Instrument Amplifier INA 125P with Load Cell

Projects General Guidance
1

im using a INA125P to amplify a load cell. the load cell output goes to a 16 bit ADC (measurin differential voltage) that communicates with an UNO. the Uno, adc, and amplifier share a common ground. its a 2000lb load cell with a 3 mv/v slope. Granted its oversize for what i need but it still should work. i am using a single sided power supply (can make it a dual with resistors if needed...) and have tried both configurations in the manual (figure 1 and 6) but neither work as expected. I believe its because figure 1 needs a dual sided power supply. The configuration in figure 6 does work but doesn't work as i expect. It has the following attributes:

  1. when i change Rg, the zero load offset changes significantly. the voltage will go from -2v to -1.5v if i increase the gain.
  2. if i go to a super high gain the gain doesn't actually increase.
  3. if i use a 10 ohm resistor the output peak to peak doesn't go up.
  4. the noise is insanely bad. granted i am not using best of power supplies but i was under the impression this amp had great noise rejection.
  5. the noise can be as bad as 5% of the peak to peak change when adding 50 lbs.
  6. noise only gets worse when adding the cap between Vin and ground.
  7. i am supplying 14v to pins 1 and 2. pin 4 gets power from pin 14. i am measuring voltage between pin 11 and ground. pin 12 is grounded. pin 10 to 11 is shorted. Rg between 8 and 9. S+ and S- to 6 and 7 respectively. Bridge excitation powered by pin 4 and ground. 3 to ground. 5 either to ground or 4 depending on the setup (figure 1 vs 6)

i was having issues while using a proto board so i made a pcb and things improved. noise is better but is still an issue. i have a pcb for both layouts.

any ideas? i know there is a lot here but i suspect i am using the amp outside its intended range?

2

Hi,

Can you post your code and a copy of your circuit diagram please?

What are fig1 ? fig 6?

Thanks... Tom.... :slight_smile:

3

Attached are the reference figures from the ina 125 datasheet.

The code is at home so I can post it later but I'm using the adc from adafruit and using their example code. I'm measuring between pins 2 and 3 on the adc which wire into 12 and 5 on the ina 125. I do not have the load resistor between 11 and 5 as I assumed that is supposed to be the impedance of the adc.

I can post a sketch of the system when I get off work.

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Screenshot_20181112-130859_Drive.jpg720×1480 116 KB

4

Attached is a sketch of my pcb, adc, and uno. Also on the side is results of different resistor values. On the left is the static offset in bits. The adc is 16 bits with 15 bits of voltage resolution and the last bit is plus/minus. On the right is what the voltage goes to when adding 50lbs to the load cell. The noise is getting amplified as the gain goes up. I guess that's expected.

The adc is measuring between ground and the output. I should be measuring between output and my voltage reference but the deltas should remain the same?