WM8200
Product Preview
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.
Control
Register
10-Bit DAC
Vclamp
CLAMP
+
SW1
-
CIN
RIN
AINP
S/H
AINN
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.
Line sync
Black
level
VIDEO AT
AINN
CLAMP
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:
V
DAC = REFB + (REFT – REFB) x (0.006 +0.988 x (DAC_code)/1024)
DAC codes can range from 0 to 1023. Figure 5 shows the clamp DAC output voltage versus the
DAC code.
PP Rev 1.22 March 2002
9
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
