THS4502 Datasheet, PDF (24/49 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
APPLICATION CIRCUITS USING FULLY
DIFFERENTIAL AMPLIFIERS
Fully differential amplifiers provide designers with a
great deal of flexibility in a wide variety of
applications. This section provides an overview of
some common circuit configurations and gives some
design guidelines. Designing the interface to an ADC,
driving lines differentially, and filtering with fully
differential amplifiers are a few of the circuits that are
covered.
BASIC DESIGN CONSIDERATIONS
The circuits in Figures 96 through 100 are used to
highlight basic design considerations for fully
differential amplifier circuit designs.
Table 4. Resistor Values for Balanced Operation
in Various Gain Configurations
ǒ Ǔ Gain
V_OD
V_IN
R2 & R4 (Ω) R1 (Ω) R3 (Ω)
R_T (Ω)
1
392
412
383
54.9
1
499
523
487
53.6
2
392
215
187
60.4
2
1.3k
665
634
52.3
5
1.3k
274
249
56.2
5
3.32k
681
649
52.3
10
1.3k
147
118
64.9
10
6.81k
698
681
52.3
NOTE: Values in this table assume a 50 Ω source impedance.
R1
R2
R_S
V_S
R3
R_T
V_n
-
+
+-
V_P
R4
Figure 96.
V_out+
V_out-
V_OCM
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 equations 6 through 9.
R_T +
1
1– ^K
1
R_S
–
2(1)K)
R3
K
+
R2
R1
R2 + R4
(6)
R3 + R1 * ^ǒR_s || R_T^Ǔ
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β₁
+
R1
R1 ) R2
β₂
+
R3 )
R3 ) R_T
R_T ||
|| R_S
R_S
) R4
(7)
ǒ Ǔ ǒ Ǔ ^VOD
V_S
+
2
1–β₂
β₁ ) β₂
R_T
R_T ) R_S
(8)
ǒ Ǔ ^VOD
V _IN
+
2
1–β₂
β₁ ) β₂
(9)
For more detailed information about balance in fully
differential amplifiers, see 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 (e.g., 50Ω ).
• Design a symmetric printed-circuit board 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
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