AD9280
The allowable voltage range that can be applied to CLAMPIN
depends on the operational limits of the internal clamp ampli-
fier. The recommended clamp range is between 0.5 volts and
2.0 volts.
The input capacitor should be sized to allow sufficient acquisi-
tion time of the clamp voltage at AIN within the CLAMP inter-
val, but also be sized to minimize droop between clamping
intervals. Specifically, the acquisition time when the switch is
closed will equal:
T
ACQ
=
R
IN
C
IN
V
ln
C
V
E
back porch to truncate the SYNC below the AD9280’s mini-
mum input voltage. With a C
IN
= 1
µF,
and R
IN
= 20
Ω,
the
acquisition time needed to set the input dc level to one volt
with 1 mV accuracy is about 140
µs,
assuming a full 1 volt V
C
.
With a 1
µF
input coupling capacitor, the droop across one
horizontal can be calculated:
I
BIAS
= 22
µA,
and t = 63.5
µs,
so dV = 1.397 mV, which is less
than one LSB.
After the input capacitor is initially charged, the clamp pulse
width only needs to be wide enough to correct small voltage
errors such as the droop. The fine scale settling characteristics
of the clamp circuitry are shown in Table II.
Depending on the required accuracy, a CLAMP pulse width of
1
µs–3 µs
should work in most applications. The OFFSET val-
ues ignore the contribution of offset from the clamp amplifier;
they simply compare the output code with a “final value” mea-
sured with a much longer CLAMP pulse duration.
Table II.
where
V
C
is the voltage change required across C
IN
, and V
E
is
the error voltage.
V
C
is calculated by taking the difference be-
tween the initial input dc level at the start of the clamp interval
and the clamp voltage supplied at CLAMPIN. V
E
is a system
dependent parameter, and equals the maximum tolerable devia-
tion from V
C
. For example, if a 2-volt input level needs to be
clamped to 1 volt at the AD9280’s input within 10 millivolts,
then V
C
equals 2 – 1 or 1 volt, and V
E
equals 10 mV. Note that
once the proper clamp level is attained at the input, only a very
small voltage change will be required to correct for droop.
The voltage droop is calculated with the following equation:
CLAMP
8
µs
4
µs
3
µs
2
µs
1
µs
OFFSET
<1 LSB
<2 LSBs
2 LSBs
5 LSBs
9 LSBs
dV
=
I
BIAS
t
C
IN
()
where
t
= time between clamping intervals.
The bias current of the AD9280 will depend on the sampling
rate, F
S
, and the difference between the reference midpoint,
(REFTS–REFBS)/2 and the input voltage. For a fixed sampling
rate of 32 MHz, Figure 14 shows the input bias current for a
given input. For a 1 V input range, the maximum input bias
current from Figure 14 is 22
µA.
For lower sampling rates the
input bias current will scale proportionally.
If droop is a critical parameter, then the minimum value of C
IN
should be calculated first based on the droop requirement.
Acquisition time—the width of the CLAMP pulse—can be
adjusted accordingly once the minimum capacitor value is cho-
sen. A tradeoff will often need to be made between droop and
acquisition time, or error voltage V
E
.
Clamp Circuit Example
AD9280
CLAMP IN
CLAMP
CIN
RIN
AIN
SW1
TO
SHA
Figure 24a. Clamp Operation
AIN
0.1 F
REFTF
REFTS
10 F
0.1 F
0.1 F
A single supply video amplifier outputs a level-shifted video
signal between 2 and 3 volts with the following parameters:
horizontal period = 63.56
µs,
horizontal sync interval = 10.9
µs,
horizontal sync pulse = 4.7
µs,
sync amplitude = 0.3 volts,
video amplitude of 0.7 volts,
reference black level = 2.3 volts
The video signal must be dc restored from a 2- to 3-volt range
down to a 1- to 2-volt range. Configuring the AD9280 for a
one volt input span with an input range from 1 to 2 volts (see
Figure 24), the CLAMPIN voltage can be set to 1 volt with an
external voltage or by direct connection to REFBS. The CLAMP
pulse may be applied during the SYNC pulse, or during the
AD9280
REFBF
REFBS
AVDD
2
MODE
CLAMP
SHORT TO REFBS
OR EXTERNAL DC
CLAMPIN
Figure 24b. Video Clamp Circuit
REV. D
–13–
AD [ ANALOG DEVICES ]
