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Proton Soup
Nov13-08, 12:00 PM
P: 1,070
Quote Quote by marcusl View Post
This description does not get to the purpose of the resistor network, and in any case the reality is more complicated. First, the input impedance of the instrumentation amps used in this application are incredibly high and bias currents are incredibly low--10 Gohm and 500 pA are typical even for cheap plastic-packaged IC's--so resistors in the leads are not there for balance. In fact the I, II and III leads are usually direct-coupled. (Of course there are three amplifiers, one for each lead pair).

Since patient safety is critical, the amps are commonly followed by optically-coupled isolation amps eliminating any possible connection to earth ground or power even if the ECG machine is AC powered.

Finally, the input amps are ground-referenced to yet another electrode which is standardized to be the right leg. It is common to actually drive the right leg to minimize common-mode potentials, since the patient is floating with respect to earth ground.

Which brings us back to the resistors; they form a signal-weighting network that transforms the RA-LA-LL "coordinate system" to another coordinate system with increased sensitivity to certain abnormalities and disease states, as I stated earlier.
if there's no current flow going on here, then how do you suppose the weighting of the resistors factors in? should be simple to show how changing R/2 to R will change it, no? i didn't see where your ppt presentation addresses this.

rather, i think it's about CMRR.