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Help - ADC1115 with external op-amp
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Help - ADC1115 with external op-amp

by amagro on Thu Aug 08, 2013 8:55 pm

Hi,

I’m a civil engineering student and I’m currently developing my master’s thesis, for which I need to gather data through some piezoelectric sensors and load cell sensor.

I have a load cell sensor (5mVout,max) with 50Kg capacity, although I’m only using 20Kg; I used the op-amp INA125 to amplify its signal, and with an analogReference of 1,1 (Arduino Uno) I ended up getting a resolution of approximately 16g/step.

Recently I purchased an ADC1115 (ADAFRUIT Board) with the purpose of getting until 1g of resolution, and here’s where the problems begin.

Initially, I connected directly the load cell to the ADC1115, with “voltage input range = 0,256V” (PGA gain = 16), and a resolution of 0,008mV/step, but since the load cell has 5mVout,max I could only read 184steps, out of the 65536 possible.


So I realized I needed to amplify the signal before sending it to the ADC1115. Here’s my line of thinking: since the “maximum analog input voltage of ADC1115 is 0,3V” I calculated the necessary gain so that the INA125 reached 0.3V.

• ADC1115: max analog in = 0,3V (used 0,2V)
• Load Cell: mVout, max = 5mV
• Gain = 0,2/0,005 = 40
• Rg = 60000 / (Gain - 4) = 1,7 kOhm

And therefore, with this resistor I could amplify the signal until 0.2V (theoretically and according to my thinking, which I don’t know if is correct).

• ADC1115: max analog in = 0,2V
• ADC1115 resolution: 0,008mV (voltage input range = 0,246V)

0,2V / 0,000008V = 25 000 steps

25 000 steps ---- 20 000gr
1 step ---- X X = 0,8 gr (as I intended)

The truth is that after all these calculations and after putting the system all together, the results are not what I expected, the number of counts starts on 8700, and pressing the maximum force on the load cell it only goes to 8728, so something’s not right… where could I have done something wrong?


Doubts:
- When I connected directly the load cell to the ADC1115, the connection was the “differential” type. On the other hand, when I used the INA125, since it only has one Vout, the connection to the ADC1115 was the “single-ended” type. Is it possible to connect the INA125 to the ADC1115 in the “differential” type?

I got the arduino code for ADC1115 in this post in arduino forum :
http://forum.arduino.cc/index.php/topic ... #msg508428

By the way, the examples in Adafruit_ADS1X15 library are by default "ADC Range: +/- 6.144V (1 bit = 3mV)", to change this settings, must I rewrite the Adafruit_ADS1015.cpp and the Adafruit_ADS1015.h ?


I really appreciate your attention.

Thanks,
Carlos Esteves
Last edited by amagro on Fri Aug 09, 2013 5:24 am, edited 2 times in total.
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Re: Help - ADC1115 with external op-amp

by adafruit_support_bill on Fri Aug 09, 2013 5:18 am

when I used the INA125, since it only has one Vout, the connection to the ADC1115 was the “single-ended” type. Is it possible to connect the INA125 to the ADC1115 in the “differential” type?

You can connect a single ended output to a differential input. Just connect the ground of the output to A0 and the output signal to A1.

the examples in Adafruit_ADS1X15 library are by default "ADC Range: +/- 6.144V (1 bit = 3mV)", to change this settings, must I rewrite the Adafruit_ADS1015.cpp and the Adafruit_ADS1015.h ?

That is correct.

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Re: Help - ADC1115 with external op-amp

by amagro on Fri Aug 09, 2013 8:01 am

Hi Bill,

Thank you for responding so quickly.

Could I ask your personal opinion on the amplification issue (ie, the need to previously amplify the signal before connecting to ADS1115)?
That is my really big problem...

You can connect a single ended output to a differential input. Just connect the ground of the output to A0 and the output signal to A1.


And will I still get the 16bit resolution?


