AN1048

LCD drive signals

9/21

2.3.2

Contrast calculation

The performance of an LCD driving system is defined by the contrast:

Contrast(D) = Vrms(ON) / Vrms(Off)

For the quadruplex signal as described on the previous page:

Contrast(D) = Vrms(ON) / Vrms(OFF) = 0.661Vcc / 0.43Vcc = 1.52

Vrms ON

()

1

T

--- f t

()

2

t

d

0

T

∫

=

Vrms ON

()

1

T

---

Vcc

()

2

t

d

0

T

8

---

∫

Vcc

()

2

t

d

T

8

---

2T

8

-------

∫

Vcc

2

----------

⎝⎠

⎛⎞

2

t

d

2T

8

-------

T

∫

++

=

Vrms ON

()

1

T

()

2 T

8

---

Vcc

()

2 T

8

---

Vcc

()

2

4

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

6T

() 8

⁄

⋅

+

⋅

+

⋅

⎝⎠

⎜⎟

⎛⎞

=

Vrms ON

()

7

16

------ Vcc

()

2

=

Vrms ON

()

0.661Vcc

=

Vrms OFF

()

1

T

---

0

() t

d

0

T

8

---

∫

0

() t

d

T

8

---

2T

8

-------

∫

Vcc

2

----------

⎝⎠

⎛⎞

2

t

d

2T

8

-------

T

∫

++

=

Vrms OFF

()

3

16

------ Vcc

()

2

=

Vrms OFF

()

0.43Vcc

=