THS4502_16 Datasheet, PDF (25/48 Pages) Texas Instruments – Wideband, Low-Distortion Fully Differential Amplifiers

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common-mode voltage control.
• Do not terminate the output unless required. Many
open-loop, class-A amplifiers require 50-Ω
termination for proper operation, but closed-loop
fully differential amplifiers drive a specific output
voltage regardless of the load impedance present.
Terminating the output of a fully differential
amplifier with a heavy load adversely effects the
amplifier's linearity.
• Comprehend the V_OCM input drive requirements.
Determine if the ADC's voltage reference can
provide the required amount of current to move
V_OCM to the desired value. A buffer may be
needed.
• Decouple the V_OCM pin to eliminate the antenna
effect. V_OCM is a high-impedance node that can
act as an antenna. A large decoupling capacitor
on this node eliminates this problem.
• Be cognizant of the input common-mode range. If
the input signal is referenced around the negative
power supply rail (e.g., around ground on a single
5 V supply), then the THS4500/1 accommodates
the input signal. If the input signal is referenced
around midrail, choose the THS4502/3 for the
best operation.
• Packaging makes a difference at higher
frequencies. If possible, choose the smaller,
thermally enhanced MSOP package for the best
performance. As a rule, lower junction
temperatures provide better performance. If
possible, use a thermally enhanced package,
even if the power dissipation is relatively small
compared to the maximum power dissipation
rating to achieve the best results.
• Comprehend the effect of the load impedance
seen by the fully differential amplifier when
performing system-level intercept point
calculations. Lighter loads (such as those
presented by an ADC) allow smaller intercept
points to support the same level of intermodulation
distortion performance.
EXAMPLE ANALOG-TO-DIGITAL
CONVERTER DRIVER CIRCUITS
The THS4500 family of devices is designed to drive
high-performance ADCs with extremely high linearity,
allowing for the maximum effective number of bits at
the output of the data converter. Two representative
circuits shown below highlight single-supply operation
and split supply operation. Specific feedback resistor,
gain resistor, and feedback capacitor values are not
specified, as their values depend on the frequency of
interest. Information on calculating these values can
be found in the applications material above.
THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
C_F
R_S
R_g
R_f
5V
V_S
R_T
5V
10 µF
+-
V_OCM
0.1 µF _Riso
IN _ADS5410
12 Bit/80 MSps
-+
1 µF
THS4503
IN
R_iso
CM
R_g
-5 V ^{10 µF 0.1 µF}
R_f
0.1 µF
C_F
Using the THS4503 With the ADS5410
Figure 97.
C_F
R_S
R_g
R_f
5V
V_S
R_T
5V
10 µF 0.1 µF _Riso
+-
IN ^ADS5421
V_OCM
14 Bit/40 MSps
-
+
IN
1 µF
THS4501
R_iso
CM
R_g
R_f
C_F
Using the THS4501 With the ADS5421
Figure 98.
0.1 µF
FULLY DIFFERENTIAL LINE DRIVERS
The THS4500 family of amplifiers can be used as
high-frequency, high-swing differential line drivers.
Their high power supply voltage rating (16.5 V
absolute maximum) allows operation on a single 12-V
or a single 15-V supply. The high supply voltage,
coupled with the ability to provide differential outputs
enables the ability to drive 26 V_PP into reasonably
heavy loads (250 Ω or greater). The circuit in
Figure 99 illustrates the THS4500 family of devices
used as high speed line drivers. For line driver
applications, close attention must be paid to thermal
design constraints due to the typically high level of
power dissipation.
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