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PICList Thread
'Log A to D Convertors'
1998\09\07@233042 by Sean Breheny

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Hi all,

I'm pretty sure the answer to this one is no, but do any of the PICs have
internal Logarythmic ADCs? If not, can anyone recommend to me a Log ADC
which can, say, compress 4 decades into 8 bits, preferably serial and in an
8 pin package? If not, I would settle for any Log convertor recomendations.


I'm trying to use photodiodes to measure light levels which vary a great
deal and I need good resolution, even at low levels.

Thanks very much,

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
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1998\09\08@093451 by Reginald Neale

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Sean asked:

>
>I'm pretty sure the answer to this one is no, but do any of the PICs have
>internal Logarythmic ADCs? If not, can anyone recommend to me a Log ADC
>which can, say, compress 4 decades into 8 bits, preferably serial and in an
>8 pin package? If not, I would settle for any Log convertor recomendations.
>
>
>I'm trying to use photodiodes to measure light levels which vary a great
>deal and I need good resolution, even at low levels.
>

 One possibility would be to use a log op-amp ahead of the A/D. There are
 lots of examples of suitable circuits in the literature. Also, the TI
 TSL230 series detectors have programmable sensitivity and direct digital
 output. It should be possible to cycle through several levels of
 sensistivity and select the one that is mid-scale for current conditions.

 Reg Neale

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