THS4503-EP_17 Datasheet, PDF (26/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4503−EP
SGLS291A − APRIL 2005 − JANUARY 2012
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 103
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 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.
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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.
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 104 and Figure 105.
P_O P_O
Δf_c = f_c − f1
Δf_c = f2 − f_c
IMD₃ = P_S − P_O
P_S
P_S
f_c − 3Δf f1 f_c f2 f_c + 3Δf
f − Frequency − MHz
Figure 104