Rail to rail op amps

[cloudberry]

Hi folks,


Is using a rail to rail op amp to limit the output voltage of a voltage adder A Bad Idea?
If so, why?
I'd like to ensure CV outputs are, say, 0-5V, regardless of inputs outside that range, to a DC mixer.
Allowing for falling short by a few millivolts, and nonlinearities close to the rails, feeding steady high impedance inputs....
I'm looking at LT1490a.

[cloudberry]

Ah no. Some zeners in the feedback loop ought to do it.

[Grumble]

Ah yes, that is one possibilities to limit the output voltage of a voltage adder, but make sure the inputs are also rail 2 rail and still be sure the rail doesn't exceed the VCC or the GND levels

[cloudberry]

Thanks for the reply!

Yes, hopefully got those covered, and the lt1490a seems to be able to go well beyond the inputs' rails. 'Over the top' they call it. We'll see.
My problem seems to be trying to do level shifting, scaling, and limiting all with one op amp. One must be optimistic. But....

[diablojoy]

look at the mutable instruments site and some of schematics for idea's
mostly they are using MCP6002/4's
but they are doing essentially what you want

[cloudberry]

Thanks, I'll take a look.

[fonik]

[cloudberry]

[fonik]

[Grumble]

[fonik]

while we are on it, what could be an advantage of using an opamp ISO diodes?

[fonik]

[cloudberry]

When I first posted I'd forgotten my rules of inverting op amp adders; thus, at least in theory, the voltage differential at the inputs will be tiny. However, the question of limiting still arises if one wants to ensure an upper output limit of, say, 5V, or scaling and limiting to say 2.5 to 10V.


That MT-036 document is very useful, thanks!

[Grumble]