Let's say, for example, I have an Op-amp set up with a gain of 1, if I have
2.5V at the - input and 2.7V at the + input, would the output be 0.2V or 2.7V? If 2.7V, how do you set this up for an A-D converter?My actual applications is wanting to take transducer signals, amplify them and read them with A/D converters. Many transducers already have a useable analog output but I'm also interested in load cells, thermocouples, and RTD's.
Thanks!
RogerN
Your example does not make sense. If the opamp is "setup" for a gain of 1 (voltage follower) then it's only got one input. So you can't just feed 2.5V to the "- input".
I'd suggest you read some basic tutrials on how opamp work and can be configured. Like these for starters:
BTW, if you did take your example literally and feed two voltages into the inverting and non-inverting inputs, the opamp will act like a compartor and your output will saturate close to your + supply rail.
You'll probably end up needing precision instrumentatio amplifiers and the like. Measuring small transducer signals accurately can be non-trivial.
Dave.
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-- If it\'s set up as a difference amplifier with a gain of 1, then the output would be 0.2 volts. Is that how you have it set up?
He has to go back and rewrite the question. That's the only way it makes sense, but then it loses sense when the A/D conversion comes in.
Which then makes me think he wants to use the op-amp as a comparator to do conversion, but that's only guessing since his question is completely lacking in sense.
Michael
Michael Black expounded in news: snipped-for-privacy@darkstar.example.net:
The question may not be well formed, but I think the jist of his question re the output is all about his power supply arrangement (and source of confusion):
Whether output is:
1) 0.2V relative to ground with split +, - supplies with a real ground in the middleor
2) 2.7V relative to ground, where ground is -ve and a virtual ground is established at 2.5V.Given his example uses 2.5V and 2.7V as input, it sounds like he is using power supply option 2 above (single ended).
The simple answer for the OP may be just to opt for option 1 above and then the offset problem goes away.
Snark.
-- That\'s what he wants to do. Check out the thread on sed (same subject) to which he multi-posted instead of crossposting.
Sorry for my incorrect question, the way I had it would have driven the output to saturation V+. I was partly thinking inverting amp and writing non-inverting to raise 0.2V from an input raised 0.2V. The bottom line is that I want to be able to take a bridge signal with maybe 30mV difference and amplify it to maybe 0-5V so I can read it with a A/D converter with as much resolution as I can. The output from the bridge is likely to be in the range of 2.5V and 2.53V but I just want to amplify the 0.030 V difference.
In college 25+ years ago I remember 741 op amps (but not all that well as my question shows), but not much else. I probably need an instrumentation amplifier but not sure what to look for, hence my question here.
RogerN
You want to buy an instrumentation amp, or roll your own out of a handful of resistors and an op amp.
You want to pay close attention to the common mode rejection ratio of whatever you make, because your common mode itself is about 100:1; if you don't want the standing voltage on the bridge to mess up your measurement you need to make sure it doesn't get into the final result.
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