THS4211 Datasheet, PDF (24/41 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
SLOS400D – SEPTEMBER 2002 – REVISED NOVEMBER 2004
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's intermodulation distortion perform-
ance. The relationship between intermodulation dis-
tortion and intercept point is depicted in Figure 85
and Figure 86.
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 85.
P_OUT
(dBm)
1X
OIP₃
P_O
IMD₃
IIP₃
3X
P_IN
(dBm)
P_S
Figure 86.
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Due to the intercept point's ease of use in system
level calculations for receiver chains, it has become
the specification of choice for guiding distor-
tion-related design decisions. Traditionally, these sys-
tems use primarily class-A, single-ended RF ampli-
fiers as gain blocks. These RF amplifiers are typically
designed to operate in a 50-Ω environment. Giving
intercept points in dBm implies an associated im-
pedance (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
THS4211 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 50 Ω
can actually be detrimental to system performance.
The discontinuity between open-loop, class-A ampli-
fiers and closed-loop, class-AB amplifiers becomes
apparent when comparing the intercept points of the
two types of devices. Equation 2 and Equation 3
define 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
(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 their slew rate
at the expense of a higher input noise voltage. The 7
nV/√Hz input voltage noise for the THS4211 and
THS4215 is, however, much lower than comparable
amplifiers. The input-referred voltage noise, and the
two input-referred current noise terms (4 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 in-
cluded. 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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