Post by thread:[PIC:] LT1790 Voltage Reference problems (was also

Thanks again to Jan for the correction on the operation of the PIC PORTs. I made this change (to tri-state the PIC pins instead of forcing them low) and still the original problem exists. That is - if I sample every 1s (2s is borderline - sometimes works, sometimes doesn't (right now I tried and it worked at least for 15 samples)) I miss every other sample because the reference voltage doesn't come on properly. I have taken thousands (I think it was just over 9k) of consecutive samples with a 5s interval with no errors. I noticed when looking at the PIC pin that powers the reference IC with the sampling interval set to 1s that there are two alternating pulse shapes. Both quickly rise to 5V (looking at 50ms/div) and hold for about 25ms but then one will decay gradually from the full voltage with a time constant of ~50-75ms while the next will quickly decay to about half voltage and then show an exponential decay from that point. When I set the sample interval to 5s (and everything works properly) I see the second waveform on the PIC pin that powers the reference (drops to half voltage then exponential decay from there). So it seems that the variance in waveforms is just a symptom of the fact that the reference isn't delivering the proper voltage and thereby is drawing less current and discharges the cap more slowly. But why would this happen? Even with the sample interval set to 1s and tri-stating the PIC pin that powers the reference rather than forcing it low, the power to the reference goes to 0 for at least a couple of hundred ms between samples. I have checked the LT1790 datasheet ( http://www.linear.com/pdf/1790fa.pdf ) a few times and don't see anything regarding this issue. If no one has any ideas by the time I finish dinner I guess I'll try replacing the reference IC. This is my first SMD project and I have fixed a few wacky problems like this by replacing the affected IC. I am blaming it on overheating the chips when I solder as the number of problems I am having seems to diminish as I use more and more caution and develop better soldering technique. I would love to hear any other ideas though as I have already replaced the reference IC a few days ago for a different problem (which was fixed by the replacement) and I am reasonably confident that I did a good and gentle job soldering this last time. The intended application of this unit will likely use sample intervals in the range of tens of minutes so I could potentially just make 5s the minimum in software, but I'd rather get this figured out in case it is symptomatic of a larger problem. Thanks, Nick {Original Message removed}

Hi ! James Nick Sears wrote : > Thanks again to Jan for the correction on the operation of > the PIC PORTs. I made this change (to tri-state the PIC > pins instead of forcing them low) and still the original > problem exists. That is - if I sample every 1s (2s is > borderline - sometimes works, sometimes doesn't (right now I > tried and it worked at least for 15 samples)) I miss every > other sample because the reference voltage doesn't come on > properly. I have taken thousands (I think it was just over > 9k) of consecutive samples with a 5s interval with no errors. Sampling ? I must have completly missed what you actualy are trying to do and just focused on the PORT/TRIS thing... So your PIC pin is connected to the input side of a LT1790 voltage reference ? Connected according to the picture on page 1 in the data sheet ? With a 0.1uF cap on the same side as the PIC and a 1.0uF cap on the output side ? What else is connected to the output side of the LT1790 ? What is a "sample" ? > I noticed when looking at the PIC pin that powers the > reference IC with the sampling interval set to 1s that > there are two alternating pulse shapes. > Both quickly rise to 5V (looking at 50ms/div) and hold for > about 25ms but then one will decay gradually from the full > voltage with a time constant of ~50-75ms while the next will > quickly decay to about half voltage and then show an > exponential decay from that point. Sounds as your PIC pin isn't held as an active high for more then about 25 ms. After that it's probably set high-Z and you are seeing the decay of the charge in the cap on the input side of the LT1790 (if connected according to the data sheet). If the PIC pin was to be set *active* low, the cap would discarge into the PIC pin. This might not be a good thing. I'd protect the PIC pin from exessive current to/from the cap using a series resistor. {Quote hidden}> > When I set the sample interval to 5s (and everything works > properly) I see the second waveform on the PIC pin that > powers the reference (drops to half voltage then exponential > decay from there). > > So it seems that the variance in waveforms is just a symptom > of the fact > that the reference isn't delivering the proper voltage and thereby is > drawing less current and discharges the cap more slowly. Yes, but is the PIC pin still high ? Should it be ? > But why would this happen? Even with the sample interval set > to 1s and tri-stating the PIC pin that powers the reference > rather than forcing it low, the power to the reference goes > to 0 for at least a couple of hundred ms between samples. And what *should* happen here ? Should it go to 0 for a longer period ? A shorter period ? Or maybe not at all ? > I have checked the LT1790 datasheet ( > http://www.linear.com/pdf/1790fa.pdf ) a few times and don't > see anything regarding this issue. To me at least, the "issue" isn't very clear... You have olny said what actually happens, not what you'd *want* to happen. I must have missed something at the very beginning... :-) :-) Regards Jan-Erik. -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

