Electronic – Detecting a disconnected analog input

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.

Simple algebra. Plug one of them into the other and simplify:

mv = code * 5000/1024

temp = (mv - 500) / 10

temp*10 = mv - 500

temp*10 = code * 5000/1024 - 500

temp*10 = (code * 5000) / 1024 - 500

(add divisor/2 before performing an integer division for rounding)

temp*10 = (code * 5000 + 512) / 1024 - 500

temp*10 = ((code * 5000 + 512) >> 10) - 500

One multiply, one shift right by 10, one addition, and one subtraction. Then convert to BCD and display it.

However, if you have an 8-bit ADC, you're probably going to want to divide by 256 instead of 1024 (shift right by 8 instead of 10) and add 128 instead of 512.

And here is an algorithm for converting binary to BCD: http://www.eng.utah.edu/~nmcdonal/Tutorials/BCDTutorial/BCDConversion.html

Just run that, split into groups of 4 bits, and drive each digit in the display with one group (after decoding to the proper 7 segment signals, of course).