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Topic: *Instrumentational Amplifier Circuit: Persistent High Voltage Output Problem (Read 995 times) previous topic - next topic

601618

I am attempting to build an instrumentation amplifier circuit as part of a "home made EKG" project, but I am having a strange problem with the output of the circuit.
For the second time now, I have attempted to build a circuit and have tested it using a function generator at school. Using an oscilloscope, I find that a sine wave from the generator is in fact amplified according to the correct gain equations, thus suggesting the circuit is working correctly.

However, once I bring the circuit home to test the EKG portion, somehow the circuit no longer functions. Instead, I have connected it to a function generator and only receive a constant, unvarying high voltage output line on an oscilloscope. I have tried pressing down the IC op-amp chip I am using, an LM324, to no avail; I have even varied the voltage supply, although I am using the exact same supply (9v to 15 v battery eliminator) used at school when the circuit functions. I would appreciate some help, as I am getting somewhat frustrated - I have built several such circuits, even swapping out different ICs, and tested them at school only to find that the circuit no longe works at home.

Some ideas: as I am using a different function generator at home, could this one somehow be shorting/breaking the IC chip?
Could surrounding computers or other electronic devices be creating noise? I have tried turning off power supply strips to the devices to no avail.

RuggedCircuits

Could be the grounding on your function generator. Are you connecting TWO outputs from the function generator to your circuit? One is the signal output, the other should be ground.

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601618

Hm, I am in fact connecting both outputs from the function generator to the circuit, although I will try connecting one to ground; I thought I had tried this as well with no success. However, as the amplifier circuit measures the difference between the two voltage inputs, would I then see any signal at all?

Strangely enough, I had in fact previously worked with both function generator outputs each connected to a different input on the circuit and received a signal in the school setting.

However, I was using a different f.g. than the one I am using at home.

Grumpy_Mike

Quote
I have built several such circuits, even swapping out different ICs, and tested them at school only to find that the circuit no longe works at home.

No you haven't.
Your test does not reflect what you are actually doing with the circuit.
There is a world of difference between a signal generator and putting electrodes on your body.
You need three electrodes, one for a common ground, the other two for the signal.

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