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'[PIC]: 16F877 ADC Probs (What will it measure)'
2001\04\23@174623 by Graham North

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Sorry, can someone answer my other question?


>Hi I'm having some problems with the ADC on the 16F877. Am I correct in
>thinking that I can connect the sampling pin straight to a voltage (is this
>limited to 0 -> 5 Volts). Is some sort of protection recomended.
>

Thanks

Graham

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2001\04\23@181144 by Dale Botkin

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You can connect it to a voltage between 0 and Vdd plus or minus whatever
the datasheet says.  No series rsistance.  The datasheet also says the
source impedance should be 10K Ohms or less.

Dale

On Mon, 23 Apr 2001, Graham North wrote:

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2001\04\23@230833 by Spehro Pefhany

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At 05:10 PM 4/23/01 -0500, you wrote:
>You can connect it to a voltage between 0 and Vdd plus or minus whatever
>the datasheet says.  No series rsistance.  The datasheet also says the
>source impedance should be 10K Ohms or less.
>
>Dale

Note that this limitation (or a maximum input current limitation)
applies even if Vdd = 0.  IOW, if you apply an input voltage to the
analog input with the PIC not powered, the voltage must be within
the range -300mV < Vin < 300mV (or whatever the data sheet says),
OR the current must be limited to the maximum current allowable
on the data sheet (with some margin, maybe 10mA). Otherwise Bad
Things can happen, especially at high temperatures, involving
parasitic thyristors on the chip.

Best regards,
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2001\04\24@034255 by Roman Black

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Graham North wrote:
>
> Sorry, can someone answer my other question?
>
> >Hi I'm having some problems with the ADC on the 16F877. Am I correct in
> >thinking that I can connect the sampling pin straight to a voltage (is this
> >limited to 0 -> 5 Volts). Is some sort of protection recomended.


Hi Graham, first, your code sample showed you
were setting ADCON1 to "all pins digital", this
is a good possibility why your ADC is not working.

Re the ADC input pin, I suggest using the largest
resistor you can, this gives greater safety to the
PIC and reduces strain on it's protect diodes if
you mess up. With a signal that changes very fast,
you will need to keep the series resistor smaller
to reduce RC effects on the input pin, but 1k
should be fine for most things. For most slowly
changing analogue signals I use a 10k resistor.

If you need to measure a higher voltage, like a
12v battery for instance, just use two resistors
in a voltage divider. say a 4.7k and 2.2k gives
about 3:1 and will give you 0v to 15v measuring.
:o)
-Roman

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2001\04\24@040045 by Michael Rigby-Jones

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{Quote hidden}

10K is the maximum recommended impedance of your measurement source
(actually, my data book says 10 kW !!!).  It's a good idea to make this
value as low as possible for the most accurate conversions.  Due to the way
the internal ADC works, the pin can be loaded by varying amounts during
conversion (according to an FAE).  A high source impedance will give a
greater voltage change during conversion as well as increasing acquisistion
time.

Ideally use an op-amp to buffer and optionaly filter your analogue signal.

Regards

Mike

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