XRD9825
13
Rev. 1.00
Figure 7. CIS AC Coupled Application
Mode 2. AC Coupled
If the CIS signal has a black reference for the video
signal, an external capacitor C
EXT
is used. When
CLAMP (clamp) pin is set high an internal switch allows
one side of the external capacitor to be set to ground.
It then is level shifted to correspond to the bottom ladder
reference voltage of the ADC (Figure 7).
This value corresponds to the black reference of the
image sensor. When the CLAMP pin is set back to low,
the ADC samples the video signal with respect to the
black reference. The typical value for the external
RL
VRT
VRB
VDD
XRD9825
CLAMP
RINT
RED
M
U
X
C
I
S
REXT
CEXT
N/C
N/C
N/C
capacitor is 100pF. This value should be adjusted
according to the time constant (Tc) needed in a particu-
lar application. The CLAMP pin has an internal 150 ohm
impedance (R
INT
) which is in series with the external
capacitor (C
EXT
).
Therefore, Tc =1/R
INT
C
EXT
If the input to the external capacitor has a source
impedance (R
EXT
), then:
T
c
=1/(R
INT
+R
EXT
)C
EXT
