Post by thread:[PIC]: AD reading problems 16f877

Hi! I have a National LM35CZ (Precision Centigrade Temperature Sensor) connected to AN0 of my 16f877. When looking at the values with my DVM on the output from the LM, everything is stable. Putting a voltage divider pot on AN0 and reading the values, everything is great. However, putting the LM35 on AN0 and reading make the values kinda "fuzzy". Normal room temepratur should give me something like 50 from AN0 which is what I get sometimes, the problem is that reading AN0 over and over, the value I get pend from 47 to 55. I thought the LM35 couldn't devliver enuff ampere so I put a LM324 between it an AN0, same result. Im a newbie, did I miss anything? /frax -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

In a message dated 08/24/2002 18:24:03 Eastern Daylight Time, .....fredrik.axteliusKILLspam@spam@TELIA.COM writes: {Quote hidden}> However, putting the LM35 on AN0 and reading make the values kinda "fuzzy". > Normal room temepratur should give me something like 50 from AN0 which is > what I get sometimes, the problem is that reading AN0 over and over, the > value I get pend from 47 to 55. > > I thought the LM35 couldn't devliver enuff ampere so I put a LM324 between > it an AN0, same result. > > Im a newbie, did I miss anything? > > Make sure you have a 1.2K resistor in series with the LM and a .1ufd from the AD pin to ground. Sid -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

>> I have a National LM35CZ (Precision Centigrade Temperature Sensor) connected to AN0 of my 16f877. When looking at the values with my DVM on the output from the LM, everything is stable. Putting a voltage divider pot on AN0 and reading the values, everything is great. However, putting the LM35 on AN0 and reading make the values kinda "fuzzy". Normal room temepratur should give me something like 50 from AN0 which is what I get sometimes, the problem is that reading AN0 over and over, the value I get pend from 47 to 55. I thought the LM35 couldn't devliver enuff ampere so I put a LM324 between it an AN0, same result. << Sounds like noise, but it's hard to guess which of the many possible sources of noise is causing the trouble. Look at the A/D pin voltage with a scope to see what the noise looks like. What does the ground connection from the sensor back to the PIC look like? How long is the wire? What type of cable, twisted pair, runing along something else? Fortunately temperature changes slowly, so you can low pass filter the crap out of it and get rid of most noise that way. However, it is good to keep as much noise as possible out of the circuit before getting rid of it with a filter. A simple R/C with a 1/2 Hz rollof should do a lot. I almost always use software filtering with the A/D too. Again, you need to start with a clean design. Actually the noise should be quite small. A filter should only be for cleaning up the last little bit, not for compensating for bad grounding or whatever. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

Your "fuzzy" results sounds like the LM35 is not being loaded. The app notes for it show a load resistor, anything from 200R to 10k, and also RC or series R for capacitive loads (eg long cables). It's got 0R1 impedance at 1mA o/p so it should have enough oomph for the PIC's AD to read. More info here (303kB) eehttp://www.eng.ohio-state.edu/~passino/LM35.pdf -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

At 12:12 AM 8/25/02 +0200, you wrote: >Hi! > >I have a National LM35CZ (Precision Centigrade Temperature Sensor) >connected to AN0 of my 16f877. >When looking at the values with my DVM on the output from the LM, >everything is stable. Putting a voltage divider pot on AN0 and reading the >values, everything is great. > >However, putting the LM35 on AN0 and reading make the values kinda "fuzzy". >Normal room temepratur should give me something like 50 from AN0 which is >what I get sometimes, the problem is that reading AN0 over and over, the >value I get pend from 47 to 55. If you have more than a couple inches of cable, put a resistor on the output of the LM35 (or any other similar voltage-output device) BEFORE the CABLE (at the pin of the device). More than a small number of pF load can cause stability issues. Put some more resistance at the other end of the cable and a small cap to analog ground. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speffspam_OUTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu

On Sun, 25 Aug 2002, Fredrik Axtelius wrote: >Hi! > >I have a National LM35CZ (Precision Centigrade Temperature Sensor) connected to AN0 of my 16f877. >When looking at the values with my DVM on the output from the LM, everything is stable. Putting a voltage divider pot on AN0 and reading the values, everything is great. > >However, putting the LM35 on AN0 and reading make the values kinda "fuzzy". >Normal room temepratur should give me something like 50 from AN0 which is what I get sometimes, the problem is that reading AN0 over and over, the value I get pend from 47 to 55. > >I thought the LM35 couldn't devliver enuff ampere so I put a LM324 between it an AN0, same result. > >Im a newbie, did I miss anything? Check your grounds and add a decoupling capacitor across the LM35 pins. Use a shielded lead if it is longer than ~30cm. You can also make the LM324 buffer a lowpass by changing the feedback network. Peter -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> Check your grounds and add a decoupling capacitor across the LM35 pins. I meant across the PIC pins where the signal from the LM35 enters. >Use a shielded lead if it is longer than ~30cm. You can also make the >LM324 buffer a lowpass by changing the feedback network. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads