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VCA824IDG4 Datasheet(PDF) 30 Page - Texas Instruments

Part No. VCA824IDG4
Description  Ultra-Wideband, > 40-dB Gain Adjust Range, Linear in V/V Variable Gain Amplifier
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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VCA824IDG4 Datasheet(HTML) 30 Page - Texas Instruments

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VCA824
SBOS394D – NOVEMBER 2007 – REVISED JANUARY 2016
www.ti.com
10 Power Supply Recommendations
High-speed amplifiers require low inductance power supply traces and low ESR bypass capacitors. When
possible both power and ground planes should be used in the printed circuit board design and the power plane
should be adjacent to the ground plane in the board stack-up. The power supply voltage should be centered on
the desired amplifier output voltage, so for ground referenced output signals, split supplies are required. The
power supply voltage should be from 7-V to 12-V.
11 Layout
11.1 Layout Guidelines
Achieving optimum performance with a high-frequency amplifier such as the VCA824 requires careful attention to
printed circuit board (PCB) layout parasitics and external component types. Recommendations to optimize
performance include:
a. Minimize parasitic capacitance to any AC ground for all of the signal I/O pins. This recommendation
includes the ground pin (pin 2). Parasitic capacitance on the output can cause instability: on both the
inverting input and the noninverting input, it can react with the source impedance to cause unintentional band
limiting. To reduce unwanted capacitance, a window around the signal I/O pins should be opened in all of the
ground and power planes around those pins. Otherwise, ground and power planes should be unbroken
elsewhere on the board. Place a small series resistance (greater than 25-
Ω) with the input pin connected to
ground to help decouple package parasitics.
b. Minimize the distance (less than 0.25”) from the power-supply pins to high-frequency 0.1-
μF decoupling
capacitors. At the device pins, the ground and power plane layout should not be in close proximity to the
signal I/O pins. Avoid narrow power and ground traces to minimize inductance between the pins and the
decoupling capacitors. The power-supply connections should always be decoupled with these capacitors.
Larger (2.2-
μF to 6.8-μF) decoupling capacitors, effective at lower frequencies, should also be used on the
main supply pins. These capacitors may be placed somewhat farther from the device and may be shared
among several devices in the same area of the PCB.
c. Careful selection and placement of external components preserve the high-frequency performance of the
VCA824. Resistors should be a very low reactance type. Surface-mount resistors work best and allow a
tighter overall layout. Metal-film and carbon composition, axially-leaded resistors can also provide good high-
frequency performance. Again, keep the leads and PCB trace length as short as possible. Never use wire-
wound type resistors in a high-frequency application. Because the output pin is the most sensitive to parasitic
capacitance, always position the series output resistor, if any, as close as possible to the output pin. Other
network components, such as inverting or noninverting input termination resistors, should also be placed
close to the package.
d. Connections to other wideband devices on the board may be made with short direct traces or through
onboard transmission lines. For short connections, consider the trace and the input to the next device as a
lumped capacitive load. Relatively wide traces (50 mils to 100 mils, or 1.27 mm to 2.54 mm) should be used,
preferably with ground and power planes opened up around them.
e. Socketing a high-speed part like the VCA824 device is not recommended. The additional lead length
and pin-to-pin capacitance introduced by the socket can create an extremely troublesome parasitic network,
which can make it almost impossible to achieve a smooth, stable frequency response. Best results are
obtained by soldering the VCA824 device onto the board.
30
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Copyright © 2007–2016, Texas Instruments Incorporated
Product Folder Links: VCA824


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