Input protection in an electrocardiogram

When you’re building an electrocardiogram, one of the many layers of protection is making sure that there is a “dumb” resistor between the body of the patient and the inputs to the system. The reasoning for this is that there is not much that can go wrong with a resistor, and when it does, they generally fail open. This is a common sens protection similar to the requirement that a fuse be the first thing in line in any system past the power input, even before a power switch.

Since it is quite possible that your circuit will be connected to a human through a low impedance path (e.g. a wet electrode), these resistors protect the user against other failure modes. One might imagine, for example, an in circuit op-amp failing in such a manner that it simply directly connects the power rail of your system to the patient. Even in a low voltage 5V system, the low impedance and placement of wet electrodes means that you could be dumping tens of milliamps across the chest of a patient, potentially enough to stop a heart!

To protect against this, there should be high impedance in between the patient and circuit. The AAMI (Association for the Advancement of Medical Instrumentation) specifies 2.5MΩ input impedance. This is fairly conservative however, and higher impedance means more thermal voltage noise and more ambient noise pickup. A system with 1MΩ input resistors right at the input with ±9V supplies is only putting the patient at an absolute maximum 18µA of risk.

Okay, so easy: Throw some 1-2MΩ resistors at the input and we’re done, right? Nope!

Common Mode Rejection Ratio (CMRR)

Off the cuff, this might seem like no big deal. After all, your instrumentation amplifier has high input impedance, right? It’s supposed to be way higher than 1MΩ, so adding 1MΩ resistors shouldn’t make much difference.

In an ideal world, this is true. But it turns out that there’s this filthy little thing called manufacturing tolerances. One of the many magical things about integrated circuits is that, since all components are manufactured in close proximity in time and space, they tend to track together. This means that it is easy to get closely matched input impedance. This is critical because input resistance mismatch means that common mode signals — like the ubiquitous and dominant 50/60Hz line nose — get turned into differential mode signals, and amplified by your instrumentation amp, swamping out the signal you want to measure.

And the thing is, the whole point of using an instrumentation amplifier in the first place was to reject common mode signals!

Cheap resistors however,  are ±5% — actually 5% in each direction. Meaning 1MΩ is actually 950kΩ and 1.05MΩ.

Obviously, you can just pay money for closer matched resistors. Say 0.1% or even 0.001%. But that ends up being the price difference between 10¢ and $180! For a more modest 0.05%, you’re still looking at 75¢, which is reasonable for production, but sometimes you’re in a pinch and only need to build a prototype anyway.

And now we’re back to where I entered the picture. I was building an ECG system, and I needed a batch of input protection resistors. Eight of them, to be specific. The lab happened to have a bin of 970kΩ 1% resistors, and I am impatient.

My solution? Sort though the bin of 1% resistors and pick the closest matched set for my input leads, to minimize the impact on my system’s CMRR.

…to be continued.