Crystal load capacitor

Hi,

Please elaborate your question. What do you mean? Maybe give an example.

Klaus

If 10pF chip capacitor is required, for 8 MHz crystal, then which is the suitable package 0402, 0603, 0805 or 1206?
What is the selection criteria for package selection?

Techman_7 said:

If 10pF chip capacitor is required, for 8 MHz crystal, then which is the suitable package 0402, 0603, 0805 or 1206?
What is the selection criteria for package selection?

Click to expand...

You use the same package as the majority of your other low power components.
Today that is probably 0402 or 0603.

What effects would you expect ?

- from {ceramic ESR, ESL and crosstalk} over dynamic logic traces?
Answers {: none, none, some }

The "load" capacitors are really the circulating
AC current loop of the xtal "tank", by way of
ground. Stray C which takes energy elsewhere
should be minimized. C which can be "lumped"
with the placed component, is just that.

Package pin capacitance numbers are often
inclusive of protection and active circuitry
attached, often sandbagged rather than
dead-accurate, and XO pins often are special
in these respects - single stage and smaller
than output-buffer inverter, lighter ESD
protection, etc.

std_match said:

You use the same package as the majority of your other low power components.
Today that is probably 0402 or 0603.

Click to expand...

Is it the only cost saving by using lower package? Or any technical reason?

DC bias effect won't be applicable, pls correct me here.......

dick_freebird said:

The "load" capacitors are really the circulating
AC current loop of the xtal "tank", by way of
ground. Stray C which takes energy elsewhere
should be minimized. C which can be "lumped"
with the placed component, is just that.

Package pin capacitance numbers are often
inclusive of protection and active circuitry
attached, often sandbagged rather than
dead-accurate, and XO pins often are special
in these respects - single stage and smaller
than output-buffer inverter, lighter ESD
protection, etc.

Click to expand...

I did not understand what you want to say?
Question is simple which package is preferred for crystal external load capacitors and why?

Specific to the capacitor itself, look to ESR (loss
detracts from Q) and ESL (stands to attenuate
amplitude at higher freqs). The package probably
affects ESL more than ESR. And proximity to the
XTAL pins of "whatever", is likely more of an ESL
contributor than intra-package.

If you can get a realistic XO to run in simulation
with ideal XTAL model then you could explore
the effects of the nonidealities and set some
goalposts for the capacitor product selection.

Your requirements for manufacturability are unstated. Use any package size you want. It will still oscillate.
Next question?

D.A.(Tony)Stewart said:

Your requirements for manufacturability are unstated. Use any package size you want. It will still oscillate.
Next question?

Click to expand...

Yes, it will oscillate by any package, but load capacitors in pF are available in 0402 to 1206. I am able to manufacture with any package, so how to choose package that can give me technical justification.

Hi,

the answer is clearly given:

D.A.(Tony)Stewart said:

Use any package size you want.

Click to expand...

and:

std_match said:

You use the same package as the majority of your other low power components.
Today that is probably 0402 or 0603.

Click to expand...

10pF are 10pF .. as long as the XTAL datasheet does not give additional requirements.

Klaus

A comment elsewhere points out that the smaller package might cost less, but if its size gives an assembler problems working with it, he will charge you more, making the total cost greater.

Another factor is availability: Is either package harder to obtain?

Techman_7 said:

Yes, it will oscillate by any package, but load capacitors in pF are available in 0402 to 1206. I am able to manufacture with any package, so how to choose package that can give me technical justification.

Click to expand...

You must learn how costs are affected by volume, component size,, material, handling , automation NRE costs, prototype costs, respin design costs, low volume costs and high volume..

When I ran a prototype shop inside R&D, 603 was the smallest size that was used. If this is your first design, you may want to use 603 for manual Assy. There is no need to use 402 size unless rise times cause issues with ESL or SRF or ESR in RF or high speed SMPS or PC MOBO’s in high volume reduction of cost or density of layout.

Also inventory of stock and who is paying for it is a factor so use standard parts from whoever is making it. Thus I suggest you use 603 unless R power dissipation requires a bigger part when derated by 50%.

It may not matter to you but the ease or cost of your choices may affect others and that will affect you in making a prototype. So find out what limits your suppliers have.