ADCIN & 16F870 - how to avoid non linear conversion?

[CuriousOne]

Hello.

I've built a simple circuit, with 16F870 and MAX6126 as reference voltage supply.

The problem is that I can't write a code that will properly convert read data into voltage.

By using external multimeter, I've determined that with 678 read by ADC, voltage is 3 volts. Based on this, I've created a conversion formula, but it does not appears to work correctly, for example, when input is 4.1v, it reads as 4.35 volts. And when input is 1.4V, it reads as 1V. The code is as follows (based on :

Include "modedefs.bas"  ' Include serial modes
   TRISA = %11111111       ' Set PORTA to all input
   ADCON1 = %10000001      ' Set PORTA analog and right justify result

DEFINE LCD_DREG PORTB
DEFINE LCD_DBIT 4
DEFINE LCD_RSREG PORTB
DEFINE LCD_RSBIT 0
DEFINE LCD_EREG PORTB
DEFINE LCD_EBIT 1
DEFINE LCD_BITS 4 
DEFINE LCD_LINES 2
DEFINE LCD_COMMANDUS 1500
DEFINE LCD_DATAUS 44
DEFINE OSC 4

Define  ADC_BITS     10    ' Set number of bits in result
Define  ADC_CLOCK    3     ' Set clock source (3=rc)
Define  ADC_SAMPLEUS 50    ' Set sampling time in uS

adval Var word             ' Create adval to store result 
droebiti var word 'temporal value

mainloox:
   ADCIN 0, adval          ' Read channel 0 to adval
   droebiti=adval*44 'convert to volts
   
   Lcdout $fe,$c0, "Value: ", DEC adval, "    "  ' Display the decimal value 
   Lcdout $fe,$80, "Voltage: ",  DEC (droebiti / 10000), ".", DEC4 droebiti, " V " 
   Pause 100               ' Wait .1 second
Goto mainloox           ' Do it forever

What I'm doing wrong?

I'm using 10k pot with middle pin tied to RA0, one pin tied to Vss and another to voltage reference output. This might be the issue?

[HenrikOlsson]

Hi,
The MAX6126 comes in various versions but I suspect you're using the 5V one, since you're feeding in voltages above 4.096V I'm guessing you're using the 5V version of the MAX6126 - is that correct?

Your numbers don't quite add up.... With a VRef of 5V an input of 3V should give you a reading of (1024/5) * 3 = 614, you're getting 678. Have you followed the datasheet for the voltage reference regarding supply and output bypassing and have you verified that the VRef voltage actually IS what you expect it to be?

/Henrik.

[mark_s]

You have ADCON1 set to use porta.3 as an input for a positive reference. Without a reference connected its floating

Change ADCON1 to this

 
ADCON1 = %10000000

then your references will be +Vref = vdd and -Vref = vss. Or you can connect porta.3 to +5v without changing adcon1

[CuriousOne]

I'm using MAX6126 with output voltage of 4.098v and it outputs that voltage - verified with precision multimeter.
The +Vref is tied to MAX6126 output and +Vref is tied to GND. So this is why I have ADCON1=%10000001
When I tie RA0 directly to MAX6126 output, the raw output reads 1023, when I connect it to GND, it reads 0.

[HenrikOlsson]

Hi,
OK, that's a great value because then each bit in the ADC result equals exactly 4mV but your numbers still doesn't add up....

If you input exactly 3V you should get a value of 750 +/- a couple of bits - not 678 as you're apparently getting.

Have you checked the VRef voltage with a scope? Perhaps it oscillates or have a lot of noise?

What if you replace the 10k pot with a 1k pot, and put a 10nF capacitor between the ADC input and GND. Or, what if you increas the sampling time? (I'll admit, I don't really have this sampling time and Tad stuff clear in my head at the moment, need to do a little reading)

Once you get a correct ADC result for the voltage you're feeding it you can increase the resolution by oversampling. If you take 40 samples, add them together and divide the result by 10 you have a value reading in mV (well, almost - there's 4mV "missing" at the top).

adval = 0
For i = 0 to 39
  ADCIN 0, adval
  Voltage = Voltage + adval
  PAUSEUS 100
NEXT

LCDOUT $FE, 1, DEC Voltage/10000, ".", DEC3 Voltage // 10000

/Henrik.

[CuriousOne]

Ok I've double checked wiring and so on. When getting 1023, I was connecting the upper end of pot to +5V, not to MAX output. Now, when I connected it to MAX output, it reads 987, instead of 1023, so I assume, there might be some more register settings required?