lm324 offset null adjustment

Thread Starter

Chapa3524

Joined Feb 19, 2012
11
hi. does anyone knw how to adjust the offset voltage to null in lm324 when it is used as an instrumentation amp? also how to use a logarithmic amplifier circuit to linearize a signal with the instrumentation amp without getting an odd output? :confused: thanks :)
 

t06afre

Joined May 11, 2009
5,934
hi. does anyone knw how to adjust the offset voltage to null in lm324 when it is used as an instrumentation amp? also how to use a logarithmic amplifier circuit to linearize a signal with the instrumentation amp without getting an odd output? :confused: thanks :)
I agree that it is much better to use a real opamp. But if this is some academic exersise. Or some other hobbyist project that do not high level of precission. You can try looking at the INA111 datasheet and see how it is done there. Note the pin 5 ref. You can offset adjust by adding a small voltage to this pin
 

crutschow

Joined Mar 14, 2008
27,736
To adjust offset you can sum in a small DC voltage through a potentiometer from a regulated DC (from a high stability reference voltage if you need good stability) to the input. Use the potentiometer to adjust the output to zero with no input signal.
 

Wendy

Joined Mar 24, 2008
22,547
It is a common spammers trick. However, there is some doubt on this one, it appears someone who has zero experience with forums. There were two other replies that are moderated, and likely to stay that way.

Anyhow February, Welcome to AAC. When you get it figured out we'll still be here.

Chapa3524, a warm welcome. Most times it is better to buy a instrument amp on a chip, as everything is balanced and it works much better. However, if you are doing this as an experiment no big deal.

My suggestion is to post your schematic however, schematics are the real language of electronics. It can be hand drawn and scaned, anything will work.
 

Thread Starter

Chapa3524

Joined Feb 19, 2012
11
i was hoping to get a variable gain so tht's y a instrumentation amp ws designed. by using a variable resister as the gain controller of the instrumentation amp. it's actualy for a project. it's simulating in protues perfectly bt the practical situation is different it seams. :)
 

John P

Joined Oct 14, 2008
1,910
The LM324 is a pretty low-quality amplifier. If you want good performance and features beyond the basic, it's better to look elsewhere.
 

Ron H

Joined Apr 14, 2005
7,014
You can make a 3 op amp IA (a la INA111) with an LM324, and adjust the offset by injecting a voltage on the REF input, as has been mentioned. The problem is, you have to set the gain first, and then set the offset. If you set the offset first, then changing the gain will change the offset. This is because the input offsets are amplified by the gain. Changing the offset does not change the gain, but is much easier to set gain if you have zero offset before you begin.
If you are using a variable resistor to set the gain, then it's a bit of a pain if you want to set it precisely, because of the interaction between the two adjustments.
 

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