OPA690 datasheet(27/35 Pages) TI

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BOARD LAYOUT GUIDELINES

OPA2890

SBOS364A – DECEMBER 2007 – REVISED DECEMBER 2007

can add a pole and/or zero below 500MHz that can

Achieving

optimum

performance

with

effect circuit operation. Keep resistor values as low

high-frequency

amplifier

such

the

OPA2890

possible

consistent

with

load

driving

requires careful attention to board layout parasitics

considerations. The 750

Ω feedback used in the

and external component types. Recommendations

Electrical Characteristics is a good starting point for

that optimize performance include:

design. Note that a 0

Ω feedback resistor is suggested

for the unity-gain follower application.

a) Minimize parasitic capacitance to any ac ground

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

d) Connections to other wideband devices on the

the output and inverting input pins can cause

board may be made with short, direct traces or

instability: on the noninverting input, it can react with

through

onboard

transmission

lines.

For

short

the

source

impedance

cause

unintentional

connections, consider the trace and the input to the

bandlimiting. To reduce unwanted capacitance, a

next device as a lumped capacitive load. Relatively

window around the signal I/O pins should be opened

wide traces (50mils to 100mils, or 1.27mm to

in all of the ground and power planes around those

2.54mm) should be used, preferably with ground and

pins. Otherwise, ground and power planes should be

power planes opened up around them. Estimate the

unbroken elsewhere on the board.

total capacitive load and set RS from the plots of

Figure 15 and Figure 36. Low parasitic capacitive

b) Minimize the distance (< 0.25in, or 6.35mm) from

loads (< 3pF) may not need an RS because the

the

power-supply

pins

high-frequency

0.1

µF

OPA2890 is nominally compensated to operate with a

decoupling capacitors. At the device pins, the ground

2pF parasitic load. Higher parasitic capacitive loads

and power-plane layout should not be in close

without an RS are allowed as the signal gain

proximity to the signal I/O pins. Avoid narrow power

increases (increasing the unloaded phase margin;

and ground traces to minimize inductance between

see Figure 61). If a long trace is required, and the

the

pins

and

the

decoupling

capacitors.

The

6dB signal loss intrinsic to a doubly-terminated

power-supply

connections

should

always

transmission line is acceptable, implement a matched

decoupled with these capacitors. An optional supply

impedance transmission line using microstrip or

decoupling capacitor (0.1

µF) across the two power

stripline techniques (consult an ECL design handbook

supplies

(for

bipolar

operation)

improves

for microstrip and stripline layout techniques). A 50

Ω

2nd-harmonic distortion performance. Larger (2.2

µF

environment is normally not necessary on board, and

to 6.8

µF) decoupling capacitors, effective at lower

in fact, a higher impedance environment improves

frequencies, should also be used on the main supply

distortion as shown in the distortion versus load plots.

pins. These capacitors may be placed somewhat

With a characteristic board trace impedance defined

farther from the device and may be shared among

(based on board material and trace dimensions), a

several devices in the same area of the printed circuit

matching series resistor into the trace from the output

board (PCB).

of the OPA2890 is used as well as a terminating

c) Careful selection and placement of external

shunt resistor at the input of the destination device.

components

preserves

the

high-frequency

Remember also that the terminating impedance is the

performance of the OPA2890. Resistors should be

parallel combination of the shunt resistor and the

a very low reactance type. Surface-mount resistors

input impedance of the destination device; this total

work best and allow a tighter overall layout. Metal film

effective impedance should be set to match the trace

or carbon composition axially-leaded resistors can

impedance.

also

provide

good

high-frequency

performance.

e) Socketing a high-speed part such as the

Again, keep the leads and PCB traces as short as

OPA2890 is not recommended. The additional lead

possible. Never use wirewound type resistors in a

length and pin-to-pin capacitance introduced by the

high-frequency application. Because the output pin

socket can create an extremely troublesome parasitic

and inverting input pin are the most sensitive to

network that can make it almost impossible to

parasitic capacitance, always position the feedback

achieve a smooth, stable frequency response. Best

and series output resistor, if any, as close as possible

results are obtained by soldering the OPA2890 onto

to the output pin. Other network components, such as

the board.

noninverting input termination resistors, should also

be placed close to the package. Even with a low

parasitic capacitance shunting the external resistors,

excessively high resistor values can create significant

time constants that can degrade performance. Good

axial metal film or surface-mount resistors have

approximately 0.2pF in shunt with the resistor. For

resistor values > 1.5k

Ω, this parasitic capacitance

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