THS4271-EP Datasheet, PDF (26/44 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FEEDBACK AMPLIFIER

THS4271-EP
SGLS270C – DECEMBER 2004 – REVISED APRIL 2010
Intercept points are specifications long used as key
design criteria in the RF communications world as a
metric for the intermodulation distortion performance
of a device in the signal chain (e.g., amplifiers,
mixers, etc.). Use of the intercept point, rather than
strictly the intermodulation distortion, allows simpler
system-level calculations. Intercept points, like noise
figures, can be easily cascaded back and forth
through a signal chain to determine the overall
receiver chain intermodulation distortion performance.
The relationship between intermodulation distortion
and intercept point is depicted in Figure 86 and
Figure 87.
P_O P_O
∆f_c = f_c - f1
∆f_c = f2 - f_c
IMD₃ = P_S - P_O
P_S
P_S
f_c - 3∆f f1 f_c f2 f_c + 3∆f
f - Frequency - MHz
Figure 86.
P_OUT
(dBm)
1X
OIP₃
P_O
IMD₃
IIP₃
3X
P_IN
(dBm)
P_S
Figure 87.
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Due to the intercept point ease of use in system level
calculations for receiver chains, it has become the
specification of choice for guiding distortion-related
design decisions. Traditionally, these systems use
primarily class-A, single-ended RF amplifiers as gain
blocks. These RF amplifiers are typically designed to
operate in a 50-Ω environment. Giving intercept
points in dBm, implies an associated impedance
(50 Ω).
However, with an operational amplifier, the output
does not require termination as an RF amplifier
would. Because closed-loop amplifiers deliver signals
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.
ǒ Ǔ OIP₃ + P_O )
Ť_IMD3Ť
2
where
(2)
ǒ Ǔ P_O + 10 log
2R_L
V^2P
0.001
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.
(3)
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 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 88 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.
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