OPA688

13

SBOS082A

DESIGN-IN TOOLS

APPLICATIONS SUPPORT

The Texas Instruments Applications Department is available

for design assistance at 1-972-644-5580. The Texas

Instrments web site (http://www.ti.com) has the latest data

sheets and other design aids.

DEMONSTRATION BOARDS

A PC board is available to assist in the initial evaluation of

circuit performance of the OPA688U. It is available as an

unpopulated PCB with descriptive documentation. See the

demonstration board literature for more information. The

summary information for this board is shown in Table I.

c) Place external components close to the OPA688. This

minimizes inductance, ground loops, transmission line ef-

fects and propagation delay problems. Be extra careful with

d) Use high-frequency components to minimize parasitic

elements. Resistors should be a very low reactance type.

Surface-mount resistors work best and allow a tighter lay-

out. Metal film or carbon composition axially-leaded resis-

tors can also provide good performance when their leads are

as short as possible. Never use wirewound resistors for high-

frequency applications. Remember that most potentiometers

have large parasitic capacitances and inductances.

Multilayer ceramic chip capacitors work best and take up

little space. Monolithic ceramic capacitors also work very

well. Use RF type capacitors with low ESR and ESL. The

large power pin bypass capacitors (2.2

µF to 6.8µF) should

be tantalum for better high-frequency and pulse perfor-

mance.

e) Choose low resistor values to minimize the time constant

set by the resistor and its parasitic parallel capacitance. Good

metal film or surface mount resistors have approximately

0.2pF parasitic parallel capacitance. For resistors > 1.5k

Ω,

this adds a pole and/or zero below 500MHz.

Make sure that the output loading is not too heavy. The

recommended 402

Ω feedback resistor is a good starting

point in your design.

f) Use short direct traces to other wideband devices on the

board. Short traces act as a lumped capacitive load. Wide traces

(50 to 100 mils) should be used. Estimate the total capacitive

load at the output, and use the series isolation resistor recom-

Load”. Parasitic loads < 2pF may not need the isolation resistor.

g) When long traces are necessary, use transmission line

design techniques (consult an ECL design handbook for

microstrip and stripline layout techniques). A 50

Ω transmis-

sion line is not required on board—a higher characteristic

impedance will help reduce output loading. Use a matching

series resistor at the output of the op amp to drive a

transmission line, and a matched load resistor at the other

end to make the line appear as a resistor. If the 6dB of

attenuation that the matched load produces is not acceptable,

and the line is not too long, use the series resistor at the

source only. This will isolate the source from the reactive

load presented by the line, but the frequency response will

be degraded.

Multiple destination devices are best handled as separate

transmission lines, each with its own series source and shunt

load terminations. Any parasitic impedances acting on the

terminating resistors will alter the transmission line match,

and can cause unwanted signal reflections and reactive

loading.

h) Do not use sockets for high-speed parts like the OPA688.

The additional lead length and pin-to-pin capacitance intro-

duced by the socket creates an extremely troublesome para-

sitic network. Best results are obtained by soldering the part

onto the board.

LITERATURE

DEMONSTRATION

REQUEST

BOARD

PACKAGE

PRODUCT

NUMBER

DEM-OPA68xU

SO-8

OPA68xU

MKT-351

TABLE I. Demo Board Summary Information.

Contact the Texas Instruments Technical Applications Sup-

port Line at 1-972-644-5580 for availability of these boards.

OPERATING INFORMATION

THEORY OF OPERATION

The OPA688 is a voltage-feedback op amp that is unity-gain

stable. The output voltage is limited to a range set by the voltage

on the limiter pins (5 and 8). When the input tries to overdrive the

output, the limiters take control of the output buffer. This avoids

saturating any part of the signal path, giving quick overdrive

recovery and excellent limiter accuracy at any signal gain.

The limiters have a very sharp transition from the linear region

of operation to output limiting. This allows the limiter voltages to

be set very near (< 100mV) the desired signal range. The

distortion performance is also very good near the limiter voltages.

CIRCUIT LAYOUT

Achieving optimum performance with the high-frequency

OPA688 requires careful attention to layout design and

component selection. Recommended PCB layout techniques

and component selection criteria are:

a) Minimize parasitic capacitance to any AC ground for

all of the signal I/O pins. Open a window in the ground and

power planes around the signal I/O pins, and leave the

ground and power planes unbroken elsewhere.

b) Provide a high quality power supply. Use linear regulators,

ground plane and power planes to provide power. Place high-

frequency 0.1

µF decoupling capacitors < 0.2" away from each

power-supply pin. Use wide, short traces to connect to these

capacitors to the ground and power planes. Also use larger

(2.2

µF to 6.8µF) high-frequency decoupling capacitors to by-

pass lower frequencies. They may be somewhat further from the

device, and be shared among several adjacent devices.