THS4500 Datasheet, PDF (32/37 Pages) Texas Instruments – WIDEBAND, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4500
THS4501
SLOS350D − APRIL 2002 − REVISED JANUARY 2004
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 25 Ω 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.
Rf
RS
Rg
VS
VS
+−
RT
−+
−VS
Riso
CL
Riso
Riso = 10 − 25 Ω
Rf
Rg
Use of Isolation Resistors With a Capacitive Load.
Figure 118
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 high frequency return currents,
but often is not required.
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EVALUATION FIXTURES, SPICE MODELS,
AND APPLICATIONS 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 the default
component values. Unpopulated footprints are shown to
provide insight into design flexibility.
C4 _C0805
R4
R0805
PD
J1
C1
R2
V_{S 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, VOCM, and Power Down Circuitry
Not Shown
Figure 119
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 either the Texas Instruments web site
(www.ti.com) or as one model on a disk from the Texas
Instruments
Product
Information
Center
(1−800−548−6132). The PIC is also available for design
assistance and detailed product information at this
number. 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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