Datasheet
BASIC DESIGN CONSIDERATIONS
1
R =
T
1 -
K
2(1+K)
R3
-
1
R
S
K=
R2
R1
R2=R4
R3=R1 (R ||R )-
S T
R1
R1+R2
b =
1
R3+R ||R
T S
R3+R || +R4
T S
R
b =
2
(7)
V
OD
V
S
=2
1 - b
2
b b
21
+
R
T
R R
ST
+
(8)
V
OD
V
IN
=2
1 - b
2
b b
21
+
(9)
INTERFACING TO AN ANALOG-TO-DIGITAL
THS4504
THS4505
SLOS363D – AUGUST 2002 – REVISED MAY 2008 .........................................................................................................................................................
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• Use separate analog and digital power supplies
and grounds. Noise (bounce) in the power
The circuits in Figure 76 through Figure 78 are used
supplies (created by digital switching currents) can
to highlight basic design considerations for fully
couple directly into the signal path, and
differential amplifier circuit designs.
power-supply noise can create higher distortion
products as well.
Equations for calculating fully differential amplifier
resistor values in order to obtain balanced operation
• Use care when filtering. While an RC low-pass
in the presence of a 50- Ω source impedance are
filter may be desirable on the output of the
given in Equation 6 through Equation 9 .
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
(6)
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.
For more detailed information about balance in fully
• Comprehend the V
OCM
input drive requirements.
differential amplifiers, see the Fully Differential
Determine if the ADC voltage reference can
Amplifiers, referenced at the end of this data sheet.
provide the required amount of current to move
V
OCM
to the desired value. A buffer may be
needed.
CONVERTER
• Decouple the V
OCM
pin to eliminate the antenna
The THS4500 family of amplifiers are designed
effect. V
OCM
is a high-impedance node that can
specifically to interface to today's
act as an antenna. A large decoupling capacitor
highest-performance analog-to-digital converters.
on this node eliminates this problem.
This section highlights the key concerns when
• Be cognizant of the input common-mode range. If
interfacing to an ADC and provides example
the input signal is referenced around the negative
ADC/fully differential amplifier interface circuits.
power supply rail (e.g., around ground on a single
Key design concerns when interfacing to an
5 V supply), then the THS4500/1 accommodates
analog-to-digital converter:
the input signal. If the input signal is referenced
• Terminate the input source properly. In around midrail, choose the THS4502/3 for the
high-frequency receiver chains, the source best operation.
feeding the fully differential amplifier requires a
• Packaging makes a difference at higher
specific load impedance (for example, 50 Ω ).
frequencies. If possible, choose the smaller,
• Design a symmetric printed-circuit board (PCB) thermally-enhanced MSOP package for the best
layout. Even-order distortion products are heavily performance. As a rule, lower junction
influenced by layout, and careful attention to a temperatures provide better performance. If
symmetric layout will minimize these distortion possible, use a thermally-enhanced package,
products. even if the power dissipation is relatively small
compared to the maximum power dissipation
• Minimize inductance in power-supply decoupling
rating to achieve the best results.
traces and components. Poor power-supply
decoupling can have a dramatic effect on circuit • Comprehend the effect of the load impedance
performance. Since the outputs are differential, seen by the fully differential amplifier when
differential currents exist in the power-supply pins. performing system-level intercept point
Thus, decoupling capacitors should be placed in a calculations. Lighter loads (such as those
manner that minimizes the impedance of the presented by an ADC) allow smaller intercept
current loop. points to support the same level of intermodulation
distortion performance.
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