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AD8038 データシート(PDF) 9 Page - Analog Devices

部品番号 AD8038
部品情報  Low Power 350 MHz Voltage Feedback Amplifiers
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メーカー  AD [Analog Devices]
ホームページ  http://www.analog.com
Logo AD - Analog Devices

AD8038 データシート(HTML) 9 Page - Analog Devices

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REV. F
AD8038/AD8039
–9–
FREQUENCY – MHz
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
0.1
1000
1.0
10
100
TPC 37. AD8038 Input-Output Isolation (G = +2,
RL = 2 k
Ω, VS = ±5 V
LAYOUT, GROUNDING, AND BYPASSING
CONSIDERATIONS
Disable
The AD8038 in the SOIC-8 package provides a disable feature.
This feature disables the input from the output (see TPC 37 for
input-output isolation) and reduces the quiescent current from
typically 1 mA to 0.2 mA. When the
DISABLE node is pulled
below 4.5 V from the positive supply rail, the part becomes
disabled. In order to enable the part, the
DISABLE node needs
to be pulled up to above 2.5 V below the positive rail.
Power Supply Bypassing
Power supply pins are actually inputs, and care must be taken
so that a noise-free stable dc voltage is applied. The purpose of
bypass capacitors is to create low impedances from the supply to
ground at all frequencies, thereby shunting or filtering a majority
of the noise.
Decoupling schemes are designed to minimize the bypassing
impedance at all frequencies with a parallel combination of capaci-
tors. 0.01
µF or 0.001 µF (X7R or NPO) chip capacitors are
critical and should be as close as possible to the amplifier pack-
age. Larger chip capacitors, such as the 0.1
µF capacitor, can be
shared among a few closely spaced active components in the same
signal path. A 10
µF tantalum capacitor is less critical for high
frequency bypassing and, in most cases, only one per board is
needed at the supply inputs.
Grounding
A ground plane layer is important in densely packed PC boards to
spread the current minimizing parasitic inductances. However,
an understanding of where the current flows in a circuit is critical
to implementing effective high speed circuit design. The length
of the current path is directly proportional to the magnitude of
parasitic inductances, and thus the high frequency impedance of
the path. High speed currents in an inductive ground return will
create an unwanted voltage noise.
The length of the high frequency bypass capacitor leads are most
critical. A parasitic inductance in the bypass grounding will work
against the low impedance created by the bypass capacitor. Place
the ground leads of the bypass capacitors at the same physical
location. Because load currents flow from the supplies as well, the
ground for the load impedance should be at the same physical
location as the bypass capacitor grounds. For the larger value
capacitors, which are intended to be effective at lower frequencies,
the current return path distance is less critical.
Input Capacitance
Along with bypassing and ground, high speed amplifiers can be
sensitive to parasitic capacitance between the inputs and ground.
A few pF of capacitance will reduce the input impedance at high
frequencies, in turn increasing the amplifiers’ gain, causing peak-
ing of the frequency response, or even oscillations if severe enough.
It is recommended that the external passive components that
are connected to the input pins be placed as close as possible to
the inputs to avoid parasitic capacitance. The ground and power
planes must be kept at a distance of at least 0.05 mm from the
input pins on all layers of the board.
Output Capacitance
To a lesser extent, parasitic capacitances on the output can cause
peaking of the frequency response. There are two methods to
minimize this effect.
1. Put a small value resistor in series with the output to isolate
the load capacitor from the amp’s output stage; see TPCs 7,
8, 22, and 23.
2. Increase the phase margin with higher noise gains or add a pole
with a parallel resistor and capacitor from –IN to the output.
Input-to-Output Coupling
The input and output signal traces should not be parallel to
minimize capacitive coupling between the inputs and outputs,
avoiding any positive feedback.
APPLICATIONS
Low Power ADC Driver
8
1
0.1 F10 F
+5V
0.1 F
10 F
7
0.1 F
10 F
–5V
3
2
6
5
4
AD8039
1k
1k
1k
1k
VINA
VINB
REF
50
50
AD9203
1k
1k
1k
1k
VIN
0V
3V
2.5V
Figure 3. Schematic to Drive AD9203 with the AD8039
Differential A/D Driver
The AD9203 is a low power (125 mW on a 5 V supply) 40 MSPS
10-bit converter. This represents a breakthrough in power/speed
for ADCs. As such, the low power, high performance AD8039
is an appropriate choice of amplifier to drive it.
In low supply voltage applications, differential analog inputs
are needed to increase the dynamic range of the ADC inputs.
Differential driving can also reduce second and other even-order
distortion products. The AD8039 can be used to make a
dc-coupled, single-ended-to-differential driver for one of these
ADCs. Figure 3 is a schematic of such a circuit for driving an
AD9203, a 10-bit, 40 MSPS ADC.


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