[Fwd: Re: Ambient light sensing via LED response]
C. Scott Ananian
cscott at laptop.org
Sun May 10 17:13:47 EDT 2009
On Sun, May 10, 2009 at 5:01 PM, Reinder de Haan <rs at mveas.com> wrote:
> C. Scott Ananian wrote:
>> On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan <rs at mveas.com> wrote:
>>>> Absolutely not. The A/D is eight bits, with an input range spanning
>>>> 0 - 3.3V, so the best you
>>>> can hope for is about 13 mV per LSB. I would guess actual accuracy
>>>> to be closer to 26 mV.
>> I think the actual A/D reference voltage is probably *maximum* 3.3V.
>> What's the *minimum* A/D reference voltage? And can the A/D measure
>> all the way down to ground? (Sometimes there's a Vmin for the A/D
>> input, often around a diode drop above ground.)
>>> i did some more experiments and at ~20 cm from the halogen lamp i doest
>>> matter is i turn the backlight full on of off .. i dont have color
>>> at that distance the bare led gave about 250mv (maybe a bit more into a
>>> very high R fet gate...)
> that 250mv is without the light guide and lcd cover installed!
> with the light guide in place its more like ~70mv ...
> 0.07/3.3*256= 5.4 LSB and then the Rin of the ADC must be >10Mohm...
5 LSB is still okay, and it could be more if the A/D Vref can be
dropped below 3.3V.
( http://cscott.net/ )
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