help with MSP430 ADC10

Development Boards MicroMod
1

i am using an NTC thermistor and MSP430 to measure temperature between 0 to 50 degC.

ADC10 in msp is programmed to take 32 readings. the first 1-2 readings are quite away from the rest of readings. eventually the readings are stable with a couple of counts different.

Also when I power up the system first 4-5 sets of readings are quite away. they are literally off by 4-5degC, because the ADC readings are off.

has anybody faced same issue.

Also can some please commment on what is the best technique to average these readings.

2

Are you using the internal reference? If so, are you allowing enough time for the voltage to stabilize? Do you have filter caps on the reference?

gm

3

yes i am using the internal reference and i dont have caps on it.

4

Check the user guide for recommended capacitors and layout but it is typically a 10uF in parallel with a 0.1uF as close to the Vref pin as possible. I am using an MSP430F248 with the 12-bit ADC and the counts are VERY consistent. Of course, it depends upon the application and the layout. YMMV.

gm

5

Is your thermistor buffered? Source impedance will affect your initial readings.

I take 33 readings, pitch the first one, sum the remaining 32 and shift left 3 places for 12 bit resolution.

6

Also, with thermistors you need to watch out for self-heating. I use a P-channel FET to switch the voltage on only while sampling.

7

i checked the datasheet. it says that for ADC10 Vref dosnt need caps, but if you are using ADC12 then you do need the caps.

hence i didnt put the caps…

i will try with caps…