Thanks again,
Carlos Esteves
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Re: Help - ADC1115 with external op-amp

by adafruit_support_bill on Fri Aug 09, 2013 8:16 am

Load cells typically require a lot of gain. The Burr Brown INA125 is a good op-amp for the purpose. Most people interface this directly to the Aruduino analog input. But the ADC1115 should give you much better resolution. Here are a couple example load-cell/op-amp circuits:

http://edg.uchicago.edu/tutorials/load_cell/
http://arduinotronics.blogspot.com/2013 ... cells.html

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Re: Help - ADC1115 with external op-amp

by amagro on Fri Aug 09, 2013 8:34 am

Hi,

The problem is: my load cell has a maximum out voltage of 5mV. With the highest ADC1115 gain (16) I have a resolution of 0,008mV/step, i.e, in 184 steps I'll reach the 5mV, so I need to amplify those 5mV to get more resolution with ADC1115.

The truth is that I thought I was thinking correctly, but apparently something's wrong...


Thanks for the links.

Here's one very well documented that I followed for my project:
http://www.hw2sw.com/2011/10/20/weight- ... d-arduino/



Thanks,
Carlos Esteves
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Re: Help - ADC1115 with external op-amp

by adafruit_support_bill on Fri Aug 09, 2013 9:44 am

Have you verified the output of your INA125 circuit?

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Re: Help - ADC1115 with external op-amp

by amagro on Tue Aug 13, 2013 11:15 am

Hi Bill,


I’ve been experimenting different amplifications on INA125 and I got better results, for example, with RG = 520 Ohm I already got a range from 8732 to 32767 (24035 steps).

You can connect a single ended output to a differential input. Just connect the ground of the output to A0 and the output signal to A1.


I have 3 ground connections from the INA125, should I connect them to the A0?
If I connect them this way, will I get 16bit?


Thanks
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Re: Help - ADC1115 with external op-amp

by adafruit_support_bill on Tue Aug 13, 2013 12:13 pm

Connect the VrefCOM pin from the INA219 to the A0 input of the ADC1115. Then connect Vo to A1. Then, if you call readADC_Differential_0_1(), you will get 16 bit resolution.

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Re: Help - ADC1115 with external op-amp

by amagro on Wed Aug 14, 2013 2:50 pm

Hi,

Connect the VrefCOM pin from the INA219 to the A0 input of the ADC1115. Then connect Vo to A1. Then, if you call readADC_Differential_0_1(), you will get 16 bit resolution.


With the connections you indicated I only got 15 bits (negative) from - 8843 to - 32768.

I altered the Adafruit_ADS1015.ccp file to a voltage range input of 0,256V (0_256V).

I have a doubt about the multiplier 3 that appears in the arduino examples:

Serial.println("ADC Range: +/- 6.144V (1 bit = 3mV)");
Serial.print(results * 3);

Wouldn’t it be:

(6,144 * 2 ) / 65536 = 0,1875 mV/bit (instead of 3mV!)
(0,256 * 2) / 65536 = 0,0078 mV/bit


Thanks
amagro
 
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Re: Help - ADC1115 with external op-amp

by adafruit_support_bill on Fri Aug 16, 2013 8:36 am

Looks like the INA219 only outputs positive voltage. Since the measurement range of the A/D is +/- .256v, you will still only get an effective 15 bits.

Wouldn’t it be:

(6,144 * 2 ) / 65536 = 0,1875 mV/bit (instead of 3mV!)
(0,256 * 2) / 65536 = 0,0078 mV/bit


That is correct. The 3mV from the example is for the 12-bit ADS1015. This is noted in the tutorial: http://learn.adafruit.com/adafruit-4-ch ... rogramming

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Re: Help - ADC1115 with external op-amp

by harsheel on Mon Mar 03, 2014 3:49 pm

can anyone help me.i am confused how to connect INA125 to ADS115 with arduino uno.
please help me as soon as possible.
i would really appreciate anyone's help in this matter. i have connected the circuit but its not reading properly.
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Re: Help - ADC1115 with external op-amp

by adafruit_support_mike on Tue Mar 04, 2014 12:33 am

First, please don't post the same question in multiple places.

Second, you'll have much better luck getting answers if you post a new topic in the forum instead of adding a reply to an existing thread that's been dormant for weeks.

Start a new topic in the Arduino forum (posting.php?mode=post&f=25) with a photo of your hardware and your code and we'll see if we can help you.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.

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Please be positive and constructive with your questions and comments.