OP-Amps and A to D

Take it to sci.electronics.basics ;-)

...Jim Thompson

What, you don't want to tell him where the gain set pin is on an op-amp?

My education is over 25 years old now but IIRC on a 741, pin 2 was inverting, 3 non-inverting, 4 -V, 1&5 were offset, 6 output, 7 V+ and 8 n/c. Gain was set with rf/ri for inverting, rf/ri + 1 for non-inverting. I would guess the output to be 2.7V but perhaps some of the newer chips could cancel out common mode voltages. My real goal is to have a gain high enough to get a 0-5V signal with a 0 to 30mV signal from a bridge circuit in a load cell.

RogerN

H "I have an "if I have 2.5V at the - input and 2.7V at the + input, would the output be 0.2V or 2.7V?"

Think OpAmp fundamentals, then return for further tutoring ;-) ...Jim Thompson

I'm sure I could easily get what I want in the lab with a + & - supply and a separate supply for load cell excitation and a bias pot on the NI input. I was hoping to come up with something a little more practical though, like a single supply rail to rail op-amp that could offset to output the differential voltage X gain.

But to answer my own question, I'm pretty sure with a 741 op amp I'd get

RogerN

Not relevant, really. Look again at the problem statement: the phrase "set up with a gain of 1" can mean inverting gain of 1 (which means (+) input is at GND), or noninverting gain of 1 (which means (-) input is resistor-connected to GND and the output), OR could be referring to a fully-differential instrumentation amplifier (not an op amp).

Instrument amplifier requires two or three op amps, let's rule out that possibility (since it could be a uA741 according to the later comment). Unbuffered fully-differential connection uses up both op amp terminals, only resistors are connected to the inputs, so that's out, too.

In neither remaining case can one arbitrarily connect signals to BOTH (+) and (-) op amp terminals. You used up one of those connections with the gain setting components.

Roger,

I guess it would be meaningful for us to know exactly what sort of circuit configuraiton you were referring when you said "I have an Op-amp set up with a gain of 1".

Is it a voltage follower circuit with the 741-type general op-amp, an amplifer with an input signal divider network that makes over-all gain to be unity, or one with output signal divider? Or is it some kind of instrumentation op-amp?

Atsunori

If you have 2.5V _at the inverting input pin of the opamp_ and 2.7V _at the non-inverting input pin of the opamp_, then you have a broken opamp.

The rest is simply done with input and feedback resistors.

Hope This Helps! Rich

On Wed, 09 Sep 2009 22:36:18 -0500, RogerN wrote: ...

That's easy. I've used an LM324 to do exactly this.[1] Just set your input/ feedback resistors to give a gain of 5/0.03 . Admittedly, that's a lot of gain, but what's the highest freq. from your load cell? (i.e., do you have gain-bandwidth issues?)

Good Luck! Rich [1] Well, my input wasn't a load cell, it was a current shunt, but the principle is the same.