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AD8042-2004 View Datasheet(PDF) - Analog Devices

Part NameDescriptionManufacturer
AD8042(2004) Dual 160 MHz Rail-to-Rail Amplifier ADI
Analog Devices ADI
AD8042 Datasheet PDF : 16 Pages
First Prev 11 12 13 14 15 16
AD8042
OVERDRIVE RECOVERY
Overdrive of an amplifier occurs when the output and/or input
range are exceeded. The amplifier must recover from this over-
drive condition. As shown in Figure 4, the AD8042 recovers
within 30 ns from negative overdrive and within 25 ns from
positive overdrive.
+5V
+2.5V
VS = +5V
0V
VIN= +5V p-p
G = +2
RL = 1kTO +2.5V
1V
50ns
Figure 4. Overdrive Recovery
CIRCUIT DESCRIPTION
The AD8042 is fabricated on Analog Devices’ proprietary
eXtra-Fast Complementary Bipolar (XFCB) process which
enables the construction of PNP and NPN transistors with
similar fTs in the 2 GHz to 4 GHz region. The process is dielec-
trically isolated to eliminate the parasitic and latch-up problems
caused by junction isolation. These features allow the construc-
tion of high frequency, low distortion amplifiers with low supply
currents. This design uses a differential output input stage to
maximize bandwidth and headroom (see Figure 5). The smaller
signal swings required on the first stage outputs (nodes S1P,
S1N) reduce the effect of nonlinear currents due to junction
capacitances and improve the distortion performance. With this
design harmonic distortion of better than –77 dB @ 1 MHz into
100 W with VOUT = 2 V p-p (Gain = +2) on a single 5 V supply
is achieved.
VCC
VINP
VINN
I1
R26
Q4
I10
R39
Q5
R15 R2
Q13 Q17
Q40
VEE
I2 I 3 Q25
Q51
Q50
I9
Q36
Q39
I5
Q23 VEE
Q22
R23 R27
Q7
Q31
Q21 Q27
C3
VOUT
SIP
SIN
C9
Q2
Q11
Q3
Q24
C7
VEE
R5
R21 R3
I7
Q8
Q47
I8
VCC
Figure 5. AD8042 Simplified Schematic
The AD8042’s rail-to-rail output range is provided by a
complementary common-emitter output stage. High output drive
capability is provided by injecting all output stage predriver cur-
rents directly into the bases of the output devices Q8 and Q36.
Biasing of Q8 and Q36 is accomplished by I8 and I5, along with
a common-mode feedback loop (not shown). This circuit topol-
ogy allows the AD8042 to drive 40 mA of output current with
the outputs within 0.5 V of the supply rails.
On the input side, the device can handle voltages from 0.2 V
below the negative rail to within 1.2 V of the positive rail.
Exceeding these values will not cause phase reversal; however,
the input ESD devices will begin to conduct if the input volt-
ages exceed the rails by greater than 0.5 V.
DRIVING CAPACITIVE LOADS
The capacitive load drive of the AD8042 can be increased by
adding a low valued resistor in series with the load. Figure 6
shows the effects of a series resistor on capacitive drive for vary-
ing voltage gains. As the closed-loop gain is increased, the larger
phase margin allows for larger capacitive loads with less over-
shoot. Adding a series resistor with lower closed-loop gains
accomplishes this same effect. For large capacitive loads, the
frequency response of the amplifier will be dominated by the
roll-off of the series resistor and capacitive load.
1000
VS = 5V
200mV STEP WITH 30% OVERSHOOT
RS
RS = 5
CL
RS = 0
100
RS = 20
10 1
2
3
4
5
CLOSED-LOOP GAIN (V/V)
Figure 6. Capacitive Load Drive vs. Closed-Loop Gain
Single-Supply Composite Video Line Driver
The two op amps of an AD8042 can be configured as a single-
supply dual line driver for composite video. The wide signal
swing of the AD8042 enables this function to be performed
without using any type of clamping or dc restore circuit which
can cause signal distortion.
Figure 7 shows a schematic for a circuit that is driven by a
single composite video source that is ac coupled, level shifted
and applied to both + inputs of the two amplifiers. Each op amp
provides a separate 75 W composite video output. To obtain
single-supply operation, ac coupling is used throughout. The
large capacitor values are required to ensure that there is minimal
tilting of the video signals due to their low frequency (30 Hz)
signal content. The circuit shown was measured to have a differ-
ential gain of 0.06% and a differential phase of 0.06.
The input is terminated in 75 W and ac coupled via CIN to a
voltage divider that provides the dc bias point to the input.
Setting the optimal bias point requires some understanding of
the nature of composite video signals and the video performance
of the AD8042.
REV. C
–11–
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