Electrical – Reading one ADC input on two microcontrollers

If it is not a software problem, I believe Ignacio is right : that's probably a MUX or sample time problem.

The ADC has a sample-and-hold capacitor (to ground) of about 10pF that holds enough charge for one ADC conversion. This capacitor is fed through a resistor by the ADC MUX that switches from the various input pins. This MUX also has a capacitor of about 4pF. (I don't remember the exact values of the resistors).

^{simulate this circuit – Schematic created using CircuitLab}

If the output impedence of your sources is low enough, the capacitors get charged (more or less) instantly and all this behaves just like a switch to an ADC. However, if the output impedence of your temperature sensor is high, the current that flows from/to your temperature sensor between the moment you switch the MUX and the moment you read the ADC may not be enough to charge correctly the capacitors, and you would read the same value as before MUX switching.

You can correct this by either :

• waiting long enough after changing the input, so that the capacitors have had time to settle,

• adding a capacitor between your high impedence input pin and ground. The value would have to be much larger than 14pF (in a ratio determined by the desired accuracy), yet small enough for your temperature time variations to be slower than the RC time constant of your capacitor & sensor impedance.

All DACs require a voltage or current reference. The reference can be supplied by an external chip, or be built-in to the DAC.

Here is one thing you should check: Is your reference oscillating?

Causes of reference oscillation include:

1. not using a filter capacitor that a manufacturer recommends on the reference output of DACs with built-in references

   1.1 not using a filter capacitor that a manufacturer recommends on a external reference output

2. using a filter capacitor with the wrong ESR (Equivalent Series Resistance)- consult the data-sheet of the reference

3. oscillation in an op-amp circuit which "buffers" the reference due to capacitive loading or design error.

   3.1 Not using an op-amp "buffer" circuit when one is required. Are you drawing too much current from the reference?

4. Excessive noise on any power supply (try powering up your circuit from the appropriate combination of batteries)

It does not look like a digital interference problem because the disturbance seems to be very steady, and there are no "spikes" or other discontinuities in the waveform. To be sure, please measure the frequency of the disturbance and find interference that varies at the same frequency or an i/o port read or write that occurs at the same frequency