Beginner A to D Converter

Question

I'm trying to understand the readings I am receiving from the analog to digital converter (ADC).

I am using an integrated uIceblue2 microcontroller. The microcontroller in the circuit is a 18F4550. In the manual of the uIceblue2 microcontroller it says the an analog range from 0 to 3.3v +/-0.8V. It is a 10 bit ADC.

Page 16 of the manual shows the specifications for the analog voltage http://www.saelig.com/supplier/emxys/EMN0210_WE_uIceBlue2_Getting_Started.pdf

EDIT: I realized that I wasn't reading my ADRESH. So when I received 1V, my ADC readings ware 1070 or 1090.

Below are some readings I'm getting from the ADC when I apply a given voltage to the analog input. I am using Docklight to read my results in decimal. I receive to outputs for each result, which I believe coordinate to the ADRESH and the ADRESL. The first group of three digits is the ADRESH and the second group of digits is the ADRESL.

• 0.055V -> 000 020 to 000 031
• 1V -> 001 070 to 001 090
• 2V -> 002 139 to 002 160
• 3.3V -> 003 255
• 3.4V -> 003 255
• 3.5V -> 003 255

My questions are:

1. Why is there so much variability in the range of the voltage? I thought that 10 bit resolution was quite high and there for the numbers should be constant. For example, I would expect to get a fixed reading of 90 in a row, or a variation of at most 1 in the value, between 89 to 91.

2. I'm using the following formula to convert the reading back to the voltage, but I don't think it is correct for calculating the voltage from ADC reading.

3. Is it possible it is already converting to volts?

4. Should I post my code?

$$Input\,Voltage = Digital\,Reading \times \frac{Voltage\,Range}{Number\,of\,Bits}$$

Could someone please guide me to the correct formula?

Answer 1

The correct formula is \$V_{IN} = \$\$D \cdot V_{REF}\over 2^{NBITS}\$ where D is the digital value. NBITS is 10, so the denominator is 1024. That's assuming Vref(-) is 0V.

If the reference is 3.3V/0V you'd expect a 1V input to produce a digital word of 310 base 10, so something is seriously wrong. I suggest you carefully read the reference manual chapter on the 10-bit ADC as linked here. The data sheet is generally sketchy in the operation of many things.

As @Ignatio Vasquez-Abrams points out, adequate \$T_{AD}\$ is very important.

Answer 2

The ADC numbers you are giving don't make sense - it is a 10 bit ADC - why should a converted number be greater than 1024? You say that with 1V you get 1070 OR 1090. Well, neither are valid numbers and it sounds like 1V gives either 1070 OR 1090 and no value in between.

It should be about 310 so you are definitely doing something wrong in your code but what?

I'm aware this isn't a definitive answer.

Answer 3

One thing that can cause a 10-bit PIC ADC to return results above 1023 is if the conversion is set for a left-justified result. That is bit 7 in the ADCCON2 register is set to zero as per page 231 on the PIC18F4550 Datasheet. To see if that's the case you could try something like the following code that I used in something recently:

    ADCON2=0xBE;  // Slowest conversion time, right justified