OPA861
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SBOS338–AUGUST 2005
a) Minimize parasitic capacitance to any AC ground
for all of the signal I/O pins. Parasitic capacitance on
the inverting input pin can cause instability: on the
noninverting input, it can react with the source im-
pedance to cause unintentional bandlimiting. To re-
duce 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 else-
where on the board.
nections, consider the trace and the input to the next
device as a lumped capacitive load. Relatively wide
traces (50mils to 100mils) should be used, preferably
with ground and power planes opened up around
them.
e) Socketing a high-speed part like the OPA861 is
not recommended. The additional lead length and
pin-to-pin capacitance introduced by the socket can
create an extremely troublesome parasitic network
that makes it almost impossible to achieve a smooth,
stable frequency response. Best results are obtained
by soldering the OPA861 onto the board.
b) Minimize the distance (< 0.25") from the
power-supply pins to high-frequency 0.1µF decoup-
ling 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 connec-
tions should always be decoupled with these capaci-
tors. An optional supply decoupling capacitor (0.1µF)
across the two power supplies (for bipolar operation)
will improve 2nd-harmonic distortion performance.
Larger (2.2µF to 6.8µF) decoupling capacitors, effec-
tive at lower frequency, should also be used on the
main supply pins. These may be placed somewhat
farther from the device and may be shared among
several devices in the same area of the PC board.
INPUT AND ESD PROTECTION
The OPA861 is built using a very high-speed comp-
lementary bipolar process. The internal junction
breakdown voltages are relatively low for these very
small geometry devices. These breakdowns are re-
flected in the Absolute Maximum Ratings table. All
device pins are protected with internal ESD protection
diodes to the power supplies as shown in Figure 49.
+VCC
External
Pin
Internal
Circuitry
c) Careful selection and placement of external
components will preserve the high-frequency per-
formance of the OPA861. Resistors should be a
very low reactance type. Surface-mount resistors
work best and allow a tighter overall layout. Metal film
or carbon composition, axially-leaded resistors can
also provide good high-frequency performance.
Again, keep their leads and PC board traces as short
as possible. Never use wirewound type resistors in a
high-frequency application.
−
VCC
Figure 49. Internal ESD Protection
These diodes provide moderate protection to input
overdrive voltages above the supplies as well. The
protection diodes can typically support 30mA continu-
ous current. Where higher currents are possible (for
example, in systems with ±15V supply parts driving
into the OPA861), current-limiting series resistors
should be added into the two inputs. Keep these
resistor values as low as possible since high values
degrade both noise performance and frequency re-
sponse.
d) Connections to other wideband devices on the
board may be made with short, direct traces or
through onboard transmission lines. For short con-
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