ECG amplifier stage running rail voltage

[razonickj]

We are trying to build an ECG amplifier stage using the INA118P. We have the positive power supply hooked up to 6V and the negative supply hooked up to -6V (battery powered). The reference pin is hooked up to ground. The input pins are attached to insulated wires running to pennies that are used as electrodes. The right leg is attached to ground through a 10k resistor. The output of the amplifier is fed into a NI USB-6009. When we collect data and display it, sometimes we can see heart beats and other times we run rail voltage. When the electrodes are left floating the output runs either +-1V. The gain on the amplifier is set at around 500. We technically have two identical amplifiers to measure the ECG reading at two different points on the left arm. Sometimes we can get one to work and then the other but we haven't been able to get them both to work at the same time. When they are hooked up and not working, one runs rail voltage and the other output runs 0V. We have these components soldered onto a circuit board. If need anymore information please let us know! Any suggestions are appreciated.

 

[bertus]

Hello,

Can you post a schematic of the current situation?

Bertus

 

[Harald Kapp]

You should not operate instrumentation amplifiers (or any op amp) with open inputs or inputs connected to a sensor without ground path.
Yu need a way to direct input bias currents to ground, otherwise the inputs will saturate due to charge accumulation from the bias currents and the op amp will go into saturation.

See e.g. "Low Input Bias and Offset Current Errors" on page 1-8 in this document.

 

[TCSC47]

Oh dear! I'm afraid you are attempting to do something that you should not. It is clear that you do not understand all the nuances required of your ECG application because you have come here to ask your question. I'm afraid it is a case of a little bit of knowledge is a dangerous thing.

The electrical resistance of our skin is our greatest defence against getting a fatal electric shock and recording ECG requires eliminating this. Hence the large pads and conductive jelly that would be used. I assume you are using medical conductive pads. These and the associated conductive jelly are readably available on line, but the thing is, they are meant for use with professionally made and proved equipment, by people who are trained and know what they are doing.

I presume you are going to connect your amplifier to an oscilloscope to display the ECG and therein lies the biggest problem. If you paper research the precautions used in Hospitals for their ECG and other monitoring equipment you will see the extraordinary lengths they go to, to make sure the mains powered equipment and indeed even battery driven equipment, are safely isolated from the subject or patient. You will not have the facilities or budget to ensure this safe operation and as I have already said, the knowledge.

The spec sheet you reference for the INA118P shows a rudimentary circuit for measuring ECG, but this is really for illustrative purposes and meant for the professional engineer. Whilst it would work, it just would not be safe to connect somebody up to it, as it stands. Indeed I think you would even come up against legislation for dangerous engineering if you were doing it professionally.

I know that many amateurs have dabbled with this sort of thing and that it is very exciting, but they are wrong to do so, and whilst probably most have got away with it, they have been lucky. At the start of what may be your professional career, adopt a professional approach.

Incidentally there are other bodily activities you can safely measure such as blood flow or blood oxygen level through infra red transparency / absorption means. Look it up.