Power Line Noise Cancellation

Another approach (middle) is to convert the noise voltage to a common voltage by applying an instrumentational amplifier (two measuring electrodes on patient). With a 

Now the point is that the noise current does not go to M, so it is not measured, it is canceled A condition is that A is able to give the necessary current and the necessary output voltage to overcome the voltage drop in the skin impedance under the CC electrode. 

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Figure 8.18 shows a commonly used circuit for power line noise cancellation. Figure 8.19 shows three situations of power line capacitively coupled noise voltage. Suppose first (left)... 

In practice, this interference cancellation process works fairly well however, the assumption that the power-line-induced signal is common mode (uniform) over the body does not hold perfectly in all situations. Slight differences in its phase or amplitude over the subject s body when in close proximity to some electric field source or unbalanced electrode impedances can cause this cancellation process to be less than perfect. Some line noise may still pass through into the recording. Usually, improvements in skin electrode preparation can remedy this problem. 

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