If the scales are AC powered, the 0V in the scales maybe connected back to the AC earth and you are experiencing some problems there. Alternatively, when you measure the 5V on the scales, you might be connecting to -5V and 0V and convincing yourself it is +5V and 0V - this will work fine when the arduino powers it but give nonesense when you use the power from the scales.
What you should do is connect a common ground (0V) between the scales and the arduino and measure what the voltages on the scale are again. If they read negative values or are not how you expected them to be there is likely a mismatch in power supplies. Check also that you still measure about 2.5V on the midpoints.
It's a bit hit-and-miss doing stuff like this and you may get different values but if they are different to what you expect (with the common 0V) then there is a mismatch.
Another possibility is how the scales (when connected to loadcell) do their measurement. You can't rule out that they are doing an AC measurment with dc superimposed - you measure 5V but in fact there maybe a significant AC signal (probabaly between 100Hz and 10kHz) superimposed that your meter is oblivious to.
I am quite a noob (I'm actually a programmer, not an electrical engineer!) - but I'm doing something similar and maybe my discoveries will help you out.
Firstly, I suggest you read this:
http://www.instructables.com/id/Arduino-Load-Cell-Scale/
Yes - it's for a 4-wire load cell, but it's very similar.
Also, read this:
http://airtripper.com/1626/arduino-load-cell-circuit-sketch-for-calibration-test/
FIRSTLY:
The big difference between these two articles, is the latter shows exciting the load cell from the INA125 voltage reference... NOT the arduino supply. I would strongly suggest doing this - as my readings significantly stabilised (improved from 50g fluctuation to only 5g!).
SECONDLY:
In your particular circuit, you cannot use pin 15 for your voltage reference (5v) -
Page 11 (section "Precision Voltage Reference") of the specification says "Positive supply voltage must be 1.25V above the desired reference voltage."
http://www.ti.com/lit/ds/symlink/ina125.pdf
This means that because your circuit supply is 5v, you can only use a voltage reference pin that is less than 5v-1.25v=3.75v.
(Why? It appears that the IC uses 1.25v to generate those reference voltages, meaning that the 5v and 10v pins will not actually be producing 5v and 10v for you!). That leaves only the 2.5v reference pin as a candidate. Unfortunately, that also means that if you use the same voltage reference as E+, you will be running your load sensor at 2.5v - which may not be enough excitation - you will need to read your load cell spec - but they usually want around 10v to really work well.
I originally made the same mistake, and used the 5v reference pin, with a circuit supply of 5v, but then I saw this on my scope:
That spike is a 100mV pulse every 200ms. With my calibrations, it resulted in 200g worth of error!! When I switched to the 2.5Vref, that spike went away.
SECONDLY: Why is your VrefOUT (pin 4) connected to your 5v supply? This pin should ONLY be connected to your VrefIN (pin 14 for 2.5v, pin 15 for 5v, pin 16 for 10v) AND your load cell E+.
Here is my understanding of what it's for...
The amplifier needs to have a consistent voltage reference, as the circuit supply may fluctuate throughout its life (i.e. depleting battery etc), so you need to give the INA125 a known voltage reference - luckily the INA125 produces 3 of them! (2.5, 5, and 10).
THIRDLY: your amplifier gain... I don't use Arduinos, but my analog inputs are referenced against 3.3v. My load cell produces about 4.1mv when loaded with 5kg - I needed to amplify that to near 3.3v, so my required gain was around 800!!
If your cell output and Arduino requirements are anywhere near mine - then your gain resistor is FAR too big. Mine was 75 ohms. With such a huge resistor, I would expect you to see no change on your analog input.
So, to summarise:
- Feed your load-cell E+ from your INA125P pin 4 - not your circuit supply. Pin 4 will be much smoother and more consistent.
- Don't connect your pin4 to your circuit supply (marked as 5v in your diagram). I don't know why you did this.
- You amplifier gain is probably too small, as a result of your gain resistor being far too large. If you can't be bothered calculating what resistor you need, grab a potentiometer in the range of 200R and play with it.
Best Answer
Thanks for all the suggestions it helped me a lot.
Yes it seems like the problem was the bread board. After I set my PCB circuit the shacking or touching did not impair my calibration.
Working on PCB brought another problem to occur. When my hand got close to the circuit, the output value from load cell was increasing in large amounts. This problem is solved by placing two parallel parasitic capacitors, 1uF and 100uF, between Vcc and ground.