AN2822 datasheet(12/19 Pages) STMICROELECTRONICS

12 / 19 page

High speed internal oscillator trimming

AN2822

12/19

Doc ID 14983 Rev 2

1.6

Uncertainty of calibrated HSI frequency

There are two sources of uncertainty(c) associated with this calibration method. The first one

is caused by the finite resolution of the trimming value. The second is the uncertainty of the

HSI frequency measurement used to regulate HSI output frequency using the HSI trimming

The calibrated HSI oscillator frequency, fHSIm, is not measured directly. We can start with

the equation describing the estimated HSI frequency as a function of each of the factors that

can influence it to evaluate the uncertainty of the calibrated HSI output frequency given by

the calibration method(d)

As L is a constant we can identify two uncertainty contributors. The first is a number of

counted HSI pulses, NHSI, the second is a calibration period, fcal.

These input parameters can be assumed as uncorrellated quantities.

The uncertainty of NHSI is caused by the nonsynchronous frequency of the calibration signal

and HSI oscillator frequency. The first rising edge of the L-series of calibration signal periods

can occur anywhere in the interval of one HSI signal period as well as the last rising edge

which close the counter gate, resulting in the uncertainty of counted HSI pulses equal to

±1.

Assuming uniform uncertainty distribution, the standard uncertainty of NHSI can be

evaluated using the following equation

To reduce this uncertainty, the calibration frequency should be well below the HSI frequency

(at least 1000 times lower). This is easily achieved when the mains frequency is used as a

calibration signal.

The uncertainty of the calibration signal frequency, fcal, is given by the fluctuation of the

mains frequency. Normally, the accuracy of the mains frequency,

δfcal, is better than ±1%.

However, even better accuracy can be achieved by using a different frequency source, such

as a signal generator. To reduce uncertainty from this source to a minimum, the frequency

generator which gives the best accuracy should be used instead of the mains frequency.

The standard uncertainty of calibration frequency, fcal, is expressed below assuming uniform

uncertainty distribution.

From the equation describing the estimated HSI frequency, fHSI, we can calculate the

sensitivity coefficient associated with each of the input estimates.

This analysis of the HSI calibration method is based on ISO Guide to the Expression of Uncertainty in

Measurement and ANSI/NCSL Z540-2-1997. These standards offer a current international consensus of the

methods for estimating measurement uncertainty.

Estimates of uncertainty obtained in this way are usually called Type B.

HSIm

HSI

-------------f

cal

HSI

()

-------

cal

()

cal δfcal

100 3

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