THS4505 Datasheet, PDF (32/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 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 Ω 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.
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 96.
C4 _C0805
R4
R0805
J1
C1
V_S
PD
U1
R2
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 97.
32
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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 per-
formance.
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 rec-
ommended to reduce the inductance along power
supply return current paths.
4. Recommended values for power supply decoup-
ling 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.
EVALUATION FIXTURES, SPICE MODELS,
AND APPLICATIONS SUPPORT
Texas Instruments is committed to providing its cus-
tomers with the highest quality of applications sup-
port. 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. The schematic for
the evaluation board is shown in Figure 97 with
default component values. Unpopulated footprints are
shown to provide insight into design flexibility.
Computer simulation of circuit performance using
SPICE is often useful when analyzing the perform-
ance of analog circuits and systems. This is particu-
larly 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 Product Information Center (PIC) is 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 perform-
ance. Detailed information about what is and is not
modeled is contained in the model file itself.