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I'm currently messing around building an emg amplifier, that is high a gain amplifier to monitor muscle voltage, in this case on the skin, which is proportinal to the force of the muscle. These voltages can be as low as in the order of a couple of microvolts, so high gain and high noise suppression is nessessary. The voltage is picked up by two elektrodes on skin covering the muscle to be determined, a third elektrode is connected to the circuit's ground an placed on a non-active area of the body, so the main's noise at 50 or 60 Hz can be surprising easily suppressed by using a differential opamp.
In my setup I'm using an instrumentation amplifier INA128 by Burr Brown which's gain is set by one external resistance. On strong muscles, covered by less skin, fat and flesh, such as the biceps, A gain of less than one hundred is sufficiant to monitor the signal via a pc soundcard.
But with the skin under the elektrode wettened for better conductivity, especially with higher gains, I experience the output going to zero as if it was directly grounded.
Does the opamp become instable? Is the resistance between the electrodes too low? Is it due to the rather low power supply of +3 and -3 volts?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If you are going into a PC soundcard, presumably the output is AC coupled. Is the inamp input DC coupled, and if so, does wetting create such a large unbalanced galvanic voltage at the electrodes that the high DC gain saturates the amp output to the rail? If so, AC coupling the inputs should fix.
why not using on a human? If I keep the device battery powered and well isolated from mains powered devices, there should be no hazard... I will try decoupling the output although dark memories from last night say I tried that before.
edit:
It seems to work now. My current setup consists of the ina128 followed by an opa177 for further amplification. I had to decouple the output of the second opamp as well as the output of the instrumantion amplifier. Now I'm getting levels of amplification that sensitive that I even can get the signal of my heart to screen just by placing the electrode on the skin where the heart approximitely lies under. Nice.
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emg amplifier
