Introduction

If you're only here to use the calculator and run (no shame), its right below, you're welcome! Although if this is your first time here and you want to learn about load and stray capacitance's, how to choose the right capacitors for your specific oscillator and why it's so important, then keep reading.

Calculator

Why is it important?

When designing electronic circuits or PCB's its important to choose the correct components and the correct type of components for the specific application. Granted, this is not always uber important such as in the case of a micro-controllers pull up resistor, a 9kΩ or 10kΩ pull-up resistor will get the job done regardless.

In the case of crystals, choosing the correct capacitors to pair with your specific crystal is fairly important and will ensure that you get the most performance out of them.

You might be thinking right now "The single line diagram i have suggests 2 x 22pF capacitors for the 16MHz crystal, so i can just use the same right?". Well.. unfortunately not always. See, there is massive variation between crystals even when comparing those with the same frequency or from the same manufacturer and the capacitors you choose have to be matched to the requirements of the specific crystal you purchased.

Luckily choosing the right capacitor value is actually quite easy and once you've learnt some of the theory behind it (Or at least figure out where to get the correct input parameters), we've made a super simple calculator that will help you get your designs off the ground fast!

Equation

C1 and C2 are the two capacitors between the crystal and ground and they should both  be the same value

As you can see, the equation is fairly simple, no need pull out Wolfram Alpha or remember back to calculus 101, Phew! But what is CL and Cstray and where do you find them?

Load and Stray Capacitance

Load Capacitance (CL)

Lets start with the easy one. Load capacitance is the capacitance required to make the crystal oscillate and effectively creates an LC tank circuit with the crystals internal inductance (See our crystals guide for more). Luckily for us, the load capacitance is always listed in the crystals datasheet. If we take this NDK 12MHz Crystal as an example, we can see that the listed load capacitance is 8pF

Stray Capacitance (Cstray)

Stray capacitance is slightly more complicated to come up with although we have a pretty good rule of thumb you can use. Everything in and around your PCB including the traces and component leads have some amount of stray capacitance. The combined total of these we represent by Cstray.

To find Cstray you should prototype your PCB with all the final components, buy expensive high precision capacitance measurement equipment and go to work measuring all the stray capacitance on your board. Orrr... your could just guess!

Typically, Cstray is going to be in the realm of 2-5pF as long as you are following good PCB design principals. Keep the crystal as close as practical to the applicable pins on the micro-controller, keep track lengths to a minimum and avoid via's on the way. I've always split the difference and take Cstray as 3pF and its always worked for me (Famous last words)

Example

So we have the equation and we know where to find our input parameters, lets do a quick example. For this we'll take the NDK 12MHz crystal again and its listed 8pF as our load capacitance. We'll assume 3pF for the stray capacitance

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