Crystal specification.

[alphacat]

Hey,

In a reference design of an IC, they used the following crystal:
XTL OSC 4.0960MHZ HC49S 50ppm 50pF.
(-------- Frequency-Case--Tol.----load cap.).

Since we couldnt find such crystal on market, we used a crystal with the following spec:
4.096MHz (frequency).
20ppm (tolerance).
18pF (load capacitance).

The IC worked fine.

How come we exceeded the required spec. (18pF instead of 50pF), but it still works?
Is it something with crystals' load capacitance that has small effect on the IC's performance?

Thanks.

 

[Diver300]

The load capacitance of a crystal is nothing to do with how hard the crystal is driven. That is the crystal power, measured in microwatts.

A crystal acts like a very good tuned circuit, with an inductance and a capacitance in series. By adding a capacitor in series with the crystal, the frequency can be altered slightly. As a result, the crystal will be made to match the load capacitance that it will be used with.

If you use the wrong load capacitance, it will only shift the frequency by a few hundred ppm at most. In other words, less than 0.05%

If you don't need the best accuracy, load capacitance doesn't matter at all.

 

[alphacat]

Thanks fellow.

What could be the consequences of such small error?

I might get from the IC wrong data every once in a while?

 

[Diver300]

I have no idea what problems a frequency error would cause.

The frequency error from the wrong load capacitance would not cause the IC to function incorrectly, because the error is so small. There could only be problems when the timing of the IC needs to be close to some other accurate timing.

If you can tell us what the IC is used for, then we might be able to work out what problems it might cause.