----- Original Message ----- From: "Jan-Erik Soderholm XA (TN/PAC)" <jan-erik.xa.soderholmKILLspamERICSSON.COM> To: <.....PICLISTKILLspam.....MITVMA.MIT.EDU> Sent: Sunday, January 11, 2004 7:31 PM Subject: Re: [PICLIST] [PIC:] LT1790 Voltage Reference problems (was also RS232 Driv er IC Supply Noise) > Sorry !! Just saw the changed subject ! > I havn't seen the "LT1790..." subject before. > Maybe this thread was run with something else then the > [PIC] tag first ? Anyway, I can't find the original post > about the LT1790... Jan-Erik I accidentally replied to a message in the wrong thread that started the labelling error. I am sampling a temperature sensor connected to AN0 with the LT1790 connected to Vref+ with Vref- pulled to ground. The LT1790 is powered from a PIC pin (RD3). I power up the reference when I am taking a sample and leave it powered down (RD3 tri-stated) the rest of the time. From here I think the problem should be pretty well defined in the last post you replied to. Basically the LT1790 will not put out the proper 1.25V output if it is powered up every second and will fail (output barely rises) every other second. Thanks, Nick -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Sun, 11 Jan 2004 20:19:16 -0600, James Nick Sears <EraseMEjsears2027spam_OUTTakeThisOuTHOTMAIL.COM> wrote: > > From here I think the problem should be pretty well defined in the last > post > you replied to. Basically the LT1790 will not put out the proper 1.25V > output if it is powered up every second and will fail (output barely > rises) > every other second. > > Thanks, > Nick > It should however.. you shouldn't be attempting to turn it on every second as acording the page 18 of the data sheet it has a 685ms turn on time. What this means is it must be on 70% of the time at least and you should sample at the end of the 70% turn it off wait 30% of the time then turn it on so the supply settles. Turn on time will vary depending on what capacitor you placed on it, acording to the sheet it's 1uF optimized. If 50 to 80uA for a micropower reference isn't a big deal, I suggest leaving it on and saving yourself some suffering. Of course if you plan to use the pin for something else, that might be a problem. -- Stephen R. Phillips was here -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

----- Original Message ----- From: "Stephen R. Phillips" <cybermanspam_OUTSOCKET.NET> To: <@spam@PICLISTKILLspamMITVMA.MIT.EDU> Sent: Wednesday, January 14, 2004 10:55 AM Subject: Re: [PICLIST] [PIC:] LT1790 Voltage Reference problems (was also RS232 Driv er IC Supply Noise) On Sun, 11 Jan 2004 20:19:16 -0600, James Nick Sears <KILLspamjsears2027KILLspamHOTMAIL.COM> wrote: >> >> From here I think the problem should be pretty well defined in the last >> post >> you replied to. Basically the LT1790 will not put out the proper 1.25V >> output if it is powered up every second and will fail (output barely >> rises) >> every other second. >> >> Thanks, >> Nick > >It should however.. you shouldn't be attempting to turn it on every >second >as acording the page 18 of the data sheet it has a 685ms turn on >time. Thanks for the idea but the turn on time is actually specified in us. It is hard to read from the plot on p.18 but on p.3 it is listed in the data table as 250us for the 1.25V model. >What this means is it must be on 70% of the time at least and you >should >sample at the end of the 70% turn it off wait 30% of the time then >turn it >on so the supply settles. Turn on time will vary depending on what >capacitor you placed on it, acording to the sheet it's 1uF optimized. >If >50 to 80uA for a micropower reference isn't a big deal, I suggest >leaving >it on and saving yourself some suffering. Of course if you plan to use >the >pin for something else, that might be a problem. My pic sleeps most of the time, so draining 50 - 80uA constantly is pretty significant. It seems possible that the problem begins when the reference is turned on before the output capacitor on the reference has fully discharged. I thought about reconfiguring the Vref+ input to an output driving low to discharge the cap before firing up the reference to see but unless I get some extra time or a better idea of what is the problem I am just going to move on and use 5s as a minimum sample interval. Thanks again, Nick -- Stephen R. Phillips was here -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.