OPA646U BURR-BROWN [Burr-Brown Corporation], OPA646U Datasheet - Page 7

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

TYPICAL PERFORMANCE CURVES
At T
APPLICATIONS INFORMATION
DISCUSSION OF PERFORMANCE
The OPA646 provides a level of speed and precision not
previously attainable in monolithic form. Unlike current
feedback amplifiers, the OPA646’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 some 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. Cancelling offset errors (due to input bias currents)
through matching of inverting and non-inverting input resis-
tors 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 reason-
able. Such amplifiers are completely inadequate for fast
settling 12-bit applications.
The OPA646’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 OPA646.
WIRING PRECAUTIONS
Maximizing the OPA646’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
A
= +25 C, V
–60
–70
–80
–90
0
5MHz HARMONIC DISTORTION vs OUTPUT SWING
S
= 5V, R
1.0
L
= 100 , C
(G = +1, R
Output Swing (Vp-p)
L
= 2pF, R
2.0
L
= 100 )
FB
= 402 and all four power supply pins are used, unless otherwise noted. R
3f
2f
O
O
3.0
4.0
7
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 OPA646, 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
(CONT)
–50
–60
–70
–80
0
10MHz HARMONIC DISTORTION vs OUTPUT SWING
1.0
(G = +1, R
Output Swing (Vp-p)
FB
OPA646
2.0
= 25
L
= 100 )
3f
2f
O
O
for a gain of +1.
3.0
4.0
®