View previous topic :: View next topic |
Author |
Message |
ferkar
Joined: 14 Jul 2007 Posts: 38
|
LM35 sensor affecting the other analog readings. why? |
Posted: Thu Mar 13, 2008 2:35 am |
|
|
hi everyone,
after 2 weeks working on my project i could not improve on lm35 problem.
here is my (or pic's) problem.
At analog channels AN0 and AN1 there is a potansiometer(4,7K) and a lm35 sensor. nad i am displaying these data on a LCD.
when i energize the my sysytem i see that lm35 readings floats . and if i turn the potansiometer it affects lm35 readings.
it means analog channels affect each others.
by the way if i remove lm35 and replace it by another pot everything works well.!
so i am so confused ... |
|
|
Ttelmah Guest
|
|
Posted: Thu Mar 13, 2008 3:27 am |
|
|
How long are you waiting between changing analog channels?.
The ADC input on the PIC, is effectively a capacitor. This take _time_ to charge to an applied voltage. How long, depends on the impedance of the source, and what voltage was last applied. If you switch too quickly between channels, the last voltage on the capacitor, will affect the new reading...
Best Wishes |
|
|
ferkar
Joined: 14 Jul 2007 Posts: 38
|
a bit more lm35 |
Posted: Thu Mar 13, 2008 5:18 am |
|
|
my adc setup: clock_dıv_by_32
waiting between channel selection 100us
Vref+=5V
Vref-=GND
10mV< lm35 output<1500mV
and the main question:
why did i get true results from 2 different potansiometers unlike the results from 1 potonsiometer and lm35?
|
|
|
SherpaDoug
Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA
|
|
Posted: Thu Mar 13, 2008 7:31 am |
|
|
Can you tell if the LM35 might be oscillating? Make sure you are using the bypass caps recommended in the LM35 data sheet. If it is oscillating at very high frequencies it could cause strange effects. _________________ The search for better is endless. Instead simply find very good and get the job done. |
|
|
RLScott
Joined: 10 Jul 2007 Posts: 465
|
Re: a bit more lm35 |
Posted: Thu Mar 13, 2008 10:28 am |
|
|
ferkar wrote: |
why did i get true results from 2 different potansiometers unlike the results from 1 potonsiometer and lm35?
|
The results depend on the driving impedance of the voltage source. Now the LM35 has very low output impedance, so it should be no problem. But if you have connected it with a resistor in series for some reason, that will degrade the output impedance and require a longer settling time in the A/D converter.
Robert Scott
Real-Time Specialties |
|
|
|