THS6214IPWPR datasheet(30/41 Pages) TI

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BOARD LAYOUT GUIDELINES

THS6214

SBOS431 – MAY 2009 ....................................................................................................................................................................................................... www.ti.com

peaked frequency response. The 1.24k

Ω feedback

resistor used in the Typical Characteristics at a gain

Achieving

optimum

performance

with

of +10V/V on ±12V supplies is a good starting point

high-frequency

amplifier

such

the

THS6214

for design. Note that a 1.5k

Ω feedback resistor,

requires careful attention to board layout parasitic and

rather than a direct short, is recommended for a

external component types. Recommendations that

unity-gain follower application. A current-feedback op

optimize performance include:

amp requires a feedback resistor to control stability

even in the unity-gain follower configuration.

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 to cause unintentional band

connections, consider the trace and the input to the

limiting. To reduce unwanted capacitance, a window

next device as a lumped capacitive load. Relatively

around the signal I/O pins should be opened in all of

wide traces (50mils to 100mils [.050in to .100in, or

the ground and power planes around those pins.

1,27mm to 2,54mm]) should be used, preferably with

Otherwise, ground and power planes should be

ground and power planes opened up around them.

unbroken elsewhere on the board.

Estimate the total capacitive load and set RS from the

plot of Recommended RS vs Capacitive Load (see

b) Minimize the distance (less than 0.25in, or

Figure 6, Figure 24, Figure 36, Figure 48, Figure 61,

6,35mm)

from

the

power-supply

pins

and Figure 73). Low parasitic capacitive loads (less

high-frequency 0.1

µF decoupling capacitors. At the

than 5pF) may not need an isolation resistor because

device pins, the ground and power plane layout

the THS6214 is nominally compensated to operate

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

with a 2pF parasitic load. If a long trace is required,

Avoid narrow power and ground traces to minimize

and

the

6dB

signal

loss

intrinsic

inductance between the pins and the decoupling

doubly-terminated transmission line is acceptable,

capacitors. The power-supply connections should

implement a matched-impedance transmission line

always be decoupled with these capacitors. An

using microstrip or stripline techniques (consult an

optional supply decoupling capacitor across the two

ECL design handbook for microstrip and stripline

power supplies (for bipolar operation) improves

layout

techniques).

Ω

environment

not

second-harmonic

distortion

performance.

Larger

necessary on board; in fact, a higher impedance

(2.2

µF to 6.8µF) decoupling capacitors, effective at

environment improves distortion (see the distortion

lower frequencies, should also be used on the main

versus load plots). With a characteristic board trace

supply

pins.

These

capacitors

can

placed

impedance defined based on board material and

somewhat farther from the device and may be shared

trace dimensions, a matching series resistor into the

among several devices in the same area of the PCB.

trace from the output of the THS6214 is used, as well

c) Careful selection and placement of external

as a terminating shunt resistor at the input of the

components

preserve

the

high-frequency

destination

device.

Remember

also

that

the

performance of the THS6214. Resistors should be a

terminating impedance is the parallel combination of

very low reactance type. Surface-mount resistors

the shunt resistor and the input impedance of the

work best and allow a tighter overall layout. Metal film

destination device.

and carbon composition, axially-leaded resistors can

This total effective impedance should be set to match

also provide good high-frequency performance.

the trace impedance. The high output voltage and

Again, keep leads and PCB trace length as short as

current capability of the THS6214 allows multiple

possible. Never use wire-wound type resistors in a

destination devices to be handled as separate

high-frequency application. Although the output pin

transmission lines, each with their own series and

and inverting input pin are the most sensitive to

shunt terminations. If the 6dB attenuation of a

parasitic capacitance, always position the feedback

doubly-terminated transmission line is unacceptable,

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

a long trace can be series-terminated at the source

to the output pin. Other network components, such as

end only.

noninverting input termination resistors, should also

Treat the trace as a capacitive load in this case and

be placed close to the package. Where double-side

set the series resistor value as shown in the plot of

component mounting is allowed, place the feedback

RS vs Capacitive Load. However, this configuration

resistor directly under the package on the other side

does not preserve signal integrity as well as a

of the board between the output and inverting input

doubly-terminated line. If the input impedance of the

pins. The frequency response is primarily determined

destination device is low, there is some signal

the

feedback

resistor

value

described

attenuation as a result of the voltage divider formed

previously.

Increasing

the

value

reduces

the

by the series output into the terminating impedance.

bandwidth, whereas decreasing it leads to a more

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