Datasheet

THS4500
THS4501
www.ti.com SLOS350F –APRIL 2002–REVISED OCTOBER 2011
For more detailed information about balance in fully voltage regardless of the load impedance present.
differential amplifiers, see the application report, Fully Terminating the output of a fully differential
Differential Amplifiers (SLOA054), referenced at the amplifier with a heavy load adversely affects the
end of this data sheet. amplifier linearity.
• Comprehend the V
OCM
input drive requirements.
INTERFACING TO AN ANALOG-TO-DIGITAL
Determine if the ADC voltage reference can
CONVERTER
provide the required amount of current to move
V
OCM
to the desired value. A buffer may be
The THS4500 family of amplifiers are designed
needed.
specifically to interface to today's
• Decouple the V
OCM
pin to eliminate the antenna
highest-performance ADCs. This section highlights
effect. V
OCM
is a high-impedance node that can
the key concerns when interfacing to an ADC and
act as an antenna. A large decoupling capacitor
provides example interface circuits.
on this node eliminates this problem.
There are several key design concerns when
• Know the input common-mode range. If the input
interfacing to an analog-to-digital converter:
signal is referenced around the negative
• Terminate the input source properly. In
power-supply rail (for example, around ground on
high-frequency receiver chains, the source that
a single 5 V supply), then the THS4500/1
feeds the fully differential amplifier requires a
accommodates the input signal. If the input signal
specific load impedance (that is, 50 Ω).
is referenced around midrail, choose the
• Design a symmetric printed circuit board (PCB)
THS4502/3 for the best operation.
layout. Even-order distortion products are heavily
• Packaging makes a difference at higher
influenced by layout, and careful attention to a
frequencies. If possible, choose the smaller,
symmetric layout minimizes these distortion
thermally-enhanced MSOP package for the best
products.
performance. As a rule, lower junction
• Minimize inductance in power-supply decoupling
temperatures provide better performance. If
traces and components. Poor power-supply
possible, use a thermally-enhanced package,
decoupling can have a dramatic effect on circuit
even if the power dissipation is relatively small
performance. Since the outputs are differential,
compared to the maximum power dissipation
differential currents exist in the power-supply pins.
rating to achieve the best results.
Thus, decoupling capacitors should be placed in a
• Understand the effect of the load impedance seen
manner that minimizes the impedance of the
by the fully differential amplifier when performing
current loop.
system-level intercept point calculations. Lighter
• Use separate analog and digital power supplies
loads (such as those presented by an ADC) allow
and grounds. Noise (bounce) in the power
smaller intercept points to support the same level
supplies (created by digital switching currents) can
of intermodulation distortion performance.
couple directly into the signal path, and
power-supply noise can create higher distortion
EXAMPLE ANALOG-TO-DIGITAL
products as well.
CONVERTER DRIVER CIRCUITS
• Use care when filtering. While an RC low-pass
The THS4500 family of devices is designed to drive
filter may be desirable on the output of the
high-performance ADCs with extremely high linearity,
amplifier to filter broadband noise, the excess
allowing for the maximum effective number of bits at
loading can negatively impact the amplifier
the output of the data converter. Two representative
linearity. Filtering in the feedback path does not
circuits shown below highlight single-supply operation
have this effect.
and split supply operation, respectively. Specific
• AC-coupling allows easier circuit design. If
feedback resistor, gain resistor, and feedback
dc-coupling is required, be aware of the excess
capacitor values are not shown, as these values
power dissipation that can occur due to
depend on the frequency of interest. Information on
level-shifting the output through the output
calculating these values can be found in the
common-mode voltage control.
applications material above.
• 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
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