MPXA4115A picbasic code

[Acetronics2]

Hi, Mel

I found that in the MAX 1241 datasheet:

Output Coding and Transfer Function

The data output from the MAX1240/MAX1241 is binary,
and Figure 10 depicts the nominal transfer function.
Code transitions occur halfway between successiveinteger
LSB values. If VREF = +2.500V, then 1 LSB =
610μV or 2.500V/4096

This is not the first ADC datasheet where i've found that ...

from MAX 187

Output Coding and Transfer Function

The data output from the MAX187/MAX189 is binary,
and Figure 10 depicts the nominal transfer function.
Code transitions occur halfway between successive
integer LSB values. If VREF = +4.096V, then
1 LSB = 1.00mV or 4.096V/4096.

From MCP 3202

4.2 Digital Output Code

The digital output code produced by an A/D Converter
is a function of the input signal and the reference voltage.
For the MCP3202, VDD is used as the reference
voltage. As the VDD level is reduced, the LSB size is
reduced accordingly. The theoretical digital output code
produced by the A/D Converter is shown below.
where:
VIN = analog input voltage
VDD = supply voltage
Digital Output Code = 4096 * VIN / VDD

From MIC 640 ( a pre-programmed 12F675 )

· Résultat = 256 x VIN/VCC où VIN est la tension d’entrée et où résultat est arrondi à
l’entier le plus proche.
Ainsi par exemple, si VCC = 5 volts et VIN = 3,5 volts, le résultat de la conversion sera :

Résultat = 256 x 3,5 / 5 = 179,2 arrondi donc à 179 soit encore B3 en hexadécimal.

so, I'm a bit confused, here ...

Regards
Alain

[Melanie]

I'm sorry, bit I'm going to 'DISAGREE' with them ALL...

Common sense...

Let's go to the extreeme... a ONE BIT ADC... (the kind every PIC has on every I/O Channel)...

Are you going to consider your I/O pin as having TWO states ZERO and ONE?

Well, I suppose it has got two states... so the 'ADC' range is 0-1.

So therefore whenever I use TWO in my calculations I can never get any more than 50% of the true answer? So with 5v as my VREF, I'll never get more than 2.5v in the calculations with an input state of 1. I must get an input state of 2 before I get 5v... Kinda tough getting 2 out of 1 bit!!!!!!!!

Are you dividing your 'cake' into 1023 or 1024 portions?

Is your range 1024 portions ie 1/1024 all the way to 1024/1024??? No it's not because 1024 is an INVALID number, you NEVER get 1024 out of your ADC... your range is 1/1023 all the way to 1023/1023 because 0/1023 although being the 1024th value is not counted when dealing with mathematical equations.

Taking what you read as Gospel without question is what crashes Mars Landers...

Yes... this is a 'simplistic' view, because many chips have peculiarities which have to be additionally accounted for (like for example not being true rail-to-rail)... what I am arguing is pure mathematics here.

[dhouston]

Melanie,

I usually illustrate this with a short staircase of 4 equal steps. The bottom level is 0 and the top is 4. While there are 5 levels (0-4), there are only 4 steps (or intervals) each of which is 1/4 of the total height. It you let people draw and number it themselves, it sticks with them longer.

[Acetronics2]

Gosh ...

One more manufacturer ( Linear ) in error ?? ...



But here ... he shows output number vs input voltage


look at this joined piece of datasheet ...

Code "1023" is NOT corresponding to Vref ... but Vref MINUS one LSB ...

so ... Vref corresponds to code "1024" !!! and there are 1024 steps, the "1024" value never being displayed ...

May be the explanation is here ...

Alain

[Melanie]

Well, in this instance ADCMAX is correct because VREF is never attainable.

[Acetronics2]

Hi, Mel

...

and I do think it's the same for all the manufacturers ...

just have a look to µChip AN 544d ... or the Midrange Manual ... they all show code $3FF for 1023 LSB ... not Vref , which is 1024 x LSB ! ( see § 23.12 )

Would be interesting to verify which input voltage give code 255 for a 2.56v ref ( quite simple !) , ... give code 4095 for a 4.096 v Vref ( less simple ), etc, etc.

don't you think ???

Regards
Alain

[Darrel Taylor]

I agree with Melanie.

Let's forget about what the overpaid engineers and datasheets have said, and look at it from purely a results perspective.

With 10-bit A/D the resulting values go from 0 - 1023.
If Vref=5.00V and you apply 5.00 volts to the A/D you get a reading of 1023.

If you then use 1024 as the Vref divisor, the formula to calc the Voltage from the A/D reading would be ...

5.00/1024 * 1023 = 4.99 ; Not the 5V we were looking for.

But if you use 1023 ...

5.00/1023 * 1023 = 5.00 ; this is what I want it to read.
  Notice how the (/1023 * 1023) cancels out.

In the context of this thread, the results are similar.
If the manufacturer states a pressure of 115kPa at 4.7V ... using 1024 with a 4.7V input, you will only read 114.xxkPa, again not the reading we wanted.

So for this (and any other analog projects), I'll continue using the maximum ADC value for the divisor (255, 1023, 4095).
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