Chapter 4. Calculating Error
PPM Terminology
The unit ppm (parts per million) provides a number similar to error expressed with percentages, but reduces the number of decimal places required. For example, 0.001% converts to 0.00001 which is equivalent to 10 ppm. Using the ppm notation makes it easier to deal with very small deviations.
The ppm terminology is also useful for calculating PCRTC error in terms of seconds per month. Since an average month has approximately 2.63 million seconds, if the PCRTC error was 2 ppm - total error for the month would be 2 x 2.63 = 5.26 seconds.
Oscillator Error
The precision of the PCRTC is entirely dependent on the quality of the oscillator circuit. There are three sources of error in the oscillator, and understanding them will allow you to estimate the precision of the PCRTC in your application.
Calibration error
The PCRTC oscillator is calibrated at the factory to within 1 ppm (part per million) of its specified frequency at room temperature (23 ° C).
Temperature Stability
The frequency of oscillation of crystal oscillators is highly dependent on temperature. The oscillator used in the PCRTC has an extremely low temperature dependency of 5 ppm from 0 ° C to 50
°C. Since the oscillator is calibrated to 1 ppm at room temperature (23 ° C), it will only exhibit 1 ppm error if its environment is held to
this temperature. The worst case condition is if the temperature of the PCRTC is held at one of the extremes, 0 or 50 ° C. At these points, there will be an error of 5 ppm. If the temperature variation covers a smaller span, less error will be exhibited.
12 | Documentation Number PCRTC2095 Manual |
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