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2007\07\31@101040 by

hi list,
I am wondering if anyone have used this ADC. I am trying to  convert a temp from LM35DZ and I got some problem for a while:
the LSB of the ADC doesn't give a stable value (I can't tell if it's 0 or 1) and also give a half of the value that is given by LM35DZ.
here are some info:
I use a R=100Kohm and C=15pF for clocking
Vref=2.5V from the pic16f628
the LM35DZ give a 0.34V (its vero hot here in Morocco)

what I got:
MSB...LSB=0001001X ==> (2+16)*2.5/256=0.175V

it's a half of the real value.
any help would appreciated
best regards,

Mustapha ELKEJBOUT
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"Every one has to do what he can do according to how it can be truly done"
For whom the bell tolls, Ernest Hemingway.

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ELKEJBOUT Mustapha wrote:

> I am wondering if anyone have used this ADC. I am trying to  convert a temp from LM35DZ and I got some problem for a while:
> the LSB of the ADC doesn't give a stable value (I can't tell if it's 0 or 1) and also give a half of the value that is given by LM35DZ.

Hmmm, the LSB floating is IMO rather normal (some millivolt floating
around...)
The 2x value issue is more strange.
Are you sure (measured?) the VRef? Is it connected/configured properly?

--
Ciao, Dario

Mustapha,

I have used the ADC0804, but many many years ago.  If you have a 2.5
volt reference going into the VREF/2 pin, it gets boosted inside to 5
volts.  So the internal reference is actually 5 volts.  This divided by
256 gives
You ~19.5 mv per step.   Multiply this value by 18 steps that you have
gives you ~351.5 mv, which is very close to your measured output voltage
using a meter.  If you're getting half of this value, maybe you're a/D

About the LSB being unsteady may be caused by slight fluctuations is the
ref voltage supplied by the PIC.
Try connecting the VREF pin of the A/D to Vdd (5 volts).  This is okay
to do as the part was designed to allow this type of connection.  If you
get the same answer as beefore, then you probably have a bad A/D.  If it
now gives you the correct answer, then don't use the VREF from the PIC.

Hope this helps.

Regards,

Jim

{Original Message removed}
>Vref=2.5V from the pic16f628

? how do you get this vref from the 16F628 chip?

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