THS4503-EP_17 Datasheet, PDF (23/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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Table 4. Resistor Values for Balanced Operation
in Various Gain Configurations
ǒ Ǔ Gain
V_OD
V_IN
1
1
2
2
5
5
10
10
R2 and R4
(Ω)
392
499
392
1.3 k
1.3 k
3.32 k
1.3 k
6.81 k
R1 (Ω) R3 (Ω)
412
383
523
487
215
187
665
634
274
249
681
649
147
118
698
681
R_T (Ω)
54.9
53.6
60.4
52.3
56.2
52.3
64.9
52.3
NOTE^: Values in the table above assume a 50-Ω source impedance.
R_S
V_S
R1
R2
R3
R_T
V_n
−
+
+^−
V_P
R4
Figure 97
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
R_S
–
1
1–2(1K_)K)
R3
K
+
R2
R1
R2 + R4
(6)
R3
+
R1
*
ǒ_Rs
||
R
Ǔ
T
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.
THS4503−EP
SGLS291A − APRIL 2005 − JANUARY 2012
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:
D Terminate the input source properly. In high-frequency
receiver chains, the source feeding the fully
differential amplifier requires a specific load
impedance (e.g., 50 Ω).
D 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.
D 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.
D 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.
D 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.
D 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.
D 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.
D 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.
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