AD9200,pdf datasheet (CMOS AD

The AD9200ARS and AD9200KST parts feature an optional
clamp circuit for dc restoration of video or ac coupled signals.
Figure 24 shows the internal clamp circuitry and the external
control signals needed for clamp operation. To enable the
clamp, apply a logic high to the CLAMP pin. This will close
the switch SW1. The clamp amplifier will then servo the voltage
at the AIN pin to be equal to the clamp voltage applied at
the CLAMPIN pin. After the desired clamp level is attained,
SW1 is opened by taking CLAMP back to a logic low. Ignoring
the droop caused by the input bias current, the input capacitor
CIN will hold the dc voltage at AIN constant until the next
clamp interval. The input resistor RIN has a minimum recommended
value of 10 W, to maintain the closed-loop stability of
the clamp amplifier.