Datasheet

IMD
3
= P
S
− P
O
P
S
P
O
P
O
∆f
c
= f
c
− f1
∆f
c
= f2 − f
c
P
S
f
c
− 3∆f f1 f
c
f2 f
c
+ 3∆f
Power
f − Frequency − MHz
IMD
3
OIP
3
IIP
3
3X
P
IN
(dBm)
1X
P
OUT
(dBm)
P
O
P
S
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 of 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 is a
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 as key design criteria in the RF
Due to the intercept point ease-of-use in system level
communications world as a 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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