AD8129/AD8130
Such a connection, also referred to as a cable-tap amplifier, can
be simply made with an AD8130 (see Figure 143). The circuit is
configured with unity gain, and if no output offset is desired,
the REF pin is grounded. The negative differential input is
connected directly to the shield of the cable (or an associated
connector) at the point at which it wants to be tapped.
EXTREME OPERATING CONDITIONS
The AD8129/AD8130 are designed to provide high
performance over a wide range of supply voltages. However,
there are some extremes of operating conditions that have
been observed to produce suboptimal results. One of these
conditions occurs when the AD8130 is operated at unity gain
with low supply voltage—less than approximately 4 V.
+V
AD8130
At unity gain, the output drives FB directly. With supplies of
VS less than approximately 4 V at unity gain, the output can
drive FB’s voltage too close to the rail for the circuit to stay
properly biased. This can lead to a parasitic oscillation.
75Ω
0.1
μ
F
10μF
3
7
1
8
+V
+
+
PD
S
6
V
OUT
4
5
A way to prevent this is to limit the input signal swing with
clamp diodes. Common silicon-junction signal diodes like the
1N4148 have a forward bias of approximately 0.7 V when about
1 mA of current flows through them. Two series pairs of such
diodes connected antiparallel across the differential inputs can
be used to clamp the input signal and prevent this condition. It
should be noted that the REF input can also shift the output
signal; therefore, this technique only works when REF is at
ground or close to it (see Figure 145).
–V
S
2
VIDEO
IN
10μF
0.1μF
–V
75Ω
Figure 143. The AD8130 Can Tap the Video Signal at Any Point Along the
Cable Without Loading the Signal.
The center conductor connects to the positive differential input
of the AD8130. The amplitude of the video signal at this point is
unity, because it is between the two termination resistors. The
AD8130 provides a high impedance to this signal so that the
signal is not disturbed. A buffered unity-gain version of the
video signal appears at the output.
+V
AD8130
0.1
μF
10μF
V
V
3
7
IN
1N4148
1
8
+V
+
+
PD
S
6
POWER-DOWN
V
OUT
4
5
IN
–V
The AD8129/AD8130 have a power-down pin that can be used
to lower the quiescent current when the amplifier is not being
S
2
PD
used. A logic low level on the
pin causes the part to power
down. Because there is no ground pin on the AD8129/AD8130,
there is no logic reference to interface to standard logic levels.
10μF
0.1μF
–V
PD
For this reason, the reference level for the
input is VS. If
Figure 145. Clamping Diodes at the Input Limits the Input Swing Amplitude
the AD8129/AD8130 are run with VS = 5 V, there is direct
compatibility with logic families. However, if VS is higher than
this, a level-shift circuit is needed to interface to conventional
logic levels. A simple level-shifting circuit that is compatible
with common logic families is presented in Figure 144.
+V
S
7
1kΩ
+V
S
3
PD
4.99kΩ
2N2222
OR EQ
LOW =
POWER-DOWN
AD8129/
AD8130
Figure 144. Circuit that Shifts the Logic Level When VS Is Not Equal to
Approximately 5 V.
Rev. C | Page 36 of 40
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