OPA641H BURR-BROWN [Burr-Brown Corporation], OPA641H Datasheet - Page 7

• Manufacturer Part Number: OPA641H
• Description: Wideband Voltage Feedback OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

APPLICATIONS INFORMATION
DISCUSSION OF PERFORMANCE
The OPA641 provides a level of speed and precision not
previously attainable in monolithic form. Unlike current
feedback amplifiers, the OPA641’s design uses a “Classi-
cal” operational amplifier architecture and can therefore
be used in all traditional operational amplifier applica-
tions. While it is true that current feedback amplifiers can
provide wider bandwidth at higher gains, they offer some
disadvantages. The asymmetrical input characteristics of
current feedback amplifiers (i.e., one input is a low imped-
ance) prevents them from being used in a variety of
applications. In addition, unbalanced inputs make input
bias current errors difficult to correct. Bias current cancel-
lation through matching of inverting and non-inverting
input resistors is impossible because the input bias cur-
rents are uncorrelated. Current noise is also asymmetrical
and is usually significantly higher on the inverting input.
Perhaps most important, settling time to 0.01% is often
extremely poor due to internal design tradeoffs. Many
current feedback designs exhibit settling times to 0.01% in
excess of 10 microseconds even though 0.1% settling
times are reasonable. Such amplifiers are completely in-
adequate for fast settling 12-bit applications.
The OPA641’s “Classical” operational amplifier architec-
ture employs true differential and fully symmetrical inputs
to eliminate these troublesome problems. All traditional
circuit configurations and op amp theory apply to the
OPA641.
WIRING PRECAUTIONS
Maximizing the OPA641’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 OPA641, 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
7
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 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 Datasheet
for the recommended layout and component placement.
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 feedback.
3) Surface mount on the PC Board. Good component selec-
tion 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. Ordinary diodes will
not be suitable in RF circuits.
4) Whenever possible, solder the OPA641 directly into the
PC board without using a socket. Sockets add parasitic
capacitance and inductance, which can seriously degrade
AC performance or produce oscillations.
5) 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. See the demonstration board layout at
OPA641
S1
and V
S2
. Power supply
®