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AD8044ARZ-14-REEL

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
AD8044
REV. B
–11–
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 33, the AD8044 recovers
within 50 ns from negative overdrive and within 25 ns from
positive overdrive.
50ns1V
V
S
= +5V
A
V
= +2
R
F
= 2k⍀
R
L
= 2k⍀
V
IN
2V/DIV
V
OUT
1V/DIV
2V
Figure 33. Overdrive Recovery, VS + 5 V, V
IN
= 4 V Step
Circuit Description
The AD8044 is fabricated on Analog Devices’ proprietary
eXtra-Fast Complementary Bipolar (XFCB) process which
enables the construction of PNP and NPN transistors with
similar f
T
s in the 2 GHz–4 GHz region. The process is dielectri-
cally 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 34). 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 –85 dB
@ 1 MHz into 100 W with V
OUT
= 2 V p-p (Gain = +2) on a
single 5 volt supply is achieved.
The AD8044’s rail-to-rail output range is provided by a comple-
mentary common-emitter output stage. High output drive capa-
bility is provided by injecting all output stage predriver currents
directly into the bases of the output devices Q8 and Q36. Bias-
ing 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 AD8044 to drive 50 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. Ex-
ceeding these values will not cause phase reversal; however, the
input ESD devices will begin to conduct if the input voltages
exceed the rails by greater than 0.5 V.
Driving Capacitance Loads
The capacitive load drive of the AD8044 can be increased by
adding a low valued resistor in series with the load. Figure 35
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.
SIN
R21
R3
V
EE
Q11
Q3
I10
R26 R39
Q5
Q4
Q40
I7
R2R15
Q13
Q17
R5
C7
Q2
SIP
Q22
Q7
Q21
Q24
R23
R27
I2 I3
I1
Q51
Q25
Q50
Q39
Q47
Q27
Q31
Q23
I9
I5
V
EE
V
CC
I8
Q36
Q8
V
OUT
C3
C9
V
CC
V
IN
P
V
IN
N
V
EE
I11
Figure 34. AD8044 Simplified Schematic
AD8044
REV. B
–12–
A
CL
(V/V)
1000
100
10
162
CAPACITIVE LOAD (pF)
34 5
V
S
= +5V
< 30% OVERSHOOT
R
G
R
F
C
L
R
S
V
OUT
V
IN
100mV STEP
R
S
= 10⍀
R
S
= 0⍀
Figure 35. Capacitive Load Drive vs. Closed-Loop Gain
APPLICATIONS
RGB Buffer
The AD8044 can provide buffering of RGB signals that include
ground while operating from a single +3 V or +5 V supply.
When driving two monitors from the same RGB video source it
is necessary to provide an additional driver for one of the moni-
tors to prevent the double termination situation that the second
monitor presents. This has usually required a dual-supply op
amp because the level of the input signal from the video driver
goes all the way to ground during horizontal blanking. In single-
supply systems it can be a major inconvenience and expense to
add an additional negative supply.
A single AD8044 can provide the necessary drive capability and
yet does not require a negative supply in this application. Fig-
ure 36 is a schematic that uses three amplifiers out of a single
AD8044 to provide buffering for a second monitor.
The source of the RGB signals is shown to be from a set of three
current output DACs that are within a single-supply graphics
IC. This is typically the situation in most PCs and workstations
that may use either a standalone triple DAC or DACs that are
integrated into a larger graphics chip.
During horizontal blanking, the current output from the DACs
is turned off and the RGB outputs are pulled to ground by the
termination resistors. If voltage sources were used for the RGB
signals, then the termination resistors near the graphics IC
would be in series and the rest of the circuit would remain the
same. This is because a voltage source is an ac short circuit, so a
series resistor is required to make the drive end of the line see
75 W to ac ground. On the other hand, a current source has a
very high output impedance, so a shunt resistor is required to
make the drive end of the line see 75 W to ground. In either
case, the monitor terminates its end of the line with 75 W.
The circuit in Figure 36 shows minimum signal degradation
when using a single-supply for the AD8044. The circuit per-
forms equally well on either a +3 V or +5 V supply.
A
1k⍀
10␮F
+3V OR +5V
75⍀
0.1␮F
1k⍀
75⍀
RGB
MONITOR #2
V+
B
1k⍀
75⍀
1k⍀
C
1k⍀
75⍀
1k⍀
V–
75⍀
75⍀
75⍀
75⍀
75⍀
RGB
MONITOR #1
75⍀
75⍀
75⍀
R
G
B
+5V
GRAPHICS
IC
AD8044
AD8044
AD8044
Figure 36. Single Supply RGB Video Driver
Figure 37 is an oscilloscope photo of the circuit in Figure 36
operating from a +3 V supply and driven by the Blue signal of a
color bar pattern. Note that the input and output are at ground
during the horizontal blanking interval. The RGB signals are
specified to output a maximum of 700 mV peak. The output of
the AD8044 is 1.4 V with the termination resistors providing a
divide-by-two.
V
IN
GND
GND
V
OUT
10
0%
100
90
5
␮
s
500mV
500mV
Figure 37. +3 V, RGB Buffer
AD8044
REV. B
–13–
Active Filters
Active filters at higher frequencies require wider bandwidth op
amps to work effectively. Excessive phase shift produced by
lower frequency op amps can significantly impact active filter
performance.
Figure 38 shows an example of a 2 MHz biquad bandwidth
filter that uses three op amps of an AD8044 package. Such
circuits are sometimes used in medical ultrasound systems to
lower the noise bandwidth of the analog signal before A/D
conversion.
2
3
1
R1
3k⍀
V
IN
R2
2k⍀
C1
50pF
R3
2k⍀
6
5
7
R6
1k⍀
R5
2k⍀
9
10
8
AD8044
AD8044
AD8044
C2
50pF
V
OUT
R4
2k⍀
Figure 38. 2 MHz Biquad Band-pass Filter Using AD8044
The frequency response of the circuit is shown in Figure 39.
FREQUENCY
(
Hz
)
0
–10
10k 100M100k
GAIN (dB)
1M 10M
–20
–30
–40
Figure 39. Frequency Response of 2 MHz Band-pass
Biquad Filter
Layout Considerations
The specified high speed performance of the AD8044 requires
careful attention to board layout and component selection.
Proper RF design techniques and low-pass parasitic component
selection are necessary.
The PCB should have a ground plane covering all unused por-
tions of the component side of the board to provide a low im-
pedance path. The ground plane should be removed from the
area near the input pins to reduce the stray capacitance.
Chip capacitors should be used for the supply bypassing. One
end should be connected to the ground plane and the other
within 1/8 inch of each power pin. An additional large (0.47 mF
– 10 mF) tantalum electrolytic capacitor should be connected in
parallel, but not necessarily so close, to supply current for fast,
large signal changes at the output.
The feedback resistor should be located close to the inverting
input pin in order to keep the stray capacitance at this node to a
minimum. Capacitance variations of less than 1 pF at the invert-
ing input will significantly affect high speed performance.
Stripline design techniques should be used for long signal traces
(greater than about 1 inch). These should be designed with a
characteristic impedance of 50 W or 75 W and properly termi-
nated at each end.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18

AD8044ARZ-14-REEL

Mfr. #:
Buy AD8044ARZ-14-REEL
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers Quad 150MHz RR
Lifecycle:
New from this manufacturer.
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