Having Problems with OPAmp overamping... sort of.

[iso9001]

Using a Microchip 602ISN on a board I have. Pretty simple little unit, 5V supply, rail-to-rail, set up to bring in X volts and spit out X volts with more power behind it.

The + and - are tied togather with the output being read by my pic's adc. So for example 4.00V comes in from another systems volt-drop resistor switch tree (figure .05mA or so), goes into the amp, comes out and goes into the pic with a 1k resistor inbetween. So my pic's adc gets a higher strength sample.


When I tested in prototyping it worked fine. But other day I had a strange problem and come to find out that the opAmp is only working corrently for signals around 4V, nothing higher. For example:

.9V > .9V
3V > 3V
4V > 4V
4.5V > 5V
5V > 5V

I'm not sure what could have damaged the unit, or if I picked a finicky model, or whatever.

Its confusing me since it apprears to be pass 4.5V signals right to VSS.

Any ideas ?

 

[audioguru]

The + and - are tied together.

What? You tied the inputs of an opamp together?
If you want a voltage follower then why not use a voltage follower circuit?
The + opamp's input is the circuit's input and the - opamp input connects to its output. The output voltage will do whatever the input voltage does.
Isn't that what you want?

 

[iso9001]

Yea, i screwed up, its setup as a voltage follower.

Typo, lack of sleep sort of thing.

 

[Roff]

The input common mode range only extends to (VDD-1.2)V (see datasheet). The output range is rail-to-rail. When you exceed the input common mode range on an op amp, the output is generally not well-defined. It could very well slam to the positive rail (which it apparently does). You need to pick a rail-to-rail I/O op amp.

 

[audioguru]

Hi Ron,
You are very sharp! :lol:
I should clean my new glasses.
I looked at the datasheet for the MCP6021 R-to-R inputs and output opamp. :cry:

 

[iso9001]

Super fantastic. Thank You Both.



But....... For a voltage follower, Would I ever need / Could use rail-to-rail output if I can never rail-to-rail input ? :? Hmmm.... Now I just need to finad an input rail to rail in the same pinout,

 

[Roff]

Super fantastic. Thank You Both.



But....... For a voltage follower, Would I ever need / Could use rail-to-rail output if I can never rail-to-rail input ? :? Hmmm.... Now I just need to finad an input rail to rail in the same pinout,

They are available, and (almost?) all dual op amps have the same pinout. The 602 is no exception. You can get rail-to-rail I/O from companies like Maxim, Analog Devices, TI, Linear Technology, Fairchild, etc.

And yes, rail-to-rail output but not input is very useful. The unity-gain follower is basically the only case (with feedback) where input and output swings are the same. Non-inverting gain-of-X has X times less input swing than output swing. In the case of an inverting amplifier, the input common-mode swing is zero!

 

[Nigel Goodwin]

You might try consulting my analogue PIC tutorial?, where I overcame the limitations of the opamp by generating a -ve supply rail, and using a 2.5V precision voltage reference.

I consider this far better then trying to find an opamp that works perfectly from rail to rail - and it gives you a far better voltage reference as well.