ADC confused

[UnD3aD]

i want to make a digital thermometer and i'm using pic 12f683. I use Vdd as Vref and 2 small 1,5V bateries and tc1047 as sensor. Now i'm confused how to convert the voltage.
a 10 bites converter means a value from 0 to 1024
so at 0v input i have 0 returned
but when i have 1024? at 3v?

 

[Nigel Goodwin]

i want to make a digital thermometer and i'm using pic 12f683. I use Vdd as Vref and 2 small 1,5V bateries and tc1047 as sensor. Now i'm confused how to convert the voltage.
a 10 bites converter means a value from 0 to 1024
so at 0v input i have 0 returned
but when i have 1024? at 3v?

Try checking my PIC tutorials, the analogue one explains all this!.

But the range of the A2D is from 0 to 1023 (NOT 1024), it's a 10 bit value. It will read 0 with 0V input, and 1023 with an input equal to the Vref voltage - it WON'T be 3V, because the batteries are unlikely to ever read 3V, it will be higher than that with new batteries, and lower than that with old batteries - there will only be a brief period when it will actually equal 3V.

For this reason it's a good idea to use an external reference, and again my analogue tutorials do just that!.

 

[DTMF]

there is a time you must respect it before you start your conversion

be careful because

there is a capacitor on the ADC pin must charge to VIN

before you start the conversion

:wink:

 

[Oznog]

If you use a thermistor with a pullup resistor to Vdd, it won't matter what Vdd is since both the ADC reference and thermistor voltage are equally affected by Vdd changes.

 

[Someone Electro]

You cod try I2C digital teperature sesors.That use the I2C bus to comunicate whith the PIC.

They have 3.3V ones and 5V ones but if you drive it on +/- 1V the temperature output will be +/- 1°C,So you cod use an output from an regulator to give it a ouput that dosent change on batery status (You can simply add or substract that from the reding to make it acurate agen)