OP-Amp and A-D Question

R

RogerN

Guest
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
 
RogerN wrote:
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?
Click to expand...
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:
http://www.eas.asu.edu/~holbert/ece201/opamp.html
http://en.wikipedia.org/wiki/Operational_amplifier
http://en.wikipedia.org/wiki/Operational_amplifier_applications

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.

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.
Click to expand...
You'll probably end up needing precision instrumentatio amplifiers and the
like.
Measuring small transducer signals accurately can be non-trivial.

Dave.

--
================================================
Check out my Electronics Engineering Video Blog & Podcast:
http://www.eevblog.com
 
On Wed, 9 Sep 2009 22:37:56 -0500, "RogerN" <regor@midwest.net> wrote:

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?
Click to expand...
---
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?
---

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.
Click to expand...
---
http://www.omega.com/LITERATURE/
 
On Thu, 10 Sep 2009, John Fields wrote:

On Wed, 9 Sep 2009 22:37:56 -0500, "RogerN" <regor@midwest.net> wrote:


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?

---
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?
Click to expand...
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

---

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.

---
http://www.omega.com/LITERATURE/
Click to expand...
 
On Thu, 10 Sep 2009 21:05:32 +0000 (UTC), Charmed Snark
<snark@cogeco.ca> wrote:

Michael Black expounded in
news:pine.LNX.4.64.0909101312330.7374@darkstar.example.net:

On Thu, 10 Sep 2009, John Fields wrote:
On Wed, 9 Sep 2009 22:37:56 -0500, "RogerN" <regor@midwest.net
wrote:
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?

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.

Michael

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 middle

or

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).
Click to expand...
---
That's what he wants to do.

Check out the thread on sed (same subject) to which he multi-posted
instead of crossposting.
 
Michael Black expounded in
news:pine.LNX.4.64.0909101312330.7374@darkstar.example.net:

On Thu, 10 Sep 2009, John Fields wrote:
On Wed, 9 Sep 2009 22:37:56 -0500, "RogerN" <regor@midwest.net
wrote:
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?

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.

Michael
Click to expand...
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 middle

or

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.
 
"RogerN" <regor@midwest.net> wrote in message
news:ydOdnZNqENc06TXXnZ2dnUVZ_qidnZ2d@earthlink.com...
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
Click to expand...
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
 
On Thu, 10 Sep 2009 18:52:24 -0500, RogerN wrote:

"RogerN" <regor@midwest.net> wrote in message
news:ydOdnZNqENc06TXXnZ2dnUVZ_qidnZ2d@earthlink.com...

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


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
Click to expand...
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.

--
www.wescottdesign.com
 
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