Crystal load capacitance and using the wrong caps - Page 1
Forum section: electronics / beginners

author: PeteAU
author_rank: Contributor
timestamp: December 25, 2013, 08:16:31 am
content: Hi Guys,I reordered some crystals for an existing design but this time they're a different brand/make and I just realised a totally different load capacitance of 30pF. (Element14's product page says so, but it's not in the datasheet per se, depends on manufacturer order code?).I'm using these crystals with a microcontroller and 27pF caps, which I realise are now the wrong value.The crystal wants to see 30pF and I'm giving it ((27*27)/27+27)+5 = ~ 19pF.Can I get away with it? How bad will it be? If I build 100 units, could some of them refuse to fire up?

author: mrkev
author_rank: Regular Contributor
timestamp: December 25, 2013, 12:55:52 pm
content: Differences of few % in this case doesn't matter. Those capacitors don't need to be very precise, because they are used just as "kind of load", lowering the amplitude on xtal. So 27pF vs 30pF is completely OK. You could check on oscilloscope what would f.e. 10pF vs 47pF do.Quote from: PeteAU on December 25, 2013, 08:16:31 amI don't really get where you came up with this value. If it's standard connection on each side of xtal, you can't calculate it like this...

author: Unknown
author_rank: Unknown
timestamp: December 25, 2013, 03:19:22 pm
content: Quote from: mrkev on December 25, 2013, 12:55:52 pmNote that even a good 10x scope probe will have a capacitance of approximately 10 pF.Quote from: mrkev on December 25, 2013, 12:55:52 pmSeems pretty standard to me. Comparethis appnote, page 2. Note that Fig. 6 gives a typical curve of load capacitance vs (relative) frequency.

author: Hatimoooo
author_rank: Newbie
timestamp: December 25, 2013, 09:23:43 pm
content: Hello.You can check your microcontroller's datasheet. I've seen some before and they offer an interval for crystal capacitors, check if yours are in the interval

author: mrkev
author_rank: Regular Contributor
timestamp: December 25, 2013, 11:51:20 pm
content: Quote from: alm on December 25, 2013, 03:19:22 pmWell ok, my fault. I didn't get the jump from parallel capacitors to overall load capacity (as it's not really clear if those 30pF are ment to be load capacitance or two capacitors that this manufacturer recommend to use).

author: PeteAU
author_rank: Contributor
timestamp: December 26, 2013, 12:03:47 am
content: I think my post wasn't very clearHere is the excerpt from farnell.So from that, I assume I _should_ be using two 48pF caps, but that seems really high, you don't see that every day. Am I on the wrong track?

author: mariush
author_rank: Super Contributor
timestamp: December 26, 2013, 01:51:03 am
content: Where do you see 54pF, how do you figure that out? It says right there, 30 pF load capacitance...

author: sleemanj
author_rank: Super Contributor
timestamp: December 26, 2013, 02:44:40 am
content: Quote from: mariush on December 26, 2013, 01:51:03 amIndeed, according to the formula in the appnote alm linked for calculating load capacitance,Quote from: alm on December 25, 2013, 03:19:22 pmin order to get a 30 pF load capacitance, one should have 50pF capacitors.

author: mrkev
author_rank: Regular Contributor
timestamp: December 26, 2013, 10:31:50 am
content: Well, I would do this. Take one board with 27pF caps, check the frequency after the oscilator (if you have uC, just use Timer/counter output or something) with good log. counter. Then mout another 27pF caps at the top of caps that are already there (so they are parallel). Check frequency one more time. If you see big improvement, change the caps; if you don't see any improvement, don't bother...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 26, 2013, 12:02:32 pm
content: A good guideline is that a "happy" xtal oscillator will have a similar amplitude on each pin. Remember you need to use a low capacitance scope probe to test thos - x100 probe, or put a 10M resistor on the end of a x10 probe