Post by thread:[PIC:] PIC16F87X ADC channels

Tag added While on the topic of jittery A/D conversion, will the use of an external osc provide better stability in the A/D conversion as well? I'm considering using a 16F88 for a voltmeter project that is supposed to go inside a power supply and trying to weigh the options of using the internal osc to minimize components or use external osc. I guess the answer of using an external voltage reference is answered... if I want the accuracy to be better than +/-20mV then I guess I'll use the external. My other concern is I've read that crystal osc's are susceptible to vibration... and well the transformer does get some vibration going on inside the power supply. Thoughts on using an external osc or internal in this case? Keith L. Kovala spam_OUTklkTakeThisOuTrenderedelement.com > {Original Message removed}

> Thoughts on using an external osc or internal in this case? Maybe a stupid question, but if you don't need the accuracy of an external crystal why bother? Less components, and a few bonus pins! Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

>Keith L. Kovala wrote... >While on the topic of jittery A/D conversion, will the use of an external >osc provide better stability in the A/D conversion as well? Hmmm. I doubt it. The PIC's A/D is a successive-approximation type, and I wouldn't think oscillator jitter would affect it unless it got really extreme. Clock phase noise can cause poor performance in delta-sigma A/D converters, but PICs don't use those. >...My other concern is I've read >that crystal osc's are susceptible to vibration... and well the transformer >does get some vibration going on inside the power supply. Thoughts on using >an external osc or internal in this case? Crystals can be damaged by high levels of shock, but I wouldn't think the kind of vibration you're likely to get from the PS transformer would do that. Ceramic resonators are less fragile, if you need the clock frequency accuracy; or you could just use the RC oscillator if you don't. Dave D. -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

He might have ment external crystal osc vs. internal *crystal* osc. Not the internal *RC* osc. If I read his post correctly... And the question is, will the external crystal osc interfere less with the ADC ? I think not, meybe better to sleep the PIC during the conversion. Jan-Erik. > {Original Message removed}

I remember reading an application note on Microchip's site about getting steady ADC results. I think it called for putting it in sleep mode or something before the measurement. I really don't recall right now, but do a search on their site If you get the AN post the URL to the list. If I find it I will do the same. Regards, Mike -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

Michael J. Pawlowsky wrote: > I remember reading an application note on Microchip's site > about getting steady ADC results. > I think it called for putting it in sleep mode or something > before the measurement. Actualy, most (all?) data sheets for a PIC with builtin ADC has a section headed "A/D operation during sleep" or something like that. There is says by running the A/D cinversion suring sleep, you "eliminate all digital switching noise from the conversion" (this is from the 16F87x data sheet, but the wording is similar on others). Jan-Erik. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Keith L. Kovala wrote: > While on the topic of jittery A/D conversion, will the use of an > external osc provide better stability in the A/D conversion as well? I doubt it. The important thing is that the minimum Tad time requirement is met. As long as you set up the divider so that Tad is met with a comfortable margin, but not way too long, I don't see how the clock speed makes a difference. By the way, I've found the PIC A/Ds to be very stable. If youre readings are jittery, then there is noise coming in from somewhere. Is the input signal properly filtered, of low enough impedence? Have you given attention to ground return currents of other parts of the circuit to make sure they don't cause a measurement offset? In any case, it's a good idea to read the A/D as often as you can manage, then low pass filter in software for the final answer. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, 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

> He might have ment external crystal osc vs. internal *crystal* osc. There is no internal crystal oscillator. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, 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

Olin Lathrop wrote: > > He might have ment external crystal osc vs. internal *crystal* osc. > > There is no internal crystal oscillator. I ment the canned external crustal oscillators where you just feed a clock signal into the PIC. And the internal oscillator (with uses an external crystal, yes...) Jan-Erik. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Olin Lathrop wrote: > > Keith L. Kovala wrote: > > While on the topic of jittery A/D conversion, will the use of an > > external osc provide better stability in the A/D conversion as well? > > I doubt it.... I don't see how the clock speed makes a difference. I don't think he ment different clock *speed*, but having different clock *source* (int/ext osc). Jan-Erik. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On 3 Dec 2003 at 7:33, Olin Lathrop wrote: > > He might have ment external crystal osc vs. internal *crystal* osc. > > There is no internal crystal oscillator. > Then all the specsheets I've read are wrong. You should call up all those companies and complain :-) "Internal Crystal Oscillator" has been touted in every spec sheet I've read from PICs to RF chips for those chips that have them. The term Indicates that the active device is on-chip. BRs, Mike -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> In any case, it's a good idea to read the A/D as often as you can manage, > then low pass filter in software for the final answer. I agree, in fact, any time I have an A/D conversion I always try to do this, and not just with the PIC's A/D. I've found this to be a good idea even when using external A/Ds and chips that do A/D inside them (i.e. digital temperature sensors). No matter how "clean" you make things there is always the possibility of some external temporary influence from affecting your readings slightly, this method brings some protection against this. TTYL ---------------------------------- Herbert's PIC Stuff: http://repatch.dyndns.org:8383/pic_stuff/ -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Jan-Erik Soderholm XA (TN/PAC) wrote: > I don't think he ment different clock *speed*, but having > different clock *source* (int/ext osc). He specifically mentioned jitter as an issue, which is a short term variation of the clock speed. I was trying to point out that small variations in the bit resolution time (Tad) from one bit to the next shouldn't effect the A/D conversion much. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, 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