----- Original Message ----- From: "Stephen R. Phillips" To: 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 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.