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AD8145YCPZ-R7 Datasheet(PDF) 17 Page - Analog Devices

Part No. AD8145YCPZ-R7
Description  High Speed, Triple Differential Receiver with Comparators
Download  24 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD8145YCPZ-R7 Datasheet(HTML) 17 Page - Analog Devices

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AD8145
Rev. 0 | Page 17 of 24
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 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. Another
application in which external common-mode clamping is
sometimes required is 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
RS
RT
VIN
RS
+
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
VOUT
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.
RT is the differential termination resistor, and the series
resistances, RS, limit the current into the diodes. The series
resistors should be highly matched in value to preserve high
frequency CMRR.
POSITIVE CLAMP
NEGATIVE CLAMP
RS
RT
VIN
RS
+
GAIN
REF
R
R
0.01µF
0.01µF
+5V
–5V
C
OUT
VOUT
Figure 40. Using Two 1N4448 Diodes in Series as a Clamp
There are many other nonlinear devices that 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 (RS), 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.


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