OPA685 datasheet(21/31 Pages) TI2

21 / 31 page

OPA3684

SBOS241C

www.ti.com

BOARD LAYOUT GUIDELINES

Achieving optimum performance with a high-frequency am-

plifier like the OPA3684 requires careful attention to board

layout parasitics and external component types. Recommen-

dations that will optimize performance include:

Minimize parasitic capacitance to any AC ground for

all of the signal I/O pins. Parasitic capacitance on the

output and inverting input pins can cause instability; on

the noninverting input, it can react with the source

impedance to cause unintentional bandlimiting. To re-

duce unwanted capacitance, a window around the sig-

nal I/O pins should be opened in all of the ground and

power planes around those pins. Otherwise, ground and

power planes should be unbroken elsewhere on the

board.

Minimize the distance (< 0.25") from the power-supply

pins to high-frequency 0.1

µF decoupling capacitors. At

the device pins, the ground and power-plane layout

should not be in close proximity to the signal I/O pins.

Avoid narrow power and ground traces to minimize

inductance between the pins and the decoupling capaci-

tors. The power-supply connections should always be

decoupled with these capacitors. An optional supply de-

coupling capacitor (0.01

µF) across the two power sup-

plies (for bipolar operation) will improve 2nd-harmonic

distortion performance. Larger (2.2

µF to 6.8µF)

decoupling capacitors, effective at lower frequencies,

should also be used on the main supply pins. These may

be placed somewhat farther from the device and may be

shared among several devices in the same area of the

PCB.

Careful selection and placement of external compo-

nents will preserve the high-frequency performance

of the OPA3684. Resistors should be a very low reac-

tance type. Surface-mount resistors work best and allow

a tighter overall layout. Metal film and carbon composi-

tion axially-leaded resistors can also provide good high-

frequency performance. Again, keep their leads and

PCB trace length as short as possible. Never use

wirewound type resistors in a high-frequency applica-

tion. Since the output pin and inverting input pin are the

most sensitive to parasitic capacitance, always position

the feedback and series output resistor, if any, as close

as possible to the output pin. The quad amplifier pinout

allows each output and inverting input to be connected

by the feedback element with virtually no trace length.

Other network components, such as noninverting input

termination resistors, should also be placed close to the

package. The frequency response is primarily deter-

mined by the feedback resistor value as described

previously. Increasing its value will reduce the peaking

at higher gains, while decreasing it will give a more

peaked frequency response at lower gains. The 800

Ω

feedback resistor used in the Typical Characteristics at

a gain of +2 on

±5V supplies is a good starting point for

design. Note that a 800

Ω feedback resistor, rather than

a direct short, is required for the unity-gain follower

application. A current-feedback op amp requires a feed-

back resistor even in the unity-gain follower configura-

tion to control stability.

Connections to other wideband devices on the board

may be made with short direct traces or through onboard

transmission lines. For short connections, consider the

trace and the input to the next device as a lumped

capacitive load. Relatively wide traces (50mils to 100mils)

should be used, preferably with ground and power

planes opened up around them. Estimate the total ca-

pacitive load and set RS from the plot of recommended

RS vs CLOAD. Low parasitic capacitive loads

(< 5pF) may not need an RS since the OPA3684 is

nominally compensated to operate with a 2pF parasitic

load. If a long trace is required, and the 6dB signal loss

intrinsic to a doubly-terminated transmission line is ac-

ceptable, implement a matched impedance transmis-

sion line using microstrip or stripline techniques (consult

an ECL design handbook for microstrip and stripline

layout techniques). A 50

Ω environment is normally not

necessary on board, and in fact a higher impedance

environment will improve distortion, see the distortion

versus load plots. With a characteristic board trace

impedance defined based on board material and trace

dimensions, a matching series resistor into the trace

from the output of the OPA3684 is used, as well as a

terminating shunt resistor at the input of the destination

device. Remember also that the terminating impedance

will be the parallel combination of the shunt resistor and

the input impedance of the destination device; this total

effective impedance should be set to match the trace

impedance. The high output voltage and current capabil-

ity of the OPA3684 allows multiple destination devices to

be handled as separate transmission lines, each with

their own series and shunt terminations. If the 6dB

attenuation of a doubly-terminated transmission line is

unacceptable, a long trace can be series-terminated at

the source end only. Treat the trace as a capacitive load

in this case and set the series resistor value as shown

in the plot of

RS vs CLOAD. This will not preserve signal

integrity as well as a doubly-terminated line. If the input

impedance of the destination device is LOW, there will

be some signal attenuation due to the voltage divider

formed by the series output into the terminating imped-

ance.

Socketing a high-speed part like the OPA3684 is not

recommended. The additional lead length and pin-to-

pin capacitance introduced by the socket can create an

extremely troublesome parasitic network which can make

it almost impossible to achieve a smooth, stable fre-

quency response. Best results are obtained by soldering

the OPA3684 onto the board.

OPA685 Datasheet(PDF) 21 Page - Texas Instruments

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