Basic op amp instrumentation

[Guest]

I posted this to alt.engineering without much success. Perhaps someone here
can help.

I am looking for an article or short description of a general purpose
instrument conditioning circuit.

When I was taking my electronics classes (early 1980's) I recall a diagram
that
showed an opp amp that had +- offset and variable amplification. I have
looked in my library and all I can find is specific applications. The
references I have are all too complicated... i.e. temperature compensation
etc.

I am hoping to have a fairly crude circuit that will use 741's or something
with perhaps just two or three other components to buffer many sensor lights
with varing voltage trip points (and scales). I would like to use one
circuit many times over with predictable component values.

I just moved from Alaska to Seattle and I looked through all my references
as I packed but to no avail. I was hoping that someone on this list was
involved in instrumentation, hopefully it should be a common task

---

For example, a thermocouple may only differ by a few millivolts from "cold"
to trip point. Therefore it is necessary to expand the sensitivity "window"
to a couple volts.

Suppose the full response of a sensor is 15mv (i.e. 75mv-90mv), if we
amplify it 200x then we have a 3v difference. Typically we would like the
trip point at some standard value such 6V so we offset the 3v window as
needed.

I believe I once had a circuit to do this that had 3 or 4 resisters and a
2n2222. I think the circuit had the bonus of single rail power too; perhaps
12-15V.


Thanks for the help!

JZ

 

[JeffM]

...to buffer many sensor lights with varing voltage trip points (and
scales).

For example, a thermocouple may only differ by a few millivolts from "cold"
to trip point. Therefore it is necessary to expand the sensitivity "window"
to a couple volts.
JZ

If you want better help, stop asking how you can do something
when you've *already figured it all out except the details*.
1) Give a better idea of *exactly* what you're trying to do.
2) Give some specifications--temperature range as a minumum.

Having said that, thermocouples are probably a terrible way to do
this.
(You're already aware of one big problem.)
Here's another:
http://www.google.com/search?&q=thermocouple+cold-junction+in-the-instrument+reference

Look into thermisters.