THS4505 Datasheet, PDF (26/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 2004
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 84.
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 oper-
ation. 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 unpre-
cedented distortion performance for monolithic fully
differential amplifiers. This section focuses on the
fundamentals of distortion, circuit techniques for re-
ducing nonlinearity, and methods for equating distor-
tion 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 pri-
mary design goal in many applications.
Intercept points are specifications that have long
been used as key design criteria in the RF communi-
cations 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 distor-
tion, 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 de-
picted in Figure 85 and Figure 86.
P_O P_O
∆f_c = f_c − f1
∆f_c = f2 − f_c
IMD₃ = P_S − P_O
P_S
P_S
f_c − 3∆f f1 f_c f2 f_c + 3∆f
f − Frequency − MHz
Figure 85.
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