Is there any simple way of limiting the op amp swing to a little under the rails? Since most op amps have some issues with rail to rail swing I would like to artificially clip the swing before the output gets to the rails. This is more for precaution for using various op amps without having to chose specific op amps that work well with rail to rail output.
I don't mind wasting a little headroom if it means that the op amp will behave better. I'm sure I've seen some type of circuit to do something like this.. I think in AOE...
Point-to-point series Zeners in the feedback work. If you want the clipping a little "softer" a resistor in series with the Zeners works, too. The gain when clipped is set the normal way (Rf/Ri).
1 Use a greater supply and an op amp that can that handle the extra voltage. iows increase the dynamic range.
2 Use small signals
3 Run your signal through a diode clamp circuit.
Complicated Ways
1 Use a op amp with pin for gain control
2 Feedback back more at clip level
3 3 channel analog multiplexer + threshold detectors
4 clip > Is there any simple way of limiting the op amp swing to a little under
On CMOS rail-to-rail OpAmps it's not the output that's the problem, it's the input stage... control the common-mode range and it'll recover nicely. ...Jim Thompson
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There are opamps which have extra inputs to limit a signal between certain levels. They are often intended to limit input signals going into ADCs.
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Really? It has always worked for me but I use schottky diodes which have a lower forward and on top of that, if you check the data sheets on most CMOS the latching does not take place until it gets a littler higher than your 0.6 level. Over all this is a better choice because if the rails sag for some reason at the chip rails, and you do connect it there, it'll follow along.
Using the diodes on the inputs like you say would not work very well if you are using the circuit for a voltage comparator, if I understand you correctly?
As for the back to back diode arrangement that is found many times, we do that in cases where the op-amp being used as a gain circuit with (-) feed back, we put 2 back to back diodes across the inputs, since these inputs should never see anything near .6 volts DT to start with.
Every one has their own ideas, I stick with mine until it fails and then we investigate :)
Oh, btw, a few years ago I made a stupid circuit that took advantage of this dilemma as a safety shut down trigger. We actually used this functionality as a over voltage pulse detector in a CMOS buffer circuit. By attaching the Vdd to the rail via a R as a current sink with a by pass on the feed side near by, this Vdd was a reference to another circuit that initiated other operations that needed to be performed when the input over swung and forced a latch operation.
With the properly current sink R to the Vdd, no damage would exist after a reset to this area of the circuit. As far as know, that circuit is still operating today taking advantage of that.
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