Voltage Fluctuations in Arduino-opamp-thermistor circuit

Discussion in 'The Projects Forum' started by ktizzle, May 21, 2012.

  1. ktizzle

    Thread Starter New Member

    May 21, 2012
    1
    0
    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 (picture of schematic attached).

    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.

    Attached is a graph, 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.
     
  2. mcasale

    Member

    Jul 18, 2011
    210
    12
    Your circuit doesn't show a connection at AREF. I think that is the reference for these on-board ADCs. You need to ensure this is low noise.

    Also, you need to ensure the output signal from the amplifier stays above ground. The amplifier is a single supply, and the ADCs can't handle a negative input.
     
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