Datasheet

IMD
3
= P
S
- P
O
P
S
P
O
P
O
∆f
c
= f
c
- f1
∆f
c
= f2 - f
c
P
S
f
c
- 3∆f f1 f
c
f2 f
c
+ 3∆f
Power
f - Frequency - MHz
OIP
3
+ P
O
)
ǒ
Ť
IMD
3
Ť
2
Ǔ
where
P
O
+ 10 log
ǒ
V
2
P
2R
L
0.001
Ǔ
IMD
3
OIP
3
IIP
3
3X
P
IN
(dBm)
1X
P
OUT
(dBm)
P
O
P
S
THS4271
THS4275
SLOS397F –JULY 2002–REVISED OCTOBER 2009 www.ti.com
Intercept points are specifications long used as key Due to the intercept point ease of use in system level
design criteria in the RF communications world as a calculations for receiver chains, it has become the
metric for the intermodulation distortion performance specification of choice for guiding distortion-related
of a device in the signal chain (e.g., amplifiers, design decisions. Traditionally, these systems use
mixers, etc.). Use of the intercept point, rather than primarily class-A, single-ended RF amplifiers as gain
strictly the intermodulation distortion, allows simpler blocks. These RF amplifiers are typically designed to
system-level calculations. Intercept points, like noise operate in a 50-Ω environment. Giving intercept
figures, can be easily cascaded back and forth points in dBm, implies an associated impedance (50
through a signal chain to determine the overall Ω).
receiver chain intermodulation distortion performance.
However, with an operational amplifier, the output
The relationship between intermodulation distortion
does not require termination as an RF amplifier
and intercept point is depicted in Figure 85 and
would. Because closed-loop amplifiers deliver signals
Figure 86.
to their outputs regardless of the impedance present,
it is important to comprehend this when evaluating
the intercept point of an operational amplifier. The
THS4271 yields optimum distortion performance
when loaded with 150 Ω to 1 kΩ, very similar to the
input impedance of an analog-to-digital converter
over its input frequency band.
As a result, terminating the input of the ADC to 50Ω
can actually be detrimental to systems performance.
The discontinuity between open-loop, class-A
amplifiers and closed-loop, class-AB amplifiers
becomes apparent when comparing the intercept
points of the two types of devices. Equation 2 and
Equation 3 give the definition of an intercept point,
relative to the intermodulation distortion.
(2)
Figure 85.
(3)
NOTE: P
O
is the output power of a single tone, R
L
is the load resistance, and V
P
is the peak
voltage for a single tone.
NOISE ANALYSIS
High slew rate, unity gain stable, voltage-feedback
operational amplifiers usually achieve the slew rate at
the expense of a higher input noise voltage. The
3-nV/√Hz input voltage noise for the THS4271 and
THS4275 is, however, much lower than comparable
amplifiers. The input-referred voltage noise, and the
two input-referred current noise terms (3 pA/√Hz),
combine to give low output noise under a wide variety
of operating conditions. Figure 87 shows the amplifier
noise analysis model with all the noise terms
included. In this model, all noise terms are taken to
be noise voltage or current density terms in either
nV/√Hz or pA/√Hz.
Figure 86.
26 Submit Documentation Feedback Copyright © 2002–2009, Texas Instruments Incorporated
Product Folder Link(s): THS4271 THS4275