hi joep,
thanks for explanations, indeed no magic once you know. the trick !
cheers
seb
--
Sébastien Lelong
Le 7 avr. 2010 à 20:37, Joep Suijs <[email protected]> a écrit :
Hi Seb,
This is to be expected. The adc has an internal capacitor. At
converion, it is connected to a pin for some time to 'copy' the
voltage level. The higher the input resistance, the longer this takes.
Next, the capacitor is disconnected from the pin and the actual
conversion is executed.
So if you read a pin with very high input resistance (one that is not
connected), the capacitors voltage will not (hardly) change and the
conversion result is highly influenced by the previous measurement.
No magic ;)
Joep
2010/4/7 Sebastien Lelong <[email protected]>:
Hi guys,
I've setup a sample with 2 ADC channels, for 18f4550. Both are read
regularly, results are sent through serial. ADC channel #0 is
connected to a
voltage divider (1K/1K). The other ADC channel #1 is connected to
another
voltage divider (22K/1K). In this configuration, I got the following
results:
Result #0: 508
Result #1: 1020
Result #0: 508
Result #1: 1020
...
So far so good...
Now I disconnect channel #1. Pin is floating in the air, not
connected
anymore to voltage divider... I would expect some kind of random
data on
this channel, but I got exactly the same result as the one from
channel #0:
Result #0: 508
Result #1: 508
Result #0: 508
Result #1: 508
Result #0: 508
...
Doing the opposite give the same result from channel #1 (most of
the time, I
admit):
Result #0: 1020
Result #1: 1020
Result #0: 1012 <-- this one is exceptional
Result #1: 1020
Result #0: 1020
Result #1: 1020
Result #0: 1020
Result #1: 1020
Attached is the running sample. I really don't know what's going
on. Is my
assertion about expected random values wrong ? Is there something
with PIC's
internal ?
Cheers,
Seb
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