THS4501CDG4 datasheet(28/48 Pages) TI1

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IMD3 = PS − PO

∆fc = fc − f1

∆fc = f2 − fc

fc − 3∆f

f1 fc f2

fc + 3∆f

f − Frequency − MHz

IMD3

OIP3

IIP3

(dBm)

POUT

(dBm)

THS4500

THS4501

SLOS350F

– APRIL 2002 – REVISED OCTOBER 2011

www.ti.com

SAVING POWER WITH POWER-DOWN

etc.). Use of the intercept point, rather than strictly the

FUNCTIONALITY

intermodulation

distortion,

allows

for

simpler

system-level calculations. Intercept points, like noise

The THS4500 family of fully differential amplifiers

figures, can be easily cascaded back and forth

contains devices that come with and without the

through a signal chain to determine the overall

power-down option. Even-numbered devices have

receiver chain intermodulation distortion performance.

power-down capability, which is described in detail

The relationship between intermodulation distortion

here.

and intercept point is depicted in Figure 108 and

Figure 109.

The power-down pin of the amplifiers defaults to the

positive supply voltage in the absence of an applied

voltage (that is, an internal pull-up resistor is present),

putting the amplifier in the power-on mode of

operation. To turn off the amplifier in an effort to

conserve power, the power-down pin can be driven

towards the negative rail. The threshold voltages for

power-on and power-down are relative to the supply

rails and given in the specification tables. Above the

enable threshold voltage, the device is on. Below the

disable threshold voltage, the device is off. Behavior

between these threshold voltages is not specified.

Note that this power-down functionality is just that;

the amplifier consumes less power in power-down

mode. The power-down mode is not intended to

provide a high-impedance output. In other words, the

power-down functionality is not intended to allow use

as a 3-state bus driver. When in power-down mode,

the impedance looking back into the output of the

amplifier is dominated by the feedback and gain

Figure 108. 2-Tone and 3rd-Order

setting resistors.

Intermodulation Products

The time delays associated with turning the device on

and off are specified as the time it takes for the

amplifier to reach 50% of the nominal quiescent

current. The time delays are on the order of

microseconds because the amplifier moves in and out

of the linear mode of operation in these transitions.

LINEARITY: DEFINITIONS, TERMINOLOGY,

CIRCUIT TECHNIQUES, AND DESIGN

TRADEOFFS

The

THS4500

family

devices

features

unprecedented distortion performance for monolithic

fully differential amplifiers. This section focuses on

the fundamentals of distortion, circuit techniques for

reducing nonlinearity, and methods for equating

distortion of fully differential amplifiers to desired

linearity specifications in RF receiver chains.

Amplifiers are generally thought of as linear devices.

In other words, the output of an amplifier is a linearly

scaled version of the input signal applied to it. In

reality, however, amplifier transfer functions are

nonlinear.

Minimizing

amplifier

nonlinearity

primary design goal in many applications.

Figure 109. Graphical Representation of 2-Tone

and 3rd-Order Intercept Point

Intercept points are specifications that have long

been

used

key

design

criteria

the

Due to the intercept point ease-of-use in system level

communications

world

metric

for

the

calculations for receiver chains, it has become the

intermodulation distortion performance of a device in

specification of choice for guiding distortion-related

the signal chain (for example, amplifiers, mixers,

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