# Beginner A to D Converter

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

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?

• How good is your reference voltage? Are you giving enough time for the given resolution? Are you giving it too much time, such that the S+H circuit leaks too much? – Ignacio Vazquez-Abrams Mar 17 '14 at 19:23
• How do I know how much time I'm suppose to give it? Is there a formula or is it trial and error? – user3385015 Mar 17 '14 at 19:34

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. • Thank for the response. I don't think I'm using the right values, at 3.3 volts it maxes out at 255. I uploaded the integrated board manual, page 16 outlines my values. – user3385015 Mar 17 '14 at 19:44
• That is approximately the right answer if you use only the most significant 8 bits. 310/4 = 77.5 and you got 70-90. That part of your problem is firmware-related. – Spehro Pefhany Mar 17 '14 at 19:46
• I realized I wasn't reading my ADRESH, so my values were actually 1070-or 1090. I'm assuming that is directly in volts, however, it is still off from the value. I know it is directly in volts, because as soon as I went to 2 volts, the number went to 2133. – user3385015 Mar 17 '14 at 20:01
• 2133 is a somewhat unlikely output from a 10-bit ADC – Spehro Pefhany Mar 17 '14 at 20:39
• PIC ADC's sometimes default to 8 bits unless set to 10-bits – Scott Seidman Mar 17 '14 at 22:31

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.

• Could be a left-justified 10 bit – Scott Seidman Mar 17 '14 at 22:28
• I use docklight to receive my results. I get two results for each voltage reading. My impression is the first set of digits is the ADRESH and the second set of digits is ADRESL. For example, when my input voltage was 1 volt. On docklight, I saw the result, 001 090 or 001 073 or 001 078 or 001 086. When I set it to 2 volts. The output was 002 133 or 002 145 or 002 156 etc. – user3385015 Mar 18 '14 at 18:15
• @user3385015 - the numbers don't stack up dude. Lets say 1V = 1080 (averaged), then I'd expect 2V to be 2160 but it looks a tad light on the numbers. I can't explain why. It's just an observation that things aint what they should be either numerically or linearly. The "range" of numbers for 1V represents maybe +/-1% is also a little high so I guess you should check how you feed volts into the ADC. – Andy aka Mar 18 '14 at 18:21
• It is 2160 at 2 volts or relatively close. At 3.3 volts it maxes out 003 255. Any higher and it repeats the same thing. Should I post my code? – user3385015 Mar 18 '14 at 18:31
• I'm not a code man but if you post the circuit how you connect signals into the PIC (aka bluetooth thing) then I might be able to tell you why there are discrepancies and non-linearities. – Andy aka Mar 18 '14 at 18:35

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