Hello,
I’ve been doing some layout and I decided i’d like to use the SMD crystals show here:
http://www.sparkfun.com/commerce/produc … ts_id=7846
I’ve looked at the data sheets and it does not appear that I need to have any caps on the crystals… is this correct or should I still add in some small 39pF caps?
Thanks
According to the data sheet, it is just a crystal. Whether or not you need caps depends on what you are doing with it. If you are using it with your standard inverter-based oscillator in most ICs, you will need to add the two external caps.
directgumby:
I’ve looked at the data sheets and it does not appear that I need to have any caps on the crystals… is this correct or should I still add in some small 39pF caps?
Greetings directgumby,
Yes, as long as the total capcitance on the crystal is
12pF, as stated in the datasheet.
The crystal manufacturer used 12pF as the calibration
capacitance, and other values will pull the crystal
off frequenmcy, which may be important to your
application.
Also, too much capacitance will load the oscillator,
draw more power, possibly stall the oscillator, or
prevent it from starting up at power on.
39pF caps seems very high, how did you arrive
at that value? Depending upon layout and the
package for the IC with the active oscillator, you
may only require 4 to 6pF on each pin to get a
net of 12pF from crystal pin to crystal pin.
Remember 1/C1 + 1/C2 = 1/ctotal for series
connected capacitors.
Comments Welcome!
Well, to be honest I was looking at a few refernce designs and noticed that they used that CAP… so, not a great way to derive the value… thus, that is why I was asking both if I needed it and secondarily you were nice enough to point out what I should be using…
So, thank you very much for the help and information!
Brett
I forgot to mention… I did go back and look at the data sheet again… and yes there is was plain as day to use the 12pF cap… thanks again for the help!
Brett