EKG's 40Hz oscillation investigation

This morning I carried on comparing the ekg noise with 3 different PSUs -

(1) Switching PSU 5V 10A regulated to 3V3,

(2) Switching PSU 12V 2A regulated to 9V, 5V, and 3V3.

(3) Lead acid recharageable batteries 6V * 2 = 12V, regulated to 9/5/3.3, by the same regulators.

I also compared the noise level with and without switching on the little digital volt meters.

To my surprise, I found the 200VAC to 12V/5V switching PSUs and the battery powered PSU have the same noise problem.

Also the little digital volt meters do not have any effect to the noise level either.

The noise period is roughly 25mS, or 1/0.025 = 1000/25 = 40 Hz.

I remember the olimex ekg module's last filter/amplifier 3rd order Besselworth low pass filter conrner frequency is 40Hz (and gain 3.56).

Now I guess that the 40 Hz low pass filter is perhaps not stable and oscillating instead of working properly.

.END