OPA644U BURR-BROWN [Burr-Brown Corporation], OPA644U Datasheet - Page 9

• Manufacturer Part Number: OPA644U
• Description: Low Distortion Current Feedback OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

R
R
R
G = Closed-loop gain
WIRING PRECAUTIONS
Maximizing the OPA644’s capability requires some wiring
precautions and high-frequency layout techniques. Oscilla-
tion, ringing, poor bandwidth and settling, gain peaking, and
instability are typical problems plaguing all high-speed
amplifiers when they are improperly used. In general, all
printed circuit board conductors should be wide to provide
low resistance, low impedance signal paths. They should
also be as short as possible. The entire physical circuit
should be as small as practical. Stray capacitances should be
minimized, especially at high impedance nodes, such as the
amplifier’s input terminals. Stray signal coupling from the
output or power supplies to the inputs should be minimized.
All circuit element leads should be no longer than 1/4 inch
(6mm) to minimize lead inductance, and low values of
resistance should be used. This will minimize time constants
formed with the circuit capacitances and will eliminate
stray, parasitic circuits.
Grounding is the most important application consideration
for the OPA644, as it is with all high-frequency circuits.
Oscillations at high frequencies can easily occur if good
grounding techniques are not used. A heavy ground plane
(2 oz. copper recommended) should connect all unused
areas on the component side. Good ground planes can
reduce stray signal pickup, provide a low resistance, low
inductance common return path for signal and power, and
can conduct heat from active circuit package pins into
ambient air by convection.
Supply bypassing is extremely critical and must always be
used, especially when driving high current loads. Both
power supply leads should be bypassed to ground as close as
possible to the amplifier pins. Tantalum capacitors (2.2 F)
with very short leads are recommended. A parallel 0.01 F
ceramic must also be added. Surface-mount bypass capaci-
tors will produce excellent results due to their low lead
inductance. Additionally, suppression filters can be used to
isolate noisy supply lines. Properly bypassed and modula-
tion-free power supply lines allow full amplifier output and
optimum settling time performance.
Points to Remember
1) Making use of all four power supply pins will lower the
effective power supply inductance seen by the input and
output stages. This will improve the AC performance in-
cluding lower distortion. The lowest distortion is achieved
when running separated traces to V
bypassing with 0.01 F and 2.2 F surface-mount capacitors
is recommended. It is essential to keep the 0.01 F capacitor
very close to the power supply pins. Refer to the demonstra-
tion board figure in the DEM-OPA64X data sheet for
the recommended layout and component placements.
FB
FF
I
= (424 + 8G)/G (inverting)
= (424 – 8G)/(G – 1) (non-inverting)
= 424 8G (+ for inverting and – for non-inverting)
S1
and V
S2
. Power supply
9
(2) Whenever possible, use surface mount. Don’t use point-
to-point wiring as the increase in wiring inductance will be
detrimental to AC performance. However, if it must be used,
very short, direct signal paths are required. The input signal
ground return, the load ground return, and the power supply
common should all be connected to the same physical point
to eliminate ground loops, which can cause unwanted feed-
back.
3) Surface mount on the backside of the PC Board. Good
component selection is essential. Capacitors used in critical
locations should be a low inductance type with a high quality
dielectric material. Likewise, diodes used in critical loca-
tions should be Schottky barrier types, such as HP5082-
2835 for fast recovery and minimum charge storage. Ordi-
nary diodes will not be suitable in RF circuits.
4) Use a small feedback resistor (usually 25 ) in unity-gain
voltage follower applications for the best performance. For
gain configurations, resistors used in feedback networks
should have values of a few hundred ohms for best perfor-
mance. Shunt capacitance problems limit the acceptable
resistance range to about 1k on the high end and to a value
that is within the amplifier’s output drive limits on the low
end. Metal film and carbon resistors will be satisfactory, but
wirewound resistors (even “non-inductive” types) are abso-
lutely unacceptable in high-frequency circuits. Feedback
resistors should be placed directly between the output and
the inverting input on the backside of the PC board. This
placement allows for the shortest feedback path and the
highest bandwidth. A longer feedback path than this will
decrease the realized bandwidth substantially. Refer to the
demonstration board layout at the end of the data sheet.
5) Surface-mount components (chip resistors, capacitors,
etc.) have low lead inductance and are therefore strongly
recommended. Circuits using all surface-mount components
with the OPA644U (SO-8 package) will offer the best AC
performance.
6) Avoid overloading the output. Remember that output
current must be provided by the amplifier to drive its own
feedback network as well as to drive its load. Lowest
distortion is achieved with high impedance loads.
7) Don’t forget that these amplifiers use
Although they will operate perfectly well with +5V and
–5.2V, use of 15V supplies will destroy the part.
8) Standard commercial test equipment has not been de-
signed to test devices in the OPA644’s speed range. Bench-
top op amp testers and ATE systems will require a special
test head to successfully test these amplifiers.
9) Terminate transmission line loads. Unterminated lines,
such as coaxial cable, can appear to the amplifier to be a
capacitive or inductive load. By terminating a transmission
line with its characteristic impedance, the amplifier’s load
then appears purely resistive.
10) Plug-in prototype boards and wire-wrap boards will not
be satisfactory. A clean layout using RF techniques is
essential; there are no shortcuts.
OPA644
5V supplies.
®