How i ken read 7.2v with pic

[Darrel Taylor]

Interesting,

I was purposely trying to keep it Under 10K to stay within the "maximum recommended impedance for analog sources".

I can see the advantage with a battery application, but would you also need to increase ADC_SAMPLEUS?
Or is it OK as long as R1 stays below 10K?
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[NavMicroSystems]

Darrel,

I use this concept in many of my designs, and with "R1" being equal or below 10k I have never seen any problem at all.

[Melanie]

For most PICs the recommended maximum input impedance is 10K... although it does vary (example 12F675 it's 2.5K). I've actually used the 12F675 with the same 10K input circuitry I use with a 16F876 and the same default 50uS settling time without problems. I don't use ADCIN but prefer to set and poll the GO/DONE bit for a completed conversion. Like with all things, it's a case of try it and see, and if it works for you - use it. For Battery-Low applications with Comparator Inputs, I've had 470K/220K R1/R2 combinations before now just to keep the current down.

[Darrel Taylor]

Great, thanks you two!

Well, I've learned my 1 thing for today. I guess I can take the rest of the day off.

But before I do that, let's see if we can finish off jetpr's question.

Assuming now that R1 can always be 10K (or 2.5K), we can forget about the guessing part and just calculate R2 with the same formula.

Vin = 7.5 <-- max Input to voltage divider
Vout = 5 <-- max Output voltage to pic
R1 = 10000

R2 = R1/(1-(Vout/Vin))-R1
R2 = 10000/(1-(5/7.5))-10000
R2 = 20000 ohms

Hmmm, Nice round numbers too.

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