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?
- 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:
http://forum.arduino.cc/index.php/topic … #msg508428
I really appreciate your attention.