Electronic – Shared bias voltage for quad op-amp

There are a few problems with your current circuit.

1. The heartbeat signal is lower than the frequency response of your microphone.

2. As everyone else has said, you need to lower the critical frequency of the high pass filter on the input. If it was me, I would replace it all together with an equal component Sallen-Key High Pass filter simply because the input signal is going to be small. The extra op amp is already in the package.

3. While maybe a non-issue, +/- 6V rails is pushing the head requirements of the 324.

4. There is no way you're going to drive an 8 ohm speaker with a general purpose op amp. It just can't source the current. With the 50 ohm resistor in series, you're dropping most of the voltage across the resistor. You'll need something like an LM386 to drive the speaker. I've only used this particular part with a 12V supply, and don't guarantee it will work with split 6V supplies. Notice that the gain is adjustable.

Looks like mains hum. (I'm assuming the annotations on the graph show you are changing the input gain during acquisition, so that the sinewave is not continuous).

Now we don't know:

• the period of the observed "sine" wave
• the ADC sampling rate
• your mains frequency but you do, so you can work out if that's a possible hypothesis.

For example if you are sampling at 50Hz in a 50Hz mains country, and seeing a frequency less than 0.5Hz, chances are that you are aliasing mains hum down to the observed frequency.

And given the high input impedance opamp inputs, they look like a target for electrostatic coupling, rather than magnetic.

Now, on which input?

If you remove the sensor and short the input to ground, does the noise disappear? Then the sensor connection must be more carefully screened. Or you need a buffer amp at the sensor to reduce the sensor's output impedance.

If the noise remains: reduce R21 to 100k and R66 to 100 ohms. Did that attenuate the noise tenfold? If so, it is the negative input that's picking up. More likely both inputs are picking up noise, since they are both fairly high impedance points.

You can reduce electrostatic pickup by screening : experimentally, surround the amp with tinfoil (and ground the tinfoil to the amp's ground)