Loadcell output signal amplification

[Thunderhorse312]

Hi! I'm designing a digital weighing scale based on this previous design


however, the signal that I am getting is insufficient for the 10 bit ADC that i was going to use. What amplifier can i add to further increase my signal?
Please provide me detailed explanation.. thank you!

 

[ud23]

use INA 125 load cell amplifier
https://www.ti.com/lit/ds/symlink/ina125.pdf

 

[Thunderhorse312]

use INA 125 load cell amplifier
https://www.ti.com/lit/ds/symlink/ina125.pdf

Question, what do i connect to the negative voltage input(-Vin pin) on the INA125 from the AD620? can i just use an op amp to further amplify the signal and what op amp can i use? thanks!

 

[ud23]

Question, what do i connect to the negative voltage input(-Vin pin) on the INA125 from the AD620? can i just use an op amp to further amplify the signal and what op amp can i use? thanks!

if you already using ad620 then you have to replace that one to INA125 bcz both are instrument amplifier .

what is the gain setting resistor do you have in ad620? from your previous post its shown 333 ohm i guess .
what output voltage you getting now from your current setups?

if your having low output you can make simple amplifier using op amp lm324 bcz you using single supply.

 

[Thunderhorse312]

if you already using ad620 then you have to replace that one to INA125 bcz both are instrument amplifier .

what is the gain setting resistor do you have in ad620? from your previous post its shown 333 ohm i guess .
what output voltage you getting now from your current setups?

if your having low output you can make simple amplifier using op amp lm324 bcz you using single supply.

I have set my gain first to 333 then i got 2.40 something volts output. However, this voltage output is very small for my low resolution ADC of 1024 my results would then be very inaccurate.. I tried to increase the gain to 9999 by using a 5 ohm resistor but the ouput still wasn't enough to cover the 5/1024 or 5mV per step.. furthermore i am only going to utilize 0 to 100 grams and their corresponding outputs that is why i am trying to stick to the low resolution ADC.. How do i connect the lm324 after the AD620?

 

[Kurenai_ryu]

what load cell are you using? is it lower than 1kg like the https://www.ptglobal.com/category/160-ptasp6-b.html
the 300 gram version will give you 1mV per V of supply, lets say you supply with a 5VCC it will give you 5mV at 300 grams, and 1.67mV at 100 grams,
then you need to amplify by x3000 to get a full 0-5V voltage output... ( ithink it's too much and remember that you can't get full voltage swing easily, and it will be easier to drop the ADC voltage reference to (let's say) half or 2.4V so you only need a x1500 amplifier..)

---------- Post added at 11:44 ---------- Previous post was at 11:37 ----------

and BTW you need a 33 ohm resistor to get a x1500 amplifier in the ad620

 

[Thunderhorse312]

what load cell are you using? is it lower than 1kg like the https://www.ptglobal.com/category/160-ptasp6-b.html
the 300 gram version will give you 1mV per V of supply, lets say you supply with a 5VCC it will give you 5mV at 300 grams, and 1.67mV at 100 grams,
then you need to amplify by x3000 to get a full 0-5V voltage output... ( ithink it's too much and remember that you can't get full voltage swing easily, and it will be easier to drop the ADC voltage reference to (let's say) half or 2.4V so you only need a x1500 amplifier..)

My load cell has a maximum weight of 3kg and its resistor values are all 750 ohms. I've tried to put an excitation voltage of 5 volts and the 2.40 something output was the result. I dont know the voltage sensitivity of the loadcell because there were no data sheets about it. I'll try to reduce my voltage reference and see what will happen. so i probably don't need another amplifier?

 

[paulfjujo]

hello

did you expect to use Delta sigma ADC converter ?

with a MCP3424 ( or other device like this)
you can choose the resolution 14 up to 18 bits
and also programmable Gain 1x up to 8x

it could simplifie the probleme.
I tested it with the microchip evaluation board , easy to use with pinout spacing of 0,1" and cost not so much.

or i tested also the AD7715 16 bits ADC (maybe old now).. but very stable
or MAX132 18bits ADC , but need +5 and -5V power supply.

or 18F2523 with 12 bits ADC instead of 10bits , maybe enough ?

 

[Kurenai_ryu]

@Thunderhorse312 :

mmmm let's say it's a 3kg load cell with 2mv/v like this one mmm i think its too big to meause a 300gram span...

let's see:
with 5V excitation: at 3Kg we will have 10mV... at 300gram we will have ONLY 1mV at full span (with a common error of 2uV or max 500 divitions in the load cell)

we need to amplify x5000 to get an 5V output... that would be... mmm 9.8 ohm? mmm lets say 10 ohm...
again, i think it will be suffice to amplify to x2500 and that will be around 20ohm resistor...


and adding to what @paulfjujo said, i can recommend the AD7730 as a good loadcell Sigma analog to digital converter...