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I have tried two designs and both are giving the same flawed results. I was using the internal voltage
reference:
Then I tried the external voltage reference:
In both cases the results were not correct. I approached this as if it were any other ADC converter. I
have discovered the following from the data sheet:
The SD24 converters can operate with internal reference (SD24REFS = 1) or with external reference
(SD24REFS = 0). When SD24 operates with internal reference the VREF pin must not be loaded
externally. Only the recommended capacitor value, C VREF must be connected at VREF pin to AVSS (see
Table 5-19)
And:
Z Input impedance I (pin A+ or A- to AV SD24GAINx = 1, 16 3 V 200 kΩ SS)(3)
So, it appears that the first design will not work because the internal voltage reference can have a load on
it. This begs the question as to why it is brought to the outside. In both cases, external and
internal, there is a 200K impedance to ground in parallel with the thermistor. The thermistor is 2349.8K
at -40C and 10K at 25C. This means that at -40C (as simulated using a decade box) the ADC is seeing
around 122K and my code is reading -28C.
So, what have missed? I thought this would be the simple part of the design. In both cases this should
be a simple voltage divider feeding a DC voltage into an ADC. I am trying to get 0.1C accuracy from this
design.