[Fwd: Re: Ambient light sensing via LED response]

[C. Scott Ananian]

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
>>> anymore....
>>> 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.
 --scott

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