Trouble with ADS1115 precision using Load Cells

Thread Starter

Meca10

Joined Jun 15, 2021
14
Hello everyone.

I used ADS 1115 ADC to digitalize resultant signal of four LC parallel connected via excitation trim. However I have trouble precision with ADC and I don't know the cause. I used 0.1875mV/bit resolution and measured AD620's output. The measurement is 65mV but I saw measurement using Arduino's serial monitor and output value is around 0.228 V. People on foros said ADS 1115 isn't the best option to digitalize LC's signals and purposed other option as ADS1231/2.

I hope anyone give me any answer for my problem.

Regards.

Meca.
 

kaindub

Joined Oct 28, 2019
90
The output of load cells is too low for the analog input of the Arduino. Use the hx711 signal conditioner and a serial input to your Arduino.
 

Thread Starter

Meca10

Joined Jun 15, 2021
14
hi Meca,
How did you connect the Load Cells to the ADS1115.?
Do you have a sketch to post.?
E
Hello Eric
I connected Instrumental Oamp Output to A3 pin ADS1115. I am used ESP32 and read ADS conversion value wrong. I Tested output with multimeter its value is 65mV. When I see Analogic conversion value with serial monitor is 0.2v
1628260488264.png
 

Thread Starter

Meca10

Joined Jun 15, 2021
14
The output of load cells is too low for the analog input of the Arduino. Use the hx711 signal conditioner and a serial input to your Arduino.
Hello kaindub
I made other test using Hx711 but need a high sample speed. The Hx max sample speed is 80. How can I conditioner Instrumental Oamp signal to read small signal values with ADS?
 

ericgibbs

Joined Jan 29, 2010
13,881
hi Meca,
What Gain setting do you have for the ADS1115.?
E
Clip:
The ADS111x perform conversions at data rates up
to 860 samples per second (SPS). The PGA offers
input ranges from ±256 mV to ±6.144 V, allowing
precise large- and small-signal measurements.


BTW: the C2 47n when sourced from R1/2 30K could cause response problems when using the Platform as an athletic force measurement.

What are the LC element resistance values.?
 

Ian Rogers

Joined Dec 12, 2012
935
FWIW... Loadcell amps "normally" amplify signals in the range of 0 ~ 20mV Where do you measure 65mV?

0.65mV on a 10 bit ADC will generate about 13 bit counts on an Arduino..
 

Thread Starter

Meca10

Joined Jun 15, 2021
14
hi Meca,
What Gain setting do you have for the ADS1115.?
E
Clip:
The ADS111x perform conversions at data rates up
to 860 samples per second (SPS). The PGA offers
input ranges from ±256 mV to ±6.144 V, allowing
precise large- and small-signal measurements.


BTW: the C2 47n when sourced from R1/2 30K could cause response problems when using the Platform as an athletic force measurement.

What are the LC element resistance values.?
OK Eric
I used ADS1115 RobTillaart's library. The ADS1115 is singleend mode. Besides I set 475 SPS and setting Gain setting in 0 (±6.144 V). But the minimum input range is ±256 mV. Could do I add offset to AD620 for increase its output to minimum ADS range and read any right ADC value?

I connected C2 47nF with R1/2 30K for reduce external noise. This circuit works as low pass filter according AD620 Datasheet but I don´t know if circuit could cause response problems.

The 4 LC's resistance values have output resistance 351 ohms for each LC and Input average resistance365.25 ohms.
 

ericgibbs

Joined Jan 29, 2010
13,881
hi M,
I see the Gain is approx 500, so you are expecting +5V out for a 10mV signal from the LC array, at [ 4*500kG],,, 2000kG full platform load.??

E
 

ericgibbs

Joined Jan 29, 2010
13,881
hi,
So this is a simplified version of your circuit.

What I would suggest that you make up a simple potentiometer/ resistor circuit and inject a Test offset into the INA input, so that you can output known voltages from the AD620.
Do you know how to do that.?

BTW have you 'zeroed' the OPA to correct for any platform weight on the LC's.

E
ESP_ 657 Aug. 06 18.21.png
 

Thread Starter

Meca10

Joined Jun 15, 2021
14
hi,
So this is a simplified version of your circuit.

What I would suggest that you make up a simple potentiometer/ resistor circuit and inject a Test offset into the INA input, so that you can output known voltages from the AD620.
Do you know how to do that.?

BTW have you 'zeroed' the OPA to correct for any platform weight on the LC's.

E
View attachment 245129
Hi eric
I would made tension divisor to settling around 400mV and put them on AD620 5 pin and enable ADC's convertion.
I plan to do the tare by software
 
Top