Hai,
I am new to Usenet groups. I so far used only bulletin boards.Forgive me if anything is wrong in my operating procedure. - Thanks
I need to monitor temperature from 12 Lm35s, So I read them one by one sequentially. My hardware line up is like this . Lm35 at the end of a 2 meter shielded 2+1 cable- input to a HEF4067 multiplexer - and from that it goes to a low pass sallen key 2 pole butterworth filter made around OP07 opamp The ADC is from a PIC18F452. with the intention of filtering out even AC hum i kept the cut off frequency of LPF at around 20HZ(unaware of the settling time issue of LPF)
For the software part I tried to made a interrupt in every millisecond, on the first state I select the channel through multiplexor- next interrupt I start conversion - and on next I read data. on next interrupt the same process is done for the next LM35. But the filter cannot settle at this rate so obviously values are not anywhere near expected. So I incresed the time between interrupts and somewhere in 100s of millisecond I am getting satisfactory results. I would like to know- What will be the cut off frequency for LPF which is optimum in such a setup using LM35 and long cables? I have read about some precautions to take while using cables for LM35. Actually I can afford more time between readings but I am interested to know How these things are done professionally. Thanks for reading.
Serpent
=93Not by accident, you may be sure, do the Christian Scriptures make the father of knowledge a serpent-slimy, sneaking and abominable.=94 Henry Louis Mencken