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# [Moved] How to choose resistance for amplifier (LM324)

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#### Prashant_2102

##### Newbie level 4
Hello,

I am trying to wire up a load cell to an ADC.

Following is what I have done:
1) I have the following load cell

Since this is a half bridge load cell, I have connected two of these to make a wheatstone bridge.

Now the output voltage for these load cells is 1mv/V, I need to amplify it 1000x to feed it to the ADC (which operates on 5V).

2) For this I have chosen the LM324 amplifier
https://www.ti.com/product/lm324

Now, this is a quad amplifier meaning I guess that it can be used to amplify 4 different inputs. I am going to use only one of it..

My question is how to get this amplifier to give a 1000x voltage gain. From what I have read I think I need to add some resistance in the circuit.. Basically I believe I need to create a circuit like this:

my question is how do I decide what should be the value for resistors R1,R2,R3,R4..

I am a programmer and have zero electronics background... So, I'm sorry if my questions are very basic..

BR,
Prashant

LM324 is a bad choice for a load cell amplifier because it has large input offset and offset drift. Think about a precision OP like OP07.

The simple single OP differential amplifier can be used, but a three OP instrumentation amplifier or a dedicated instrumentation amplifier like AD620 would be better. The shown circuit has a gain of 250 which isn't bad as a starting point. It can be easily varied by changing two resistors. The design equations can be found e.g. here https://en.wikipedia.org/wiki/Differential_amplifier#Operational_amplifier_as_differential_amplifier

Prashant_2102

### Prashant_2102

Points: 2
LM324 is a bad choice for a load cell amplifier because it has large input offset and offset drift. Think about a precision OP like OP07.

The simple single OP differential amplifier can be used, but a three OP instrumentation amplifier or a dedicated instrumentation amplifier like AD620 would be better. The shown circuit has a gain of 250 which isn't bad as a starting point. It can be easily varied by changing two resistors. The design equations can be found e.g. here https://en.wikipedia.org/wiki/Differential_amplifier#Operational_amplifier_as_differential_amplifier

Hi Fvm,

Thanks for your response. I get the point that I need a more precise amplifier for my design..

However I'm still not able to figure out how to choose the resistances for a gain of 1000x (5mv - 5V) ...? :roll:

BR,
Prashant

My suggestion would be use an instrumentation amplifier like INA333.
https://www.ti.com/solution/weigh_scale

This will ease out lot of things for you and you can do the gain setting with a single resistor.

My suggestion would be use an instrumentation amplifier like INA333.
https://www.ti.com/solution/weigh_scale

This will ease out lot of things for you and you can do the gain setting with a single resistor.

Hi Ravindragudi,

thanks for your suggestion... But cost is also an issue for me... I need to have atleast 20 of these... and INA333 seems much higher priced that OP07/AD623..

BR,
Prashant

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