AN2648

STLM20 temperature-to-output transfer function

9/15

Equation 4

This has the form:

Equation 4b

But, rather than using this equation, it is better to filter the temperature, to average multiple

samples together for display. This has two benefits. It smoothes the changes resulting in a

displayed value which updates more gradually. And it increases the effective resolution of

the sensor. By adding two more bits, this essentially makes the A/D appear to have 10-bit

resolution.

So, we use a modified version of the previous math, but sum four consecutive samples

together:

T

R1

R3

0.01169

R1

R3

1.8663

R1

R2)

(R4

R4

R1)

R3

(

V2

255

V

V

CC

ADC

⋅

⋅

=

⋅

+

+

+

⋅

−

⋅

T

R3

R1

R1

R3

0.01169

R3

R1

R1

R3

1.8663

R3

R1

R1

R2)

(R4

R4

R1)

R3

(

V2

R3

R1

255

V

V

CC

ADC

⋅

⋅

⋅

=

⋅

⋅

+

⋅

+

+

⋅

−

⋅

⋅

T

0.01169

1.8663

R3

R2)

(R4

R4

R1)

R3

(

V2

R3

R1

255

V

V

CC

ADC

⋅

=

+

+

+

⋅

−

⋅

⋅

0.01169

1.8663

0.01169

1

R3

R2)

(R4

R4

R1)

R3

(

V2

0.01169

1

R3

R1

255

V

V

T

T

CC

ADC

MEASA

+

⋅

+

+

⋅

−

⋅

⋅

⋅

=

=

O

S

ADC

MEASA

K

K

V

T

+

⋅

=

V

V

255

INT

V

OUT

CC

ADC

⎥

⎦

⎤

⎢

⎣

⎡

=

T

R1

R3

0.01169

R1

R3

1.8663

R1

R2)

(R4

R4

R1)

R3

(

V2

V

255

INT

V

CC

ADC

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

⋅

+

⋅

−

+

+

⋅

=