I have an electrocardiogram sensing circuit that uses an INA321 amplifier for common mode rejection on two measurement electrodes. The device is meant for hand-to-hand measurement of the heart rate and powered by a low voltage.
If supplied by batteries, the circuit works well. However, the device is now connected to a small computer and screen for demonstration purposes. The switiching power supplies of such devices tend to couple about one half of the mains voltage into the common ground of the devices. I guess they use two equally large capacitors between ground and the two mains supply lines for some unknown purpose. The INA321 can never reject 110V of course, in respect to the heart signal of about 1mV and having it powered with 3 to 5 volts.
Even if I connect the system ground to the mains protective earth, there is still a voltage up to 50V between mains protective earth and the body standing on the ground.
So is there any way to supply computer and screen without having them tied to some hefty earth capacitance?