OPA646U BURR-BROWN [Burr-Brown Corporation], OPA646U Datasheet - Page 8

• Manufacturer Part Number: OPA646U
• Description: Low Power, Wide Bandwidth OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

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 impedance 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
on the topside of the PC Board is recommended. It is
essential to keep the 0.01 F capacitor very close to the
power supply pins. Refer to the DEM-OPA64X Data Sheet
for the recommended layout and component placements.
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 backside of PC Board. Good compo-
nent 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
locations 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. Refer to the demonstration board layout
at the end of the data sheet. A longer feedback path than
this will decrease the realized bandwidth substantially.
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 OPA646U (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.
®
OPA646
S1
and V
S2
. Power supply
8
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
designed to test devices in the OPA646’s speed range.
Benchtop 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.
OFFSET VOLTAGE ADJUSTMENT
If additional offset adjustment is needed, the circuit in
Figure 1 can be used without degrading offset drift with
temperature. Avoid external adjustment whenever possible
since extraneous noise, such as power supply noise, can be
inadvertently coupled into the amplifier’s inverting input
terminal. Remember that additional offset errors can be
created by the amplifier’s input bias currents. Whenever
possible, match the impedance seen by both inputs as is
shown with R
the amplifier’s offset current.
FIGURE 1. Offset Voltage Trim.
INPUT PROTECTION
Static damage has been well recognized for MOSFET de-
vices, but any semiconductor device deserves protection
NOTE: (1) R
bias currents.
20k
+V
–V
3
is optional and can be used to cancel offset errors due to input
Output Trim Range
CC
CC
3.
This will reduce input bias current errors to
10µF
R
47k
Trim
R
1
V
IN
+V
or Ground
CC
R
R
Trim
2
(1)
OPA646
to –V
R
3
R
= R
2
CC
1
5V supplies.
|| R
R
R
Trim
2
2