Datasheet logical error ? - Page 1
Forum section: electronics / beginners

author: kellogs
author_rank: Regular Contributor
timestamp: December 10, 2024, 12:09:06 am
content: Hello, It seems to me that all of these chips are calling for the same type of crystal; but which load capacitance is needed,6 pF or 12 pF?Page 35QuotePage 15QuoteAfter a couple of not so useful emails exchanged with support at Analog Devices, I got pointed to the eval boards documentations, which are usinga 9pFcrystal for both radio chips... -https://www.digikey.com/en/products/detail/epson/TSX-3225-16-0000MF18X-AC0/5260855Eval board for MAX4147xEval board for MAX4146x

author: u666sa
author_rank: Frequent Contributor
timestamp: December 10, 2024, 07:52:04 am
content: You need to measure capacitance on your prototype board. Connect to legs of your cap (cap not installed) which goes to XTAL1 and XTAL2 pins, and measure capacitance using NanoVNA H4 or LiteVNA 64. The value you get is the value of your capacitor...you can temporarily solder ipex connector in place of the cap, for ease of measurements.

author: PGPG
author_rank: Frequent Contributor
timestamp: December 10, 2024, 09:56:24 am
content: I didn't tried to look what these ICs are.In both descriptions they say that from each pin they have 12pF internal capacitance to GND. So if you were able to design PCB having absolutely 0pF than you should use crystal with CL=6pF.But as you can't design such PCB you will add some extra pF so using 9pF crystal seems reasonable.If generator in IC in enough 'strong' the better frequency stability I would expect when using 16pF crystal and adding extra capacitors out of IC. Assuming that you have better control on capacitors capacity than at PCB capacity the higher capacity comes from capacitors the resulted capacity fluctuations are smaller.In my life I meet 6pF crystals only for 32768Hz. The task is to have in RTC as small supply current (in a range of hundreds nA) as possible and the smaller C you can drive it with lighter generator consuming less Icc.For higher frequencies higher CL are more common, I think.In my opinion in both descriptions there are error when they write "is typically below its specified frequency". It should say 'over', I think.

author: xvr
author_rank: Frequent Contributor
timestamp: December 10, 2024, 11:03:57 am
content: QuoteFrom crystal point of view capacitors on its pins connected in series, so it seen half of it (6pF of load capacitance for 12pF capacitors)

author: kellogs
author_rank: Regular Contributor
timestamp: December 10, 2024, 03:58:13 pm
content: Does that mean that the first quote is calling for the incorrect crystal, purely in the context of that paragraph ?

author: xvr
author_rank: Frequent Contributor
timestamp: December 10, 2024, 04:06:21 pm
content: Quote from: kellogs on December 10, 2024, 03:58:13 pmNo. It says, that chip itself provides 12pF capacitance from each of crystal pin to ground (including PCB parasitics). So total load capacitance (for crystal) will be 12/2 = 6pFIt is possible to use crystal with larger load capacitance, but in this case you should provide additional capacitors at PCB.

author: kellogs
author_rank: Regular Contributor
timestamp: December 10, 2024, 04:13:02 pm
content: Then the second datasheet is wrong. Otherwise I am totally at loss.

author: xvr
author_rank: Frequent Contributor
timestamp: December 10, 2024, 04:22:32 pm
content: Eval board includes 2 additional capacitors (5pF each), attached to crystal. So total load capacitance is 6+5/2=8.5pF This is quite aligned with 9pF required to Seiko-Epson crystal.

author: kellogs
author_rank: Regular Contributor
timestamp: December 10, 2024, 04:31:20 pm
content: I shall try an simplify things as they currently stand in my mind:First datasheet:"12 pF at XTAL pins to GND, we account for typical PCB parasitics => we choose a 6 pF crystal"Second datasheet:"12 pF at XTAL pins to GND, we account for typical PCB parasitics => we choose a 12 pF crystal"Can these be simultaneously true? What am I getting wrong ?

author: xvr
author_rank: Frequent Contributor
timestamp: December 10, 2024, 04:35:24 pm
content: What datasheets are you talking about? I think it's 2 quotes in first post, am I wrong? In both of them load capacitance is 6pF (described in the same words, exactly)

author: kellogs
author_rank: Regular Contributor
timestamp: December 10, 2024, 04:39:59 pm
content: Hope I am not getting trolled.first quote from first linked datasheet"Accounting for typical board parasitics, a 16MHz crystal with 6pF specified CL is recommended."second quote from second linked datasheet"Accounting for typical board parasitics, a 16MHz, 12pF crystal is recommended."

author: xvr
author_rank: Frequent Contributor
timestamp: December 10, 2024, 04:43:59 pm
content: First quote:QuoteSecond:QuoteAaa, I see. Last sentence. I think it just typo.