Data Sheet AD8145
INPUT CLAMPING
The differential input that is assigned to receive the input signal
includes clamping diodes that limit the differential input swing
to approximately 5.5 V p-p at 25°C. Because of this, the input
and feedback stages should never be interchanged.
The supply current drawn by the AD8145 has a strong dependence
on the input signal magnitude because the input transconductance
stages operate with differential input signals that can be up to a
few volts peak-to-peak. This behavior is distinctly different
from that of traditional op amps, where the differential input
signal is driven to essentially 0 V by negative feedback.
For most applications, including receiving RGB video signals,
the input signal magnitudes encountered are well within the
safe operating limits of the AD8145 over its full power supply
and operating temperature ranges. In some extreme applications
where large differential and/or common-mode voltages are
encountered, external clamping may be necessary. External
common-mode clamping is also sometimes required when an
unpowered AD8145 receives a signal from an active driver. In
this case, external diodes are required when the current drawn
by the internal ESD diodes cannot be kept to less than 5 mA.
Figure 39 shows a general approach to external differential-mode
clamping.
POSITIVE CLAMP NEGATIVE CLAMP
R
S
R
T
V
IN
R
S
–
+
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
V
OUT
06307-040
Figure 39. Differential-Mode Clamping with G = 1
The positive and negative clamps are nonlinear devices that exhibit
very low impedance when the voltage across them reaches a
critical threshold (clamping voltage), thereby limiting the voltage
across the AD8145 input. The positive clamp has a positive
threshold, and the negative clamp has a negative threshold.
A diode is a simple example of such a clamp. Schottky diodes
generally have lower clamping voltages than typical signal diodes.
The clamping voltage should be larger than the largest expected
signal amplitude, with enough margin to ensure that the received
signal passes without being distorted.
A simple way to implement a clamp is to use a number of diodes in
series. The resultant clamping voltage is then the sum of the
clamping voltages of individual diodes.
A 1N4448 diode has a forward voltage of approximately 0.70 V
to 0.75 V at typical current levels that are seen when it is being
used as a clamp, and 2 pF maximum capacitance at 0 V bias.
(The capacitance of a diode decreases as its reverse-bias voltage
is increased.) The series connection of two 1N4448 diodes,
therefore, has a clamping voltage of 1.4 V to 1.5 V. Figure 40
shows how to limit the differential input voltage applied to an
AD8145 amplifier to ±1.4 V to ±1.5 V (2.8 V p-p to 3.0 V p-p).
Note that the capacitance of the two series diodes is half that of
one diode. Different numbers of series diodes can be used to
obtain different clamping voltages.
R
T
is the differential termination resistor, and the series
resistances, R
S
, limit the current into the diodes. The series
resistors should be highly matched in value to preserve high
frequency CMRR.
POSITIVE CLAMP NEGATIVE CLAMP
R
S
R
T
V
IN
R
S
–
+
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
V
OUT
06307-041
Figure 40. Using Two 1N4448 Diodes in Series as a Clamp
Many other nonlinear devices can be used as clamps. The best
choice for a particular application depends upon the desired
clamping voltage, response time, parasitic capacitance, and
other factors.
When using external differential-mode clamping, it is important
to ensure that the series resistors (R
S
), the sum of the parasitic
capacitance of the clamping devices, and the input capacitance
of the AD8145 are small enough to preserve the desired signal
bandwidth.
Rev. B | Page 17 of 21
