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AD8010ARZ-16

P1-P3P4-P6P7-P9P10-P12P13-P13
AD8010
–9–REV. B
APPLICATIONS
Video Distribution Amplifier
The AD8010 is optimized for the specific function of providing
excellent video performance when driving multiple video loads
in parallel. Significant power is saved and heat sinking is greatly
simplified because of the ability of the AD8010 to obtain this
performance when running on a ±5 V supply. However, due to
the high currents that flow when driving many parallel video
loads, special layout and bypassing techniques are required to
assure optimal performance.
When designing a video distribution amplifier with the AD8010, it
is very important to keep in mind where the high (ac) currents
will flow. These paths include the power supply pins of the chip
along with the bypass capacitors and the return path for these
capacitors, the output circuits and the return path of the output
current from the loads.
In general, any loops that are formed by any of the above paths
should be made as small as possible. Large loops are both gen-
erators and receivers of magnetic fields and can cause undesired
coupling of signals that lowers the performance of the amplifier.
Effects that have not been seen before in other op amp circuits
might arise because of the high currents. Most op amp circuits
output, at most, tens of milliamps and do not require extremely
tight video specifications, while a video distribution amplifier
can output hundreds of milliamps and require extremely low
differential gain and phase errors.
The bypassing scheme that is used for the AD8010 requires
special attention. It was found that the conventional technique
of bypassing each power pin individually to ground can have an
adverse effect on the differential phase error of the circuit. The
cause of this is attributed to the fact that there is an internal
compensation capacitor in the AD8010 that is referenced to the
negative supply.
The recommended technique is to connect parallel bypass
capacitors from the positive supply to the negative supply and
then to bypass the negative supply to ground. For high fre-
quency bypassing, 0.1 µF ceramic capacitors are recommended.
These should be placed within a few millimeters of the power
pins and should preferably be chip type capacitors.
The high currents that can potentially flow through the power
supply pins require large bypassing capacitors. These should be
low inductance tantalum types and at least 47 µF. The ground
side of the capacitor that bypasses the negative supply should be
brought to a single point ground that is the common for the
returns of the outputs.
Figure 30 shows a circuit for making an N-channel video distri-
bution amplifier. As a practical matter, the AD8010 can readily
drive eight standard 150 Ω video loads. When driving up to 12
video loads, there is minimal degradation in video performance.
Another important consideration when driving multiple cables
is the high frequency isolation between the outputs of the
cables. Due to its low output impedance, the AD8010 achieves
better than 46 dB of output-to-output isolation at 5 MHz driv-
ing back terminated 75 Ω cables.
AD8010
V
IN
150
75
R
L1
499499
+5V
–5V
75
75
75
75
75 R
L2
75 R
LN
FB
FB
C1
C2
Figure 30. An N-Channel Video Distribution Amplifier Using An AD8010.
NOTE: Please see Figure 29 for Recommended Bypassing Technique.
AD8010
–10–
REV. B
Differential Line Driver
Twisted pair transmission lines are more often being used for
high frequency analog and digital signals. Over long distances,
however, the attenuation characteristics of these lines can
degrade the performance of the transmission system. To com-
pensate for this, larger signals are transmitted, which after the
attenuation, will still have useful signal strength.
The high output current of two AD8010s can be used along
with a transformer to create a high power differential line driver.
The differential configuration effectively doubles the output
swing, while the step-up transformer further increases the out-
put voltage.
In the circuit in Figure 31 the A device is configured as a gain-
of-two follower, while the B device is a gain-of-two inverter.
These will produce a differential output signal whose maximum
value is twice the peak-to-peak value of the maximum output
of one device. For this circuit a 12 V peak-to-peak output can
be obtained.
The op amps drive a 1:2 step-up transformer that drives a
100 Ω transmission line. Since the impedance reflected back to
the primary varies as the square of the turns ratio, it will appear
as 25 Ω at the primary. This source terminating resistor is split
as a 12.4 Ω resistor at the output of each device.
The circuit shown is capable of delivering 12 V p-p to the line
and operates with a –3 dB bandwidth of 40 MHz. The peak
current output of either op amp is 100 mA.
AD8010
V
IN
499
499
150
402
AD8010
150
806
12.4
12.4
+6
–6
1:2
100
Figure 31. High Output Differential Line Driver Using Two AD8010s.
NOTE: Please see Figure 29 for Recommended Bypassing Technique.
AD8010
–11–REV. B
Table I. –3 dB Bandwidth and Slew Rate vs. Closed-Loop
Gain and Resistor Values
Package: N-8
Closed-Loop –3 dB BW Slew Rate
Gain R
F
()R
G
() (MHz) (V/s)
+1 453
∞
285 900
+2 374 374 255 900
+5 348 86.6 200 800
+10 562 61.9 120 550
Package: R-16
Closed-Loop –3 dB BW Slew Rate
Gain R
F
()R
G
() (MHz) (V/s)
+1 412
∞
245 900
+2 392 392 220 900
+5 392 97.6 160 800
+10 604 66.5 95 550
Package: SO-8
Closed-Loop –3 dB BW Slew Rate
Gain R
F
()R
G
() (MHz) (V/s)
+1 392
∞
345 950
+2 374 374 305 1000
+5 348 86.6 220 1000
+10 499 54.9 135 650
1. V
O
= 0.2 V p-p for –3 dB Bandwidth.
2. V
O
= 2 V p-p for Slew Rate.
3. Bypassing per Figure 29.
150
50
R
F
R
G
V
OUT
18.75
V
IN
Figure 32. Test Circuit for Table I
Closed-Loop Gain and Bandwidth
The AD8010 is a current feedback amplifier optimized for use
in high performance video and data acquisition applications.
Since it uses a current feedback architecture, its closed-loop
–3 dB bandwidth is dependent on the magnitude of the feedback
resistor. The desired closed-loop bandwidth and gain are obtained
by varying the feedback resistor (R
F
) to set the bandwidth, and
varying the gain resistor (R
G
) to set the desired gain. The char-
acteristic curves and specifications for this data sheet reflect the
performance of the AD8010 using the values of R
F
noted at the
top of the specifications table. If a greater –3 dB bandwidth
and/or slew rate is required (at the expense of video performance),
Table I provides the recommended resistor values. Figure 32
shows the test circuit and conditions used to produce Table I.
Effect of Feedback Resistor Tolerance on Gain Flatness
Because of the relationship between the 3 dB bandwidth and the
feedback resistor, the fine scale gain flatness will, to some extent,
vary with feedback resistor tolerance. It is therefore recommended
that resistors with a 1% tolerance be used if it is desired to main-
tain flatness over a wide range of production lots. In addition,
resistors of different construction have different associated para-
sitic capacitance and inductance. Metal-film resistors were used
for the bulk of the characterization for this data sheet. It is pos-
sible that values other than those indicated will be optimal for
other resistor types.
Quality of Coaxial Cable
Optimum flatness when driving a coax cable is possible only
when the driven cable is terminated at each end with a resistor
matching its characteristic impedance. If the coax was ideal,
then the resulting flatness would not be affected by the length of
the cable. While outstanding results can be achieved using inex-
pensive cables, it should be noted that some variation in flatness
due to varying cable lengths may be experienced.
P1-P3P4-P6P7-P9P10-P12P13-P13

AD8010ARZ-16

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Buy AD8010ARZ-16
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers Low Power Hi Crnt Distribution
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