WM8200

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PP Rev 1.22 March 2002

9

DC CLAMP

The WM8200 incorporates a clamp function for restoring the DC reference level of AC coupled input

signals. When the clamp input pin is held high the internal clamp amplifier forces the voltage at

AINP to equal the clamp reference voltage, setting the DC level at AINP.

The clamp reference voltage comes from the on-chip 10-bit Clamp Level DAC by default, however it

can be applied to the AINN pin if the CLPSEL register bit is set high.

CLAMP

AINN

-

+

AINP

10-Bit DAC

SW1

Control

Register

S/H

R

IN

C

IN

Vclamp

Figure 3 Schematic of Clamp Circuitry

Figure 4 shows an example of using the clamp to restore the black level of a composite video input

AC coupled to AINN. While the clamp pin is held high, the clamp amplifier forces the voltage at AINN

to equal the clamp reference voltage, setting the DC voltage at AINN for the video black level.

VIDEO AT

AINN

CLAMP

Line sync

Black

level

Figure 4 Example Waveforms for Line-Clamping to a Video Input Black Level

If the CLAMP amplifier is not required it can be disabled for power saving purposes by setting the

CLPDIS register bit to high.

CLAMP DAC OUTPUT VOLTAGE RANGE AND LIMITS

Important: When using the internal clamp DAC in Top/Bottom or Centre Span Mode, the user must

ensure that the desired DC clamp level at AIN lies within the voltage range REFB to REFT. This is

because the clamp DAC voltage is constrained to lie within this range REFB to REFT. Specifically:

DAC codes can range from 0 to 1023. Figure 5 shows the clamp DAC output voltage versus the

DAC code.