Xtal load caps

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I'm working on a project that has a xtal in a similar configuration to the ones on an AVR, with  load caps on either side.

 

The DS for the xtal states Load Capacitance as 12pf, but the circuit has a 30pf cap on each side of the xtal for c1,c2.

The DS for the chip shows C1, C2, but only states the Load Capacitance, not the values for c1, c2......

Given the LC,  what should the values of c1,c2 be???

a) 1/2 of LC each

b) value of LC each

c) 2x value of LC each

See diagram below,

 

Thanks

 

Jim

 

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The net LOAD capacitance is the SERIES combination of the capacitors to ground at each end of the crystal (plus IC stray capacitance). I usually figure strays around 3pf to 7pf. So, with 30pf capacitors, the crystal sees a nominal ( 5pf plus (30pf/2) ) or about 20pf total.

 

Most modern crystals expect 12pf-15pf load capacitance, as your spec sheet shows.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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There is a nice discussion on the load capacitors in the App Note AVR042.

 

Bottom line:

 

Each of the external caps has a value of C.

C = (2*Cload) - Cstray.

 

So, for example, assume 10 pF for the layout's stray capacitance.

Cload = 10 pF.

 

C = (20-10) = 10 pF.

 

With the +/- 2pF on the Load cap, using an external cap of 12 pF makes good sense.

 

JC

 

 

 

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Thanks guys, just what I needed!

 

Jim

 

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From the SAMD21 Xplain schematic:

 

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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ki0bk wrote:

Thanks guys, just what I needed!

.. and remember to check the actual Osc value on a few units, when done.

 

Your spec mentions a total ppm budget of ±50ppm, so you want to check you are in the middle of that, not off to one side.

That also likely means choosing a crystal that is better than ±50ppm - plenty out there of ±10ppm,±20ppm,±25ppm,±30ppm etc

 

For finer adjustments, you can also skew the C values slightly.

 

If there is no easy divided-osc probe pin/UART signal etc, a ~10k resistor to the  Oscillator driver pin can help isolate the Scope/Counter probe C, and give a better measurement.

 

Missing from the spec is tolerated ESR, one detail some overlook, is that smaller SMD Crystals, have worse ESR values.