More times than not when talking to a customer about clock accuracy and I mention a spec in units of parts per million (PPM) the response is, “Huh? What’s PPM?” Fair enough, but first some background:
Behind every great clock there’s a crystal, a piezoelectric device that vibrates at a precise and known frequency. There are other ways to generate frequencies (a resistor and capacitor combination is one of them), but none are more accurate.
Many of our data logger products provide a built-in date and time clock that the instrument uses to time and date stamp recorded data. If you record temperature and humidity, for example, you’ll be able to determine the date and time of occurrence to a precision that is determined by the accuracy of crystal that drives the date-and-time chip that’s embedded in the instrument.
For reasons known only to crystal manufacturers, crystal accuracy is speced in units of PPM. Lower PPM crystals cost more than higher PPM, and manufacturers like us who use crystals in our products make a price/performance judgement call and then simply spec time-and-date clock accuracy at whatever PPM number is associated with the choice. So how do you use PPM to put the figure into the context of your application? I’ll answer that with an example.
The de facto standard in the industry for crystal inaccuracy is 20 PPM, which is always interpreted as a plus or minus number (±20 PPM). In a general sense, for this inaccuracy figure we can state that after 1 million actual parts, the registered number may be 999,980 to 1,000,020. In the context of a date and time clock, “parts” can be anything that you want it to be: days, hours, minutes, but most likely seconds since it doesn’t make sense to spec inaccuracy over 1 million days (270+ centuries). So, after 11 days, 13 hours, 46 minutes, and 40 seconds (i.e. 1,000,000 seconds) the date-and-time chip driven by the ±20 PPM crystal will register an actual time of this value, ±20 seconds.
You can also express PPM as a percentage: ±20/1,000,000 = ±0.002%. So after 30 days (2,592,000 seconds) we can expect the clock to drift about ±52 seconds; after 60 days about ±104 seconds, and so on.