Kevin,

Just a couple of things you should think about
when comparing readings:

1.  How accurate is your DVM?  When was it last
     calibrated? How can you be sure the DVM was
     right and the PIC wrong?

2.  If the PIC power supply is not exactly 5 volts,
     your calculation will be incorrect. 5.1V or 5.001V?
     And how was that voltage measured?

3. The reading of 145 from the ADC - was it one
    reading or an average?  All clocks off, etc.

4. Noise from other portions of your circuit or from
    the PIC itself may cause loss of accuracy in the
    last few bits of the ADC.  What is the noise floor
    on the circuit board itself?

> How would I calculate the acceptable bit error ?

I'm not sure what you mean here.  An acceptable error
is basically whatever you want it to be - a design specification,
not what your particular hardware is doing or is
even capable of.

Ken



----- Original Message -----
From: <kben@DCA.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, February 16, 2004 4:11 PM
Subject: [PIC:] 12F675 AD Resolution ?


> Hi,
>
> I am using a 12F675 to read a LM34DZ Temp Sensor.
> If I read the sensor with my DMM I get 0.719
> I calculate the reading I should get from the
> pic as,  1024/5V = 204.8
> 204.8 * .719 = 147.25.
> When I read the A/D from the pic I get 145.
> So, I am off by 2.25. Is this an acceptable error ?
>
> The LM34DZ is a high precision temp sensor of
> the range 32F to 212F, outputting 10mv/Degree F.
> I am powering the PIC with 5V from a bench supply
> and have ample decoupling caps. I am using Vref as
> the reference voltage.
> How would I calculate the acceptable bit error ?
>
> Thanks in advance,
>                Kevin
>
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