Help with the conversion math

[HenrikOlsson]

Just for grins I took a 10K resistor and fed the A/D input pin with 5 volts. The reading I got was 65472!

Yes, because you have the freaking thing configured wrong for the way that ADCIN works for crying out loud :-)
Just set the justification bit correct, as Ioannis shows AND as we've already discussed in the other thread where we went thru all this resolution and justification stuff last time.

OK, so it's a 0.07psi and not 7psi as I based my code around..... Well, then we need to change the scaling but lets see if it works at all first.

/Henrik.

[richard]

that works out pretty nicely every adc 100 count is almost exactly .01 psi

0=110 , 0.07 = 820 couldn't be easier


assuming I read the data sheet properly and that 0.07 psi = 4.0 volts and vref=5 v of course

[EarlyBird2]

that works out pretty nicely every adc 100 count is almost exactly .01 psi

0=110 , 0.07 = 820 couldn't be easier


assuming I read the data sheet properly and that 0.07 psi = 4.0 volts and vref=5 v of course

Not quite Richard. In theory

0=102.4 at 0.5 volts and 0.07=921.6 at 4.5 volts.

assuming as you say the vref=5 v.

102.4 is the zero offset and 0.07/(921.6-102.4) is the slope.

to convert to psi

psi=(reading-offset)*slope

psi=(reading-102.4)*0.07/819.2

psi=(reading-102.4)*0.0000854492.....

I do seem to remember this is for a depth gauge on a model submarine and am wondering what depth of water is 0.07 psi. The answer is 100 mm of water (99.54mm).

Giving

mm of water=(reading-102.4)*0.12207028.....

just to check at maximum reading = 921.6

therefore

mm of water=(921.6-102.4)*0.12207028

mm of water=99.9999

I am now wondering if this gauge is suitable?

[Acetronics2]

I am now wondering if this gauge is suitable?

0.07 psi gauge surely is a differential one ... so, one could reasonnably think it's for measuring SPEED with a pitot tube ...

just my two cents ...

Hi, Ed

Why not download that jewel ???
http://curveexpert.software.informer...nload/?ca13e51

Alain

[Ramius]

Hi! So if I understand correctly I need to use ADCON2 instead of ADCON1 with the same bit configuation? Yes Alian your are completely correct! It is a differntial sensor for my submarine. As the sub dives you need to subtract out the pressure at a given depth for the incoming pressure to stay meaningful.

[Ioannis]

You mean that at page 156 of the datasheet you saw ADCON1 and at page 157 ADCON2 and concluded that are the same at bit 7 ? They are not and must be setup accordingly to your needs ALL of them. DO NOT rely on power up defaults.

You have to set if not all, the most registers for the PIC to work as expected.

ADCON0, ADCON1, ADCON2 should ALL be set as YOU desire, NOT the PIC's default state. Each register has its own bits for different setting. Otherwise why have dozens of registers if they were all the same?

Ioannis

[Ramius]

Hi Ioannis,
This is my very first attempt at using the PIC's A/D.
I think that ADCON0 should be 000100?1 as I am not sure what "GO/DONE" should be set at.
ADCON2 looks like it should be 11101010 and again I am not certain of the last 3 bits.

Thanks, Ed