Re: OP-Amps and A to D


"Jim Thompson" <To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote in
message news:4rtga5lcvnd94e75e0c1olb412tfhcq8n5@xxxxxxxxxx
On Wed, 9 Sep 2009 22:36:18 -0500, "RogerN" <regor@xxxxxxxxxxx> wrote:


"Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message
news:bs6dnV0x6uBb_DXXnZ2dnUVZ_tti4p2d@xxxxxxxxxxxxxxx
On Wed, 09 Sep 2009 19:02:21 -0700, Jim Thompson wrote:

On Wed, 9 Sep 2009 19:35:49 -0500, "RogerN" <regor@xxxxxxxxxxx> 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?

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


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?

--
www.wescottdesign.com

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


Hint, hint... You said...

"I have an Op-amp set up with a gain of 1"

Then you said...

"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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

An engineer is supposed to have an inquisitive mind and question
unproven theories. Leftist weenies have neither attribute. Their
behavior is of a religious nature. Thus, like all religious nut-
cases, they should be culled from the fraternity and dispatched.

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
2.7V out. I'm not sure though if other op-amps and instrumentation
amplifiers are the same or how CMRR affects this.

RogerN



.

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