Datasheet
BOARD LAYOUT
INPUT AND ESD PROTECTION
External
Pin
+V
S
-V
S
Internal
Circuitry
ESDprotectiondiodesinternally
connectedtoallpins.
VCA822
SBOS343C – SEPTEMBER 2007 – REVISED DECEMBER 2008 .....................................................................................................................................
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d) Connections to other wideband devices on the
board may be made with short direct traces or
Achieving optimum performance with a
through onboard transmission lines. For short
high-frequency amplifier such as the VCA822
connections, consider the trace and the input to the
requires careful attention to printed circuit board
next device as a lumped capacitive load. Relatively
(PCB) layout parasitics and external component
wide traces (50mils to 100mils, or 1.27mm to
types. Recommendations to optimize performance
2.54mm) should be used, preferably with ground and
include:
power planes opened up around them.
a) Minimize parasitic capacitance to any ac ground
e) Socketing a high-speed part like the VCA822 is
for all of the signal I/O pins. This recommendation
not recommended. The additional lead length and
includes the ground pin (pin 2). Parasitic capacitance
pin-to-pin capacitance introduced by the socket can
on the output can cause instability: on both the
create an extremely troublesome parasitic network,
inverting input and the noninverting input, it can react
which can make it almost impossible to achieve a
with the source impedance to cause unintentional
smooth, stable frequency response. Best results are
band limiting. To reduce unwanted capacitance, a
obtained by soldering the VCA822 onto the board.
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
The VCA822 is built using a very high-speed
series resistance (greater than 25 Ω ) with the input pin
complementary bipolar process. The internal junction
connected to ground to help decouple package
breakdown voltages are relatively low for these very
parasitics.
small geometry devices. These breakdowns are
reflected in the Absolute Maximum Ratings table.
b) Minimize the distance (less than 0.25 ” ) from the
power-supply pins to high-frequency 0.1 µ F
All pins on the VCA822 are internally protected from
decoupling capacitors. At the device pins, the ground
ESD by means of a pair of back-to-back
and power plane layout should not be in close
reverse-biased diodes to either power supply, as
proximity to the signal I/O pins. Avoid narrow power
shown in Figure 94 . These diodes begin to conduct
and ground traces to minimize inductance between
when the pin voltage exceeds either power supply by
the pins and the decoupling capacitors. The
about 0.7V. This situation can occur with loss of the
power-supply connections should always be
amplifier power supplies while a signal source is still
decoupled with these capacitors. Larger (2.2 µ F to
present. The diodes can typically withstand a
6.8 µ F) decoupling capacitors, effective at lower
continuous current of 30mA without destruction. To
frequencies, should also be used on the main supply
ensure long-term reliability, however, diode current
pins. These capacitors may be placed somewhat
should be externally limited to 10mA whenever
farther from the device and may be shared among
possible.
several devices in the same area of the PCB.
c) Careful selection and placement of external
components preserve the high-frequency
performance of the VCA822. 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
Figure 94. Internal ESD Protection
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 non-inverting input
termination resistors, should also be placed close to
the package.
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