Datasheet

V
OCM
= 2.5 V
5 V
V
S
R
S
R
g1
R
g2
R
f1
R
f2
+
-
R
T
-
+
R
L
2.5-V DC
2.5-V DC
DC Current Path to Ground
DC Current Path to Ground
I
2
=
V
OCM
R
f2
+ R
g2
Depiction of DC Power Dissipation Caused By
Output Level-Shifting in a DC-Coupled Circuit
I
1
=
V
OCM
R
f1
+ R
g1
+ R
S
|| R
T
THS4502
THS4503
www.ti.com SLOS352E –APRIL 2002–REVISED OCTOBER 2011
specific source of electromagnetic or radio frequency operation. To turn off the amplifier in an effort to
interference is present elsewhere in the system. conserve power, the power-down pin can be driven
Information on the ac performance (bandwidth, slew towards the negative rail. The threshold voltages for
rate) of the V
OCM
circuitry is included in the power-on and power-down are relative to the supply
specification table and graph section. rails and given in the specification tables. Above the
enable threshold voltage, the device is on. Below the
Since the V
OCM
pin provides the ability to set an
disable threshold voltage, the device is off. Behavior
output common-mode voltage, the ability for
in between these threshold voltages is not specified.
increased power dissipation exists. While this does
not pose a performance problem for the amplifier, it Note that this power-down functionality is just that;
can cause additional power dissipation of which the the amplifier consumes less power in power-down
system designer should be aware. The circuit shown mode. The power-down mode is not intended to
in Figure 102 demonstrates an example of this provide a high-impedance output. In other words, the
phenomenon. For a device operating on a single 5-V power-down functionality is not intended to allow use
supply with an input signal referenced around ground as a 3-state bus driver. When in power-down mode,
and an output common-mode voltage of 2.5 V, a dc the impedance looking back into the output of the
potential exists between the outputs and the inputs of amplifier is dominated by the feedback and gain
the device. The amplifier sources current into the setting resistors.
feedback network in order to provide the circuit with
The time delays associated with turning the device on
the proper operating point. While there are no serious
and off are specified as the time it takes for the
effects on the circuit performance, the extra power
amplifier to reach 50% of the nominal quiescent
dissipation may need to be included in the system's
current. The time delays are on the order of
power budget.
microseconds because the amplifier moves in and out
of the linear mode of operation in these transitions.
LINEARITY; DEFINITIIONS, 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 aslinear 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 102.
Intercept points are specifications that have long
been used as key design criteria in the RF
communications world as a metric for the
SAVING POWER WITH POWER-DOWN
intermodulation distortion performance of a device in
FUNCTIONALITY
the signal chain (e.g., amplifiers, mixers, etc.). Use of
the intercept point, rather than strictly the
The THS4500 family of fully differential amplifiers
intermodulation distortion, allows for simpler
contains devices that come with and without the
system-level calculations. Intercept points, like noise
power-down option. Even-numbered devices have
figures, can be easily cascaded back and forth
power-down capability, which is described in detail
through a signal chain to determine the overall
here.
receiver chain's intermodulation distortion
The power-down pin of the amplifiers defaults to the
performance. The relationship between
positive supply voltage in the absence of an applied
intermodulation distortion and intercept point is
voltage (i.e. an internal pullup resistor is present),
depicted in Figure 103 and Figure 104.
putting the amplifier in the power-on mode of
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