter with 500Hz cut off frequency. The filters were constructed using OPA2604. I am planning to add an ISO124 isolation amplifier to the output of the low pass filter to galvanically isolate between the EMG circuit (connected to a subject) and a battery powered laptop used to display the EMG signals. I am aware that the ISO124 is not meant to be used to meet medical standards, however this is a short term class project only and not for commercialization.

I want to digitize the EMG signal and send serial values of the digitized EMG signal to a laptop COM port. I will be sending the output of this EMG circuit to an analog input pin of an arduino microcontroller (with inbuilt ADC) which digitizes the EMG signal and sends the serial values to the COM port through an RS232 to USB converter cable. I have a separate program in Labview which can read and graph serial values from the COM port.

Before I can directly connect the EMG circuit to a battery powered laptop, i want to provide galvanic isolation between the EMG circuit (connected to the subject) and the microcontroller which is connected directly to the battery powered laptop using an RS232 to USB converter cable.

I would like to ask:

a) The instrumentation amplifier and op amps in this circuit are powered by two 9V batteries providing +9V and -9V. Can I use this same power supply for +Vs1,-Vs1 and +Vs2,-Vs2 on the isolation amplifier, or do +Vs2,-Vs2 need to have a separate power supply?

b) Suppose in this circuit I want to add a 50Hz notch filter (using UAF42) and two other op amps - one for gain adjustment and the other for offset adjustment, should I place the isolation amplifier before or after the notch filter and op amp adjusters?

c) Is there any suitable dual in line package ADC chip that can directly convert EMG voltage values to serial values? I want to send these serial values to a COM port and graph the EMG using a Labview program. I want to substitute the arduino board im currently using which digitizes and reads EMG values into an array and transmits the EMG serial values to the COM port.

I  appreciate any feedback & suggestions from all of you.

Thanks!

  • 0
    Guru 21975 points

    Ahmad,

    The two 9V batteries should should not be used to power the "s2" side of the isolation amplifier. Doing this would connect the input side ground to the output side ground which would totally defeat the purpose of the isolation amp.

    I would suggest putting any additional signal processing on the output side of the isolation amplifier.

    We have many older ADC chips in DIP packages. I suggest that you write a question specific to this issue and post it on our data converter forum where it will be seen by our best experts in this area. You may want to consider using a more modern ADC and purchase an "EVM" evaluation board that would directly interface to a USB port. Adapting a serial output DAC to RS-232 may turn out to be cumbersome.

    Regards, Bruce.

  • 0
    Prodigy 130 points

    HI, I NEED A HELP ABOUT ISO 124. PLEASE. Here says: http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/156139.aspx#566041 

    I don't speak 100% english. I apologize if you find mistake in this. I bought iso 124 cause i need isolate 2 stage, and i am confused because i wanna use only one battery (9V=VCC) for my circuit, iso 124 requires 2, but i have questions, can i use voltage regulators for the other VCC and -VCC? or what supposed to going to do? and the battery has one ground, and i dont know if can i use that ground for stage 1 and stage 2? (Figure 1) My teacher told me with DC/DC Converser, i found MC34063A but this does not have its own ground, somebody can help me how can i use iso 124 with only one battery?

    Thank you, i hope your answer please! I need it soon.