THS4502 Datasheet, PDF (33/49 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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dissipation, but also dynamic power dissipation. Often
times, this is difficult to quantify because the signal
pattern is inconsistent, but an estimate of the RMS
power dissipation can provide visibility into a possible
problem.
DRIVING CAPACITIVE LOADS
High-speed amplifiers are typically not well-suited for
driving large capacitive loads. If necessary, however,
the load capacitance should be isolated by two
isolation resistors in series with the output. The
requisite isolation resistor size depends on the value
of the capacitance, but 10 to 25Ω is a good place to
begin the optimization process. Larger isolation
resistors decrease the amount of peaking in the
frequency response induced by the capacitive load,
but this comes at the expense of larger voltage drop
across the resistors, increasing the output swing
requirements of the system.
R_f
V_S
R_S
R_g
V_S
+-
R_T
-+
-V_S
R_iso
C_L
R_iso
Riso = 10 - 25 Ω
R_f
R_g
Use of Isolation Resistors With a Capacitive Load.
Figure 115.
POWER SUPPLY DECOUPLING
TECHNIQUES AND RECOMMENDATIONS
Power supply decoupling is a critical aspect of any
high-performance amplifier design process. Careful
decoupling provides higher quality ac performance
(most notably improved distortion performance). The
following guidelines ensure the highest level of
performance.
1. Place decoupling capacitors as close to the
power supply inputs as possible, with the goal of
minimizing the inductance of the path from
ground to the power supply.
2. Placement priority should be as follows: smaller
capacitors should be closer to the device.
3. Use of solid power and ground planes is
recommended to reduce the inductance along
power supply return current paths.
4. Recommended values for power supply
decoupling include 10-µF and 0.1-µF capacitors
for each supply. A 1000-pF capacitor can be
used across the supplies as well for extremely
THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
high frequency return currents, but often is not
required.
EVALUATION FIXTURES, SPICE MODELS,
AND APPLICTIONS SUPPORT
Texas Instruments is committed to providing its
customers with the highest quality of applications
support. To support this goal, an evaluation board
has been developed for the THS4500 family of fully
differential amplifiers. The evaluation board can be
obtained by ordering through the Texas Instruments
web site, www.ti.com, or through your local Texas
Instruments sales representative. Schematic for the
evaluation board is shown below with their default
component values. Unpopulated footprints are shown
to provide insight into design flexibility.
C4 _C0805
R4
R0805
J1
C1
R2
V_S
PD
U1
1
3
_
7
THS450X
4
R6
C5
J2
C0805
J2
C0805 _R0805
R1
C2
R1206
R0805
C0805
R3
8+
26
R0805
C7
C0805
5 ^R0805
PwrPad R7
C6
J3
J3
V_OCM ^-VS
R5 R0805
C0805
C3
C0805
J2
R8
43
J4
R0805
R9 5
J3
R0805
R0805
R11
R1206
R9
61
T1
Simplified Schematic of the Evaluation Board. Power
Supply Decoupling, V_OCM, and Power Down Circuitry
Not Shown
Figure 116.
Computer simulation of circuit performance using
SPICE is often useful when analyzing the
performance of analog circuits and systems. This is
particularly true for video and RF amplifier circuits
where parasitic capacitance and inductance can have
a major effect on circuit performance. A SPICE model
for the THS4500 family of devices is available
through the Texas Instruments web site (www.ti.com).
The PIC is also available for design assistance and
detailed product information. These models do a
good job of predicting small-signal ac and transient
performance under a wide variety of operating
conditions. They are not intended to model the
distortion characteristics of the amplifier, nor do they
attempt to distinguish between the package types in
their small-signal ac performance. Detailed
information about what is and is not modeled is
contained in the model file itself.
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