Mark Bellamy wrote :

> I want to display the temperature on an LCD.  I'm using a LM34CZ.  It
> outputs 10mv per degree F.

If you use an external ADC reference of 1.024 V (there are a lot of
those available) you will get an ADC reading with a resolution
of 0.1 F directly. No need to mul or div, just convert the 10-bit
ADC value to BCD and display. You can more or less forget
about the rest of your post...

1.024 V / 1024 ADC-steps = 1mV per ADC-step = 0.1 F.

B.t.w, have you thought about some of the temp sensors that
directly outputs a binary (digital) value ? Then you don't need
the ADC at all.

Best Regards,
Jan-Erik.

> On the LCD, I only expect one place after
> the decimal.  (I am probably lucky if I get accuracy to the nearest
> degree, but it's still nice to have that decimal place to see which
> way the temperature is trending.)  To make it simple for now, I decide
> to place limits of 0-100 degrees F.
> 
> Will this design work?:
> 
> Vcc is 5v.
> The PIC ADC is configured to use Vcc as the upper ref voltage.
> Take the 10bit ADC value, multiply it by .00488 (5/1024).
> That will result in the actual voltage.
> Multiply that value by 100 to get degrees. (or instead of of using
> .00488 in the previous step, just use .488)
> 
> How do I determine what fixed point format will work?   The ADC result
> is 10 bits so I will need at least two bytes.  How do you determine
> how many bits are required for the fractional part after the decimal?
> 
> On figuring out how many bits I need before the decimal point:
> Assuming a max temperature of 100 deg F, that results in 1V into the
> ADC (100 x 10mv).  1V in the ADC would correspond to 204 ( 1/.00488). 
> That 204, I believe will be the largest number, so 8 bits should work.
>  Is this correct logic?
> 
> Since I need to represent .488, I will need enough bits to hold 488.
> That ends up requiring 9 bits (2^9=512).  Is this the correct way to
> tell how many bits you need for the fractional part?  Maybe for
> simplicity I should use the 16x16 fixed point format?  Or maybe it can
> be determined that .49 will be accurate enough?  Although I don't know
> how to calculate how accurate that will be.  .488 itself is a rounded
> result of 5/1024*100.  Maybe .5 would be accurate enough?
> 
> On a slightly different topic, since I'm going to hard code the
> voltage per bit of the ADC, I need a stable and accurate voltage
> reference.  For simplicity's sake, I was going to use Vcc which is 5v.
>  What are some strategies to help guarantee a stable and accurate
> reference voltage for an ADC?
> 
> Thanks for all of your input.
> -
> 
> Mark Bellamy
> 
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