How to implement max/min functions on op-amp output without introducing error?

[uoficowboy]

Hi - for a data acquisition system I'm planning on having a number of signals low pass filtered via active filters using op-amps. These signals will then be fed into the inputs on an ADC. The op-amps will have the ability to drive their outputs above safe input levels for the ADC, so I'm trying to think of a way to protect the ADC.

I've seen people using diodes for this purpose, ie anode on ground, cathode on the output to keep it from going too negative, with a series current limiting resistor so that you don't blow out your OA. But I'm worried about the reverse leakage of the diodes causing small but measurable errors in my system (aiming for 16b accuracy).

Are there better ways of implementing a max/min function like this?

Thanks!!!

 

[godfreyl]

I think the trick here is to put the resistor/diode limiter inside the opamp's feedback loop, e.g. like this:

 

[uoficowboy]

I think the trick here is to put the resistor/diode limiter inside the opamp's feedback loop, e.g. like this:

Thanks! That makes perfect sense. That's going to be very useful for me.

Now to make it trickier! (since I forgot to mention this earlier) What if I have a signal coming in from the outside world to an OA based circuit? (LP filter or buffer). Same requirement. For example, the input to the circuit you have drawn above. How do I protect that? Again I want to minimize current through any protection devices as it could affect my measurement. Sorry I didn't mention this case earlier.

 

[godfreyl]

I think the ideal would be to put the protection diodes between the inverting and non-inverting input pins of the opamp. Then the voltage across them will be negligible except when the opamp is clipping.

This will work well for an inverting amplifier since one of the opamp's inputs is grounded, and the diodes will protect the other input.

For a unity gain buffer, it's a bit more tricky. Whether it works depends on whether the input voltage can safely exceed the power supply voltage, and whether the output voltage can be pulled beyond the supply voltages.