Trying to interface Arduino with AD623 instrumentation amplifier

[ferik]

Hi all,

I am trying to interface an AD623 with an Arduino so I can amplify a
sensor output (0-200mv) to the (0-5v) range.

Following my calculation, I am using a Rg of 4.7kOhms (1+100/4.7=22.3
and 200mv*22.3=4.45v)

The datasheet for the AD623 is here: http://www.analog.com/static/imported-files/Data_Sheets/AD623.pdf

And this is the breadboard I have built http://coldstorage.frenchguys.com/albums/Web%20Links/ad623.gif

Now I know that I don't have any capacitors on +Vs yet but I thought I
would do without for now since I don't need precision just yet.

The Arduino GND is connected to the breadboard ground. And the arduino
+5V is connected to the +5V of the Arduino. Rg seems to be 4.65 kOhms
which is good enough for this prototype (yellow, purple, red, gold). I
am reading 5.08v between GND and +5V.

The sensor currently outputs 8.9 mv. Therefore if all goes well, I
should be reading nearly 200mv between AD1 and GND. But when I measure
with a voltmeter between pin 5 and 6, I read 38mv, quite far from the
200mv expected.

I know I must be making about 34 different mistakes here. Can anyone
point a few ?

Thanks in advance,

Erik

[John Luciani]

Check the polarity of sensor and make sure you putting
the more positive pin to the positive input.

Remove the circuit from the Arduino and power it separately.

What is the voltage using the sensor?
What is the voltage between pins 2 and 3? What is the voltage
between 2 and GND and between 3 and GND?

Remove the sensor and short pins two and three. What is the output
voltage? Use a resistor and a pot to input 10mV, 100mV, 200mV
between 2 and 3. What is calculated gain?

If these tests do not uncover the errors you will need an
oscilloscope to start looking for noise.

(* jcl *)

--

You can't create open hardware with closed EDA tools.

http://www.luciani.org

[Martin Klingensmith]

Also, put a capacitor over the opamp from +V to ground. Opamps are
always looking for a reason to oscillate.
-
Martin