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OPA640

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APPLICATIONS INFORMATION

DISCUSSION OF PERFORMANCE

The OPA640 provides a level of speed and precision not

previously attainable in monolithic form. Unlike current

feedback amplifiers, the OPA640’s design uses a “Classi-

cal” 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 disadvan-

tages. 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 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 reason-

able. Such amplifiers are completely inadequate for fast

settling 12-bit applications.

The OPA640’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

OPA640.

WIRING PRECAUTIONS

Maximizing the OPA640’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 OPA640, 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

(2oz 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

µF)

with very short leads are recommended. A parallel 0.01

µF

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.

–65

–75

–85

–95

0

Output Swing (Vp-p)

10MHz HARMONIC DISTORTION vs OUTPUT SWING

(G = +1, R

1.0

2.0

3.0

4.0

2f

O

3f

O