THS6204IPWPR datasheet(32/43 Pages) TI

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THS6204

SBOS416C – OCTOBER 2007 – REVISED APRIL 2009 ................................................................................................................................................... www.ti.com

c) Careful selection and placement of external

distortion (see the distortion versus load plots). With a

components

preserve

the

high-frequency

characteristic board trace impedance defined based

performance of the THS6204. Resistors should be a

on board material and trace dimensions, a matching

very low reactance type. Surface-mount resistors

series resistor into the trace from the output of the

work best and allow a tighter overall layout. Metal film

THS6204 is used, as well as a terminating shunt

and carbon composition axially leaded resistors can

resistor at the input of the destination device.

also provide good high-frequency performance.

Remember also that the terminating impedance is the

parallel combination of the shunt resistor and the

Again, keep leads and PCB trace length as short as

input impedance of the destination device.

possible. Never use wire-wound type resistors in a

high-frequency application. Although the output pin

This total effective impedance should be set to match

and inverting input pin are the most sensitive to

the trace impedance. The high output voltage and

parasitic capacitance, always position the feedback

current capability of the THS6204 allows multiple

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

destination devices to be handled as separate

to the output pin. Other network components, such as

transmission lines, each with their own series and

noninverting input termination resistors, should also

shunt terminations. If the 6dB attenuation of a

be placed close to the package. Where double-side

doubly-terminated transmission line is unacceptable,

component mounting is allowed, place the feedback

a long trace can be series-terminated at the source

resistor directly under the package on the other side

end only.

of the board between the output and inverting input

Treat the trace as a capacitive load in this case and

pins. The frequency response is primarily determined

set the series resistor value as shown in the plot of

the

feedback

resistor

value

described

RS vs Capacitive Load (Figure 6, Figure 24, and

previously.

Increasing

the

value

reduces

the

Figure 36). However, this does not preserve signal

bandwidth, whereas decreasing it gives a more

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

peaked frequency response. The 1.24k

Ω feedback

input impedance of the destination device is low,

resistor used in the Typical Characteristics at a gain

there is some signal attenuation due to the voltage

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

divider

formed

the

series

output

into

the

for design. Note that a 1.5k

Ω feedback resistor,

terminating impedance.

rather than a direct short, is recommended for the

unity-gain follower application. A current-feedback op

e) Socketing a high-speed part like the THS6204

amp requires a feedback resistor even in the

is not recommended. The additional lead length and

unity-gain follower configuration to control stability.

pin-to-pin capacitance introduced by the socket can

create an extremely troublesome parasitic network,

d) Connections to other wideband devices on the

which can make it almost impossible to achieve a

board may be made with short direct traces or

smooth, stable frequency response. Best results are

through

onboard

transmission

lines.

For

short

obtained by soldering the THS6204 directly onto the

connections, consider the trace and the input to the

board.

next device as a lumped capacitive load. Relatively

wide traces (50mils to 100mils) should be used,

f) Use the –VS plane to conduct heat out of the

preferably with ground and power planes opened up

QFN-24 and TSSOP-24 PowerPAD packages. These

around them. Estimate the total capacitive load and

packages attach the die directly to an exposed

set RS from the plot of Recommended RS vs

thermal pad on the bottom, which should be soldered

Capacitive Load (Figure 6, Figure 24, and Figure 36).

to the board. This pad must be connected electrically

Low parasitic capacitive loads (< 5pF) may not need

to the same voltage plane as the most negative

because

the

THS6204

nominally

supply applied to the THS6204 (in Figure 82, this

compensated to operate with a 2pF parasitic load. If a

would be –12V).

long trace is required, and the 6dB signal loss

intrinsic to a doubly-terminated transmission line is

acceptable,

implement

matched

impedance

transmission

line

using

microstrip

stripline

techniques (consult an ECL design handbook for

microstrip and stripline layout techniques). A 50

Ω

environment is normally not necessary on board; in

fact, a higher impedance environment improves

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