Post by thread:[PIC]: Floating analog input causing trouble?

Yes, I did something careless yesterday... I have an external analog source 0-4.1 V that I have to use a clip lead to connect the output of one circuit into the PIC's A/D input. I have a 4.096 Vref into the PIC's Vref. However, in my haste to get a test going, I accidentally forgot to clip the analog source into the PIC's A/D input pin, so it was just left floating. Later, I noticed that my Vref is no longer accurate at 4.096, but instead is around 4.80 V. My question is: could I have partially burned out the voltage reference by leaving that pin floating? The code always attempts to read the pin, so yes, the PIC was actively trying to read the input. Not being an EE myself, I don't really understand what happens in these circumstances, I just try not to do things like this :) --Andrew _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

On Sun, 20 May 2001, Drew Vassallo wrote: > My question is: could I have partially burned out the voltage reference by > leaving that pin floating? The code always attempts to read the pin, so > yes, the PIC was actively trying to read the input. Not being an EE myself, > I don't really understand what happens in these circumstances, I just try > not to do things like this :) > I think not ( if I understand well your question ) Your reference voltage chip may be defective or you haven't use correct connections and decoupling capacitors. I've done a lot of tests with 877 analogic inputs including negative and out of spec experiments and never happent such damage, nor with external reference chips or with analogic inputs even if where floating. Check if you have used decoupling capacitors on the reference output ( and pic reference input ) this may cause troubles or if you accidentaly program pic vref as digital output. Take care, Vasile -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

> However, in my haste to get a test going, I accidentally forgot to clip the > analog source into the PIC's A/D input pin, so it was just left floating. > Later, I noticed that my Vref is no longer accurate at 4.096, but instead is > around 4.80 V. I can't say whether the PIC got damaged, but it is certainly possible. A PIC A/D input can tolerate up to 10K ohms of source impedance and still maintain accuracy. I would put a resistor in series with the PIC input to limit the current in case of connection to an out of range voltage. A total source impedance at the PIC pin of a bit under 10K (5k - 8K) is a good target. If the source has too high an impedance so that the extra resistor would come out to less than 3K or so, you should buffer the signal thru an op amp. That will also allow you to add more ESD protection without cutting into normal signals. If you do this, remember that the A/D precharge time goes up with the source impedance at the A/D pin. ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, olinKILLspamembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

> > However, in my haste to get a test going, I accidentally forgot to clip > the > > analog source into the PIC's A/D input pin, so it was just left floating. > > Later, I noticed that my Vref is no longer accurate at 4.096, but instead > is > > around 4.80 V. > > I can't say whether the PIC got damaged, but it is certainly possible. A I must be stupid, but I don't understand how leaving an input pin floating could "damage" the PIC. Perhaps you could enlighten me on this. -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

> > I can't say whether the PIC got damaged, but it is certainly possible. A > > I must be stupid, but I don't understand how leaving an input pin floating > could "damage" the PIC. Perhaps you could enlighten me on this. The floating part isn't the problem. The damage can be done when the floating wire is eventually connected to something. This is like jumping off a tall building. The falling part is rather harmless. It's the sudden stop at the end that kills you. He did say there was an allegator clip at the end. Even a small static charge is several thousand volts. If that has enough capacitance back to the PIC power supply, then you can get enough current for long enough to fry something. ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, olinspam_OUTembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads