Using bipolar Instrument Amplifier without bipolar supply

Other Hardware General Electronics
1

Hello,

I bought a pressure sensor that can measure values between 0 and 12 bars, but I only need it to measure values between 1 and 1.2 bars. It has red, black and yellow wires.

When I connect red to 5V, black to GND and yellow to A0, I get terrible precision (which is expected since (1024/12)*0.2 = 17 steps over 0.2 bars)

I would like to use an instrumentation amplifier (IA) to improve the precision. I thought of the attached circuit (c1.png attached).

I calculated that I would have V_out = Gain * V_in * (alpha - R3/(R2+R3))

I can adjust Gain using a gain resistor (as specified in the IA datasheet), and V_in=5V, and alpha is the perssure sensor reading (between 0 and 1)

My question is : will this IA (http://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf) work properly if I set +Vs=5V and -Vs=GND, or do I need a bipolar input ?

If I need a bipolar input, will the attached circuit (c2.png) work ? I would set R2 and R3 differently than the first circuit so that the output can use the wole range 0-5V and not just 2.5-5V.

Second question : should I include voltage clamping diodes in case the pressure gets greater than 0.2 bars, or lower than 1 bar, or will the Arduino diodes suffice ?

Thanks for your help !

c1.png
c1.png1089×471 27 KB

c2.png
c2.png1027×558 30.9 KB

2

Can you post a link to the data sheet for the pressure sensor?

3

I could not find an actual data sheet.

Here is the link to the product : http://www.ebay.com/itm/400797753819?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Here is the "interesting" content :

Input:DC5V
Output:0.5-4.5V
Pressure range :0-1.2 MPa
Wires connection: Red cable connect with power + (positive), black cable connect with power -(negative),yellow coonect with singal output

4

0 - 1.2bar is about 0.5volt - 0.9volt on the sensor output..
If you read that with the internal 1.1volt Aref, you will have ~400 digital steps. 0.03bar resolution.
Multiple samples could improve that resolution.
If you're happy with that, then no opamp is needed.
Leo..

5

Wow, I didn't think of Aref at all, thanks ! :slight_smile:

I guess I could even use an external Aref set at 0.9V using a resistor bridge but it makes a really tiny difference...

BTW, I think you mean 0.003 bar

:slight_smile:

6

Yes, typo, 0.003bar.
I would just use 1.1volt Aref.
Leo..