THS4502 Datasheet, PDF (27/49 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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specific source of electromagnetic or radio frequency
interference is present elsewhere in the system.
Information on the ac performance (bandwidth, slew
rate) of the V_OCM circuitry is included in the
specification table and graph section.
Since the V_OCM pin provides the ability to set an
output common-mode voltage, the ability for
increased power dissipation exists. While this does
not pose a performance problem for the amplifier, it
can cause additional power dissipation of which the
system designer should be aware. The circuit shown
in Figure 102 demonstrates an example of this
phenomenon. For a device operating on a single 5-V
supply with an input signal referenced around ground
and an output common-mode voltage of 2.5 V, a dc
potential exists between the outputs and the inputs of
the device. The amplifier sources current into the
feedback network in order to provide the circuit with
the proper operating point. While there are no serious
effects on the circuit performance, the extra power
dissipation may need to be included in the system's
power budget.
I₁ =
V_OCM
R_f1+ R_g1 + R_S || R_T
DC Current Path to Ground
R_S
R_g1
R_f1
V_S
R_T
5V
V_OCM = 2.5 V
+-
-+
2.5-V DC
R_L
2.5-V DC
R_g2
R_f2
DC Current Path to Ground
I₂ = ^VOCM
R_f2 + R_g2
Depiction of DC Power Dissipation Caused By
Output Level-Shifting in a DC-Coupled Circuit
Figure 102.
SAVING POWER WITH POWER-DOWN
FUNCTIONALITY
The THS4500 family of fully differential amplifiers
contains devices that come with and without the
power-down option. Even-numbered devices have
power-down capability, which is described in detail
here.
The power-down pin of the amplifiers defaults to the
positive supply voltage in the absence of an applied
voltage (i.e. an internal pullup resistor is present),
putting the amplifier in the power-on mode of
THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
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
in 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
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.
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.
Intercept points are specifications that have long
been used as key design criteria in the RF
communications world as a metric for the
intermodulation distortion performance of a device in
the signal chain (e.g., amplifiers, mixers, etc.). Use of
the intercept point, rather than strictly the
intermodulation distortion, allows for simpler
system-level calculations. Intercept points, like noise
figures, can be easily cascaded back and forth
through a signal chain to determine the overall
receiver chain's intermodulation distortion
performance. The relationship between
intermodulation distortion and intercept point is
depicted in Figure 103 and Figure 104.
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