THS4211 Datasheet, PDF (25/50 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
www.ti.com ................................................................................................................................... SLOS400E – SEPTEMBER 2002 – REVISED SEPTEMBER 2009
P_OUT
(dBm)
OIP₃
P_O
ǒ Ǔ OIP₃ + P_O )
Ť_IMD3Ť
2
where
(2)
1X
ǒ Ǔ ^V2
P_O + 10 log _2RL
P
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.
IMD₃
IIP₃
3X
P_IN
(dBm)
P_S
Figure 86.
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
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
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
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 define an intercept point, relative to the
intermodulation distortion.
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
included. In this model, all noise terms are taken to
be noise voltage or current density terms in either
nV/√Hz or pA/√Hz.
R_S
E_RS
4kTR_S
E_NI
I_BN
THS4211/THS4215
+
E_O
_
R_f
E_RF
4kT
R_g
I_BI
4kTR_f
R_g
4kT = 1.6E-20J
at 290K
Figure 87. Noise Analysis Model
The total output shot noise voltage can be computed
as the square of all square output noise voltage
contributors. Equation 4 shows the general form for
the output noise voltage using the terms shown in
Equation 4:
Ǹǒ Ǔ E_O +
E_NI2 ) ^ǒI_BNR_S^Ǔ2 ) 4kTR_S NG² ) ^ǒI_BIR_f^Ǔ2 ) 4kTR_fNG
(4)
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