MPXA4115A picbasic code

**Darrel Taylor** · - 17th May 2009, 01:41

I've updated the Calculator again to include a Calibration section at the bottom.

Once you have test data for a sensor, it will calculate the new slope and intercept.
Be sure to set the number of decimals at the top, it affects the calibration numbers.

Cheers,

**Acetronics2** · - 17th May 2009, 09:48

Hi, Darrel

This tool is becoming from good to better and better !

Just one point :

Don't you think, for a 10 bits ADC i.e., the LSB is Vref / 1024 and not Vref /1023 ???

as 0 is 0v and 1023 is NOT Vref ( see µChip apps ...), but Vref x 1023/1024 ...

Not so big an error ... but for those looking for the 224th bit error !!!

and it is neater tho get exact 4 mv/bit for a 4.096 v ref. ...

BTW ... I think ! ( yes ... I can ! ) ...

What about a " Mouseover " on the graph to show the Pressure and AD Count values pointed at ???

Regards

Alain

**Melanie** · - 17th May 2009, 10:15

Actually, it's 1024 bits, but you include ZERO, so the range is 0-1023.

But your math is always divided by the MAXIMUM (Full Scale) value (which is 1023).

That is because there are 1023 equal portions of whatever you are dividing up to make your whole. Zero is not a portion! I'd get the hump (big-time) if I went into Maison Alain (French Restaurant) and got the Zero portion of the Cream Cake!

**Acetronics2** · - 17th May 2009, 10:37

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** · - 17th May 2009, 11:05

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** · - 17th May 2009, 11:40

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** · - 17th May 2009, 12:18

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