Help to increase the accuracy of op-amp[lm324] as possible as

Thread Starter

booboo

Joined Apr 25, 2015
168
Hi guys
I'm working with LM324 to learn how to use Op-Amp and all aspects of it. currently my circuit is this(a simple non-inverting Amp):



And here is a table of inputs and outputs and the gain of each input:



The gain of each input is odd to me. I'm trying to increase the accuracy of this circuit as possible as. I have applied 9.6mv in the input and got 4.75v in the output. or for 11.2mv I got 5.39v. What's the problem? IIRC the max input offset voltage is 7mv. Am I right? please show me by mathematical calculation that why the output is this? the gain isn't correct. should I bias the input offset current or offset voltage? How?
 

#12

Joined Nov 30, 2010
18,210
Have you connected the other 3 amplifiers in that chip so they will stay at a stable DC voltage?
Do you have the circuit shielded with metal so it can't pick up local noise?
Do you have an oscilloscope to check for oscillation?
Why don't you try for something like a gain of 10?
How are you making the input voltage?
Is your input voltage so purely DC that you can't be amplifying a ripple voltage by 392?
 

djsfantasi

Joined Apr 11, 2010
6,718
Are you sure about your measurement at 3.1v? Other than that measurement, the values you get are very close to expected. They can be estimated by:
output=385.83*input + 1065.11​
See the graph below...
Capture.PNG
Of course, this is the purely mathematical view, per your request. #12 has a much more realistic view of your post.
 

wayneh

Joined Sep 9, 2010
16,399
Other than a suspect data point, it seems clear what's happening.
y = 385.49x + 937.46

Screen Shot 2015-04-27 at 1.42.51 PM.png

Oh wow, was I slow today.
 

#12

Joined Nov 30, 2010
18,210
I welcome your assistance.
My experience with op-amps is that they are exceedingly predictable if you know how to arrange the circuit.
Please label your graphs. That would make it easier for me to see that the gain seems to be 327 (Y= MX + B) and a few millivolts of input offset voltage can explain the B term.
 

Alec_t

Joined Sep 17, 2013
11,525
Plot a graph of the values in post #1. Extrapolate it back to zero input. What do you deduce the actual input offset voltage to be?
 

crutschow

Joined Mar 14, 2008
25,265
.........................
IIRC the max input offset voltage is 7mv. Am I right?
....................
I believe you missed the fact that the offset is amplified by the closed-loop gain, the same as the signal, so for the small signals you are using, the offset can give an output greater than the signal.
 
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Alec_t

Joined Sep 17, 2013
11,525
Given the OP's aim ("I'm working with LM324 to learn how to use Op-Amp and all aspects of it.") I see no problem with using an LM324 in this exercise. He has now learned something about offset voltage.
 

atferrari

Joined Jan 6, 2004
4,069
Anyone daring to suggest the "old lousy" 741 for the same educative purposes? :)

Seriously speaking, not bad to start with them and then going to something better.
 

#12

Joined Nov 30, 2010
18,210
Anyone daring to suggest the "old lousy" 741 for the same educative purposes? :)
The common mode input voltage is 3 volts above ground to 3 volts less than the Vcc. Not best for a beginner that is making DC amplifiers with a single supply.
 

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