Datasheet

External
Pin
+V
CC
−
V
CC
Internal
Circuitry
OPA861
SBOS338G –AUGUST 2005–REVISED MAY 2013 www.ti.com
As a worst-case example, compute the maximum T
J
c) Careful selection and placement of external
using an OPA861IDBV in the circuit of Figure 32b components will preserve the high-frequency
operating at the maximum specified ambient performance of the OPA861. Resistors should be a
temperature of +85°C and driving a –1V voltage very low reactance type. Surface-mount resistors
reference. work best and allow a tighter overall layout. Metal film
or carbon composition, axially-leaded resistors can
P
D
= 10V × 5.4mA + (15mA × 9V) = 185mW
also provide good high-frequency performance.
Again, keep their leads and PC board traces as short
Maximum T
J
= +85°C + (0.19W × 150°C/W) = 114°C.
as possible. Never use wirewound type resistors in a
Although this is still well below the specified
high-frequency application.
maximum junction temperature, system reliability
d) Connections to other wideband devices on the
considerations may require lower tested junction
board may be made with short, direct traces or
temperatures. The highest possible internal
through onboard transmission lines. For short
dissipation will occur if the load requires current to be
connections, consider the trace and the input to the
forced into the output for positive output voltages or
next device as a lumped capacitive load. Relatively
sourced from the output for negative output voltages.
wide traces (50mils to 100mils) should be used,
This puts a high current through a large internal
preferably with ground and power planes opened up
voltage drop in the output transistors.
around them.
BOARD LAYOUT GUIDELINES
e) Socketing a high-speed part like the OPA861 is
not recommended. The additional lead length and
Achieving optimum performance with a high-
pin-to-pin capacitance introduced by the socket can
frequency amplifier like the OPA861 requires careful
create an extremely troublesome parasitic network
attention to board layout parasitics and external
that makes it almost impossible to achieve a smooth,
component types. Recommendations that will
stable frequency response. Best results are obtained
optimize performance include:
by soldering the OPA861 onto the board.
a) Minimize parasitic capacitance to any AC ground
INPUT AND ESD PROTECTION
for all of the signal I/O pins. Parasitic capacitance on
the inverting input pin can cause instability: on the
The OPA861 is built using a very high-speed
noninverting input, it can react with the source
complementary bipolar process. The internal junction
impedance to cause unintentional bandlimiting. To
breakdown voltages are relatively low for these very
reduce unwanted capacitance, a window around the
small geometry devices. These breakdowns are
signal I/O pins should be opened in all of the ground
reflected in the Absolute Maximum Ratings table. All
and power planes around those pins. Otherwise,
device pins are protected with internal ESD protection
ground and power planes should be unbroken
diodes to the power supplies as shown in Figure 52.
elsewhere on the board.
b) Minimize the distance (< 0.25") from the power-
supply pins to high-frequency 0.1µF 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. An optional supply decoupling capacitor
Figure 52. Internal ESD Protection
(0.1µF) across the two power supplies (for bipolar
operation) will improve 2nd-harmonic distortion
performance. Larger (2.2µF to 6.8µF) decoupling
These diodes provide moderate protection to input
capacitors, effective at lower frequency, should also
overdrive voltages above the supplies as well. The
be used on the main supply pins. These may be
protection diodes can typically support 30mA
placed somewhat farther from the device and may be
continuous current. Where higher currents are
shared among several devices in the same area of
possible (for example, in systems with ±15V supply
the PC board.
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 response.
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