AN2822

High speed internal oscillator trimming

Doc ID 14983 Rev 2

13/19

The combined standard uncertainty of the HSI frequency measurement can be calculated

by the equation bellow.

When the calibration is finished, the oscillator output frequency will be at maximum

±1/2 of

the frequency trimming step,

The total combined uncertainty of the HSI frequency can be expressed using the equation

below.

Example

frequency 16 MHz. Evaluation of the uncertainty of input quantities and sensitivity

coefficients are the following.

e.

Assuming a uniform distribution of the HSI output frequency uncertainty.

c

N

HSI

N

HSI

∂

∂f

HSIm

f

cal

L

--------

==

c

f

cal

f

cal

∂

∂f

HSIm

N

HSI

L

-------------

==

u

c

2

f

HSIm

()

c

i

2

ui

()

2

i

∑

f

cal

L

--------

⎝⎠

⎛⎞

2

1

3

-------

⎝⎠

N

HSI

L

-------------

⎝⎠

⎛⎞

100 3

-----------------------

⎝⎠

⎛⎞

2

+

==

uf

trim

()

Δf

trim

23

--------------

=

uf

HSI

()

u

2

f

trim

() u

c

2

f

HSIm

()

+

=

uN

HSI

()

1

3

-------

1.732

≅

=

uf

cal

()

f

100 3

-----------------------

50

100 3

-----------------

0.286

≅

==

c

N

HSI

N

HSI

∂

∂f

HSIm

f

cal

L

--------

12.5

≅

==