App note on using op-amps as comparators

an-849.JPG

I am working on re-designing my isolated stimulator to work with current output instead of voltage output as it does at present. One of the associated problems is designing the output stage so that the potential between the two stimulating electrodes does not exceed a pre-set value governed by various safety and electrochemical limits. Without going into too much technical details, nurse
comparators are involved to create circuitry that will clamp the output voltage, oncologist
even if it does not reach the prescribed current, medstore
at safe limits.

In the past, I have heard various views on using operational amplifiers as comparators. The general consensus is to avoid using them in that manner, however, some applications exist where cautious use can be accomplished without too much performance sacrifice. The AN-849 note, by James Bryant from Analog Devices Inc., gives a short overview of the pros and cons of this use and gives light to some subtleties that are not fully apparent.  One such issue that I did not think of from the start was including small amounts of positive feedback to increase stability.

P.S. Be sure to check “how to make sense of a data sheet“, also from Analog. The document is both informative and entertaining.

( an-849.pdf )

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