Oscillations at frequencies of 200MHz and above 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.
APPLICATIONS INFORMATION
DISCUSSION OF PERFORMANCE
The OPA620 provides a level of speed and precision not
previously attainable in monolithic form. Unlike current
feedback amplifiers, the OPA620’s design uses a “classical”
operational amplifier architecture and can therefore be used
in all traditional operational amplifier applications. While it
is true that current feedback amplifiers can provide wider
bandwidth at higher gains, they offer many disadvantages.
The asymmetrical input characteristics of current feedback
amplifiers (i.e., one input is a low impedance) prevents them
from being used in a variety of applications. In addition,
unbalanced inputs make input bias current errors difficult to
correct. Bias current cancellation through matching of in-
verting and non-inverting input resistors is impossible
because the input bias currents 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 inadequate for fast settling 12-bit applications.
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 (1µF to
10µF) with very short leads are recommended. A parallel
0.1µF ceramic should be added at the supply pins. Surface
mount bypass capacitors will produce excellent results due
to their low lead inductance. Additionally, suppression fil-
ters can be used to isolate noisy supply lines. Properly
bypassed and modulation-free power supply lines allow full
amplifier output and optimum settling time
performance.
Points to Remember
1) Don’t use point-to-point wiring as the increase in wiring
inductance will be detrimental to AC performance. How-
ever, 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.
The OPA620’s “classical” operational amplifier architecture
employs true differential and fully symmetrical inputs to
eliminate these troublesome problems. All traditional circuit
configurations and op amp theory apply to the OPA620. The
use of low-drift thin-film resistors allows internal operating
currents to be laser-trimmed at wafer-level to optimize AC
performance such as bandwidth and settling time, as well as
DC parameters such as input offset voltage and drift. The
result is a wideband, high-frequency monolithic operational
amplifier with a gain-bandwidth product of 200MHz, a
0.01% settling time of 25ns, and an input offset voltage
of 200µV.
2) 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
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.
3) Whenever possible, solder the OPA620 directly into the
PC board without using a socket. Sockets add parasitic
capacitance and inductance, which can seriously degrade
AC performance or produce oscillations. If sockets must be
used, consider using zero-profile solderless sockets such as
Augat part number 8134-HC-5P2. Alternately, Teflon® stand-
offs located close to the amplifier’s pins can be used to
mount feedback components.
WIRING PRECAUTIONS
Maximizing the OPA620’s capability requires some wiring
precautions and high-frequency layout techniques.
Oscillation, 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.
4) Resistors used in feedback networks should have values
of a few hundred ohms for best performance. Shunt capaci-
tance problems limit the acceptable resistance range to about
1kΩ 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 absolutely
unacceptable in high-frequency circuits.
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 OPA620KU (SO-8 package) will offer the best AC
performance. The parasitic package inductance and capaci-
tance for the SO-8 is lower than the both the Cerdip and
8-lead Plastic DIP.
Grounding is the most important application consideration
for the OPA620, as it is with all high-frequency circuits.
Teflon® E. I. Du Pont de Nemours & Co.
®
9
OPA620
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