THS4505 Datasheet, PDF (22/45 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363D – AUGUST 2002 – REVISED MAY 2008 ......................................................................................................................................................... www.ti.com
BASIC DESIGN CONSIDERATIONS
The circuits in Figure 76 through Figure 78 are used
to highlight basic design considerations for fully
differential amplifier circuit designs.
Equations for calculating fully differential amplifier
resistor values in order to obtain balanced operation
in the presence of a 50-Ω source impedance are
given in Equation 6 through Equation 9.
R_T =
1
K
1 ^{1 - 2(1 + K)}
-
R_S
R3
R2
K=
R2 = R4
R1
R3 = R1 - (R_S || R_T)
(6)
R1
b₁ = _{R1 + R2}
b₂ = ^{R3 + RT || RS}
R3 + R_T || R_S + R4
(7)
V_OD
1 - b₂
R_T
=2
V_S
b₁ + b₂ R_T + R_S
(8)
V_OD
1 - b₂
=2
V_IN
b₁ + b₂
(9)
For more detailed information about balance in fully
differential amplifiers, see the Fully Differential
Amplifiers, referenced at the end of this data sheet.
INTERFACING TO AN ANALOG-TO-DIGITAL
CONVERTER
The THS4500 family of amplifiers are designed
specifically
to
interface
to
today's
highest-performance analog-to-digital converters.
This section highlights the key concerns when
interfacing to an ADC and provides example
ADC/fully differential amplifier interface circuits.
Key design concerns when interfacing to an
analog-to-digital converter:
• Terminate the input source properly. In
high-frequency receiver chains, the source
feeding the fully differential amplifier requires a
specific load impedance (for example, 50 Ω).
• Design a symmetric printed-circuit board (PCB)
layout. Even-order distortion products are heavily
influenced by layout, and careful attention to a
symmetric layout will minimize these distortion
products.
• Minimize inductance in power-supply decoupling
traces and components. Poor power-supply
decoupling can have a dramatic effect on circuit
performance. Since the outputs are differential,
differential currents exist in the power-supply pins.
Thus, decoupling capacitors should be placed in a
manner that minimizes the impedance of the
current loop.
• Use separate analog and digital power supplies
and grounds. Noise (bounce) in the power
supplies (created by digital switching currents) can
couple directly into the signal path, and
power-supply noise can create higher distortion
products as well.
• Use care when filtering. While an RC low-pass
filter may be desirable on the output of the
amplifier to filter broadband noise, the excess
loading can negatively impact the amplifier
linearity. Filtering in the feedback path does not
have this effect.
• AC-coupling allows easier circuit design. If
dc-coupling is required, be aware of the excess
power dissipation that can occur due to
level-shifting the output through the output
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 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.
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