I'm working on a board that takes measurements of voltage off a thermistor through an Arduino and amplified by an opamp (really an instrumentation amp AD623), also there are two potentiometers. See schematic below: enter image description here

I've been receiving some very strange voltage fluctuations that I can't seem to isolate to any particular component. I've tested both the voltage source and the thermistor independently of the circuit and they are both stable.

Below is a graph

enter image description here

..and on the bottom of the pic you can see the fluctuations over time with no change implemented in the system. At times they look almost oscillatory, but then stabilize at a random voltage before jumping around again. example, would go from 0.01-0.06 volts, jump around 0.3-0.4 volts, and occasionally go as high as 1-2 volts (With no change to the thermistor!)

I was wondering if anyone had any ideas for what is causing the noise/fluctuations. Also, I've heard of a floating ground (not sure what that is), and I was wondering if that was an issue here.

  • \$\begingroup\$ Show a real schematic, particularly of the analog front end. What you have is a wiring diagram at best. No, I'm not going dig out the datasheet for that chip to see which pins are which, even if I could read the chip name. Also, what is the graph of? What point of the circuit? What was the power supply doing at that time. What else was going on then? \$\endgroup\$ – Olin Lathrop May 21 '12 at 20:38
  • \$\begingroup\$ Since temperature is a very slow-changing magnitude, go ahead and add two capacitors between pins 2 and 4, and between pins 3 and 4, of your AD623. As close as possible to the pins of the IC. Ideally, ceramic (you can find with 10 uF). Or even two sets of capacitors, each being a 47 uF (or so) electrolytic in parallel with a small 100 nF ceramic. That will reduce your input noise. I don't know what's your setup, and how long your wires are. Tell us if your measured signal (whatever it is :)) gets better. \$\endgroup\$ – Telaclavo May 21 '12 at 20:48
  • \$\begingroup\$ I'm not sure what the best thing to give you for the chip would be, but maybe these pics will help. i1149.photobucket.com/albums/o599/ktizzlewat/OpAmpDiagram.png i1149.photobucket.com/albums/o599/ktizzlewat/OpAmpPins.png As for the graph it just shows our voltage vs. time (a measurement taken every 300ms) and I was just including it because it was the only graph of the data I've made so far. Nothing else was really going on at the time... \$\endgroup\$ – user9890 May 21 '12 at 20:52
  • \$\begingroup\$ the power was stable at 5.072 volts, basically I just let the entire device just sit. @Telaclavo I'll definitely try those things and get back \$\endgroup\$ – user9890 May 21 '12 at 21:00
  • \$\begingroup\$ Time to give up. \$\endgroup\$ – Olin Lathrop May 21 '12 at 21:01