THS4501CD datasheet(32/37 Pages) TI

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THS4500

THS4501

SLOS350D − APRIL 2002 − REVISED JANUARY 2004

www.ti.com

DRIVING CAPACITIVE LOADS

High-speed amplifiers are typically not well-suited for

driving large capacitive loads. If necessary, however, the

load capacitance should be isolated by two isolation

resistors in series with the output. The requisite isolation

resistor size depends on the value of the capacitance, but

10 to 25

Ω is a good place to begin the optimization

process. Larger isolation resistors decrease the amount of

peaking in the frequency response induced by the

capacitive load, but this comes at the expense of larger

voltage drop across the resistors, increasing the output

swing requirements of the system.

−

Riso

Riso = 10 − 25

Ω

−VS

Riso

Use of Isolation Resistors With a Capacitive Load.

Figure 118

POWER SUPPLY DECOUPLING

TECHNIQUES AND RECOMMENDATIONS

Power supply decoupling is a critical aspect of any

high-performance amplifier design process. Careful

decoupling provides higher quality ac performance (most

notably improved distortion performance). The following

guidelines ensure the highest level of performance.

Place decoupling capacitors as close to the power

supply inputs as possible, with the goal of minimizing

the inductance of the path from ground to the power

supply.

Placement priority should be as follows: smaller

capacitors should be closer to the device.

Use

solid

power

and

ground

planes

recommended to reduce the inductance along power

supply return current paths.

Recommended values for power supply decoupling

include 10-

µF and 0.1-µF capacitors for each supply.

A 1000-pF capacitor can be used across the supplies

as well for extremely high frequency return currents,

but often is not required.

EVALUATION FIXTURES, SPICE MODELS,

AND APPLICATIONS SUPPORT

Texas Instruments is committed to providing its customers

with the highest quality of applications support. To support

this goal, an evaluation board has been developed for the

THS4500 family of fully differential amplifiers. The

evaluation board can be obtained by ordering through the

Texas Instruments web site, www.ti.com, or through your

local Texas Instruments sales representative. Schematic

for the evaluation board is shown below with the default

component values. Unpopulated footprints are shown to

provide insight into design flexibility.

VOCM

−VS

PwrPad

R0805

C0805

R5 R0805

C0805

R0805

C0805

R0805

C0805

R1206

R11

R1206

R0805

THS450X

Simplified Schematic of the Evaluation Board. Power

Supply Decoupling, VOCM, and Power Down Circuitry

Not Shown

Figure 119

Computer simulation of circuit performance using SPICE

is often useful when analyzing the performance of analog

circuits and systems. This is particularly true for video and

RF amplifier circuits where parasitic capacitance and

inductance can have a major effect on circuit performance.

A SPICE model for the THS4500 family of devices is

available through either the Texas Instruments web site

(www.ti.com) or as one model on a disk from the Texas

Instruments

Product

Information

Center

(1−800−548−6132). The PIC is also available for design

assistance and detailed product information at this

number. These models do a good job of predicting

small-signal ac and transient performance under a wide

variety of operating conditions. They are not intended to

model the distortion characteristics of the amplifier, nor do

they attempt to distinguish between the package types in

their small-signal ac performance. Detailed information

about what is and is not modeled is contained in the model

file itself.

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