Datasheet

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
_
+
R
f
4kT = 1.6E-20J
at 290K
THS4211/THS4215
I
BN
E
O
E
RF
R
S
E
RS
I
BI
R
g
E
NI
4kTR
S
4kT
R
g
4kTR
f
E
O
+
ǒ
E
2
NI
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
Ǔ
NG
2
)
ǒ
I
BI
R
f
Ǔ
2
) 4kTR
f
NG
Ǹ
THS4211
THS4215
www.ti.com ................................................................................................................................... SLOS400E –SEPTEMBER 2002–REVISED SEPTEMBER 2009
(2)
(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
Figure 86.
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
Due to the intercept point's ease of use in system
be noise voltage or current density terms in either
level calculations for receiver chains, it has become
nV/√Hz or pA/√Hz.
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
THS4211 yields optimum distortion performance
when loaded with 150 Ω to 1 kΩ, very similar to the
Figure 87. Noise Analysis Model
input impedance of an analog-to-digital converter
over its input frequency band.
The total output shot noise voltage can be computed
As a result, terminating the input of the ADC to 50 Ω
as the square of all square output noise voltage
can actually be detrimental to system performance.
contributors. Equation 4 shows the general form for
The discontinuity between open-loop, class-A the output noise voltage using the terms shown in
amplifiers and closed-loop, class-AB amplifiers Equation 4:
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
(4)
intermodulation distortion.
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