Datasheet
SAVING POWER WITH POWER-DOWN
LINEARITY: DEFINITIONS, TERMINOLOGY,
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
THS4504
THS4505
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......................................................................................................................................................... SLOS363D – AUGUST 2002 – REVISED MAY 2008
Amplifiers are generally thought of as linear devices.
FUNCTIONALITY In other words, the output of an amplifier is a linearly
scaled version of the input signal applied to it. In
The THS4500 family of fully differential amplifiers
reality, however, amplifier transfer functions are
contains devices that come with and without the
nonlinear. Minimizing amplifier nonlinearity is a
power-down option. Even-numbered devices have
primary design goal in many applications.
power-down capability, which is described in detail
here. Intercept points are specifications that have long
been used as key design criteria in the RF
The power-down pin of the amplifiers defaults to the
communications world as a metric for the
positive supply voltage in the absence of an applied
intermodulation distortion performance of a device in
voltage (i.e. an internal pullup resistor is present),
the signal chain (for example, amplifiers, mixers,
putting the amplifier in the power-on mode of
etc.). Use of the intercept point, rather than strictly the
operation. To turn off the amplifier in an effort to
intermodulation distortion, allows for simpler
conserve power, the power-down pin can be driven
system-level calculations. Intercept points, like noise
towards the negative rail. The threshold voltages for
figures, can be easily cascaded back and forth
power-on and power-down are relative to the supply
through a signal chain to determine the overall
rails and given in the specification tables. Above the
receiver chain intermodulation distortion performance.
enable threshold voltage, the device is on. Below the
The relationship between intermodulation distortion
disable threshold voltage, the device is off. Behavior
and intercept point is depicted in Figure 85 and
in between these threshold voltages is not specified.
Figure 86 .
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
setting resistors.
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.
CIRCUIT TECHNIQUES, AND DESIGN
TRADEOFFS
Figure 85. 2-Tone and 3rd-Order Intermodulation
The THS4500 family of devices features
Products
unprecedented distortion performance for monolithic
fully differential amplifiers. This section focuses on
Due to the intercept point's ease of use in system
the fundamentals of distortion, circuit techniques for
level calculations for receiver chains, it has become
reducing nonlinearity, and methods for equating
the specification of choice for guiding
distortion of fully differential amplifiers to desired
distortion-related design decisions. Traditionally,
linearity specifications in RF receiver chains.
these systems use primarily class-A, single-ended RF
amplifiers as gain blocks. These RF amplifiers are
typically designed to operate in a 50- Ω environment,
just like the rest of the receiver chain. Since intercept
points are given in dBm, this implies an associated
impedance (50 Ω ).
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