ADval*/Quanta Question

Hello

In the formulae ADCIN0,Adval 'read ch0 into Adval
&Adval=Adval*/Quanta (using 18f4550)
where Quanta=1251 for 0-5v input
Adval returns 0-499 ie 4.99

However for a 10 bit result I require Aval to return 0-1023.
This appears to be about right when Quanta=2600

Question: Does anyone know what the correct value of Quanta should be or the formulae to calculate it accurately.

Thanks
Tom

Take VDD divided by the MAX A/D reading, and multiply times 256.

5 / 1023 * 256 = 1.251

Scale that up to however many decimal places you want.
For 3 decimal places, multiply by 1000.

= 1251

So your current number is correct for 10-bit A/D with 3 decimals. (0-4999)

If you only want 2 decimals as indicated in your post, use 125 instead. (0-499)
Or you can calculate to 3 decimals, then add 5 and divide by 10 and you'll get a rounded number. (0-500)

4999 + 5 = 5004, /10 = 500
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Thanks Darrel for your formulae. Substituting 1023 for the 5 I get
1023/1023*256 *10=2560(Quanta). I was using 2600,now from Pic this gives true 10bit number(1023) at full scale that is sent to pc to be scaled,damped,averaged etc.

Tom

What?

That's completely self defeating. The A/D converter already put's out 0-1023.

1023/1023*256 *10=2560
With the */ operator, all your formula does is multiply it times 2560 and divide by 256, which is the same as just saying ADval * 10.

So you would actually be sending 10230.
I don't think that's what you wanted.

If you want to send 0-1023 to the PC, just send the ADval.
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Thanks Darrel

Now I understand, Adval by itself returns 102? and when Adval=Adval*10 it returns 1023 which is what I am using.

Thanks again.

Tom

Don't thank me yet.
Cuz it looks like there's still some misunderstandings.

With the Analog to Digital converters, you have two choices (8-bit or 10-bit).
8-bit returns values from 0-255.
10-bit values are from 0-1023.

Those numbers come straight from the converter without any additional math needed.

Without seeing your code, I can only guess at what's happening.
My guess is that before sending to the PC, you are breaking the numbers down with the DIG operator.

DIG starts at 0, with 0 indicating the rightmost digit.
If ADval was holding the value 1023, (ADval DIG 0) would be 3.

Code:

---

DIG  3210
    1023

---

If you're thinking the rightmost digit is 1, and you used DIG 1 thru 4, it would return 0102.
Multiplying *10 first would make DIG 1 thru 4 return 1023, although the actual value is 10230.

Are you using DIG?
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Yes you are right Darrel, I am using DIG 1-4 as you thought. I have now simplified program, since adval does = 1023 directly and using DIG 0-3 to send to PC. All 8 temperature readings now work well.

Thanks again

Tom

Maybe this isn't the right thread, but when A/Ds are mentioned, everyone thinks in terms of 8 or 10 bit resolution. Some chips (8723 for example) return a 12 bit value. That has screwed me up more than once.