When a precision rectifier -- isn't

Thread Starter


Joined May 4, 2013
... or Why doesn't my precision rectifier work?

It could be those hidden diodes between the inputs of the op-amp. Many op-amps have clamp diodes across the inputs for over-voltage protection. These diodes can sometimes cause problems.

This can happen when the op-amp is operating in a non-linear fashion. One example is when the op-amp is used as a differential voltage comparator with large differential input voltages. Another, more subtle, situation is when there is a diode in the feedback loop such as in a precision detector or rectifier circuit.

It is sometimes hard, if not impossible, to tell if an op-amp has protection diodes when looking at a data sheet. The best way to tell is to look at the equivalent circuit if the data sheet has one.

Here is an LTspice simulation showing the problem. I chose an op-amp that did not have the protection diodes so I could simulate the circuit with and without diodes using the same op-amp.
I had to look at 4 different op-amp data sheets before I found one that did _not_ have the protection diodes.


(This post was prompted by a reply on another thread and I thought that it deserved its own thread since the problem is not always easy to figure out.)