Variance in ECG lead resistance ; its effect ; CMRR

Question

We are designing custom ECG electrode with flexible printed wires as to reduce wires clutter from different electrode. The manufacturer has made the patch but the resistance variation between all the three electrode varies, which is 20 ohm, 5 ohm and 30 ohm respectively.

Is this variation acceptable or resistance of the electrode and the the lead as seen by the amplifier should be same. Considering the electrode-skin interface in which variation is in the order of 1000 of ohm, is variation of lead resistance by 15 to 50 ohm acceptable. If we consider common mode noise variation across leads with different resistance, will amplifier amplify common mode noise, thus amplifying noise.

Will using the right leg drive can help to reduce the noise.

Answer 1

Considering the load is > 1M and source is 1k min, adding a tolerance of 50 Ohms has null effect considering 5% of source is good and 50 parts per million compared to load is effectively null. I might consider < 100 ohms OK.

The bigger error source is contact pressure or motion producing galvanic skin voltage from the change in pressure and change in capacitance between electrode and skin. Thus you want pads that maintain relatively constant adhesion pressure.

Answer 2

The difference you are seeing is far less than the differences in the skin-electrode interface at the different sites, and should not be your biggest source of noise by far.

Any common-mode influences will be negligible in the pass band, but there may be subtle differences in cutoff frequency of an RF attenuation. You shouldn't care about this, as there is tremendous space between your target signal and RF. You can, and should, deal with any noise remaining after the conversion to a single-ended signal by filtering.

Answer 3

You need very high input resistance, then it all will not matter. Actually, anyway skin variance is few orders of magnitude highet, so anyhow just forget it. May your CMRR be 120dB!