THS4505 Datasheet, PDF (27/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

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P_OUT
(dBm)
1X
OIP₃
P_O
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 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, just like
the rest of the receiver chain. Since intercept points
are given in dBm, this implies an associated im-
pedance (50 Ω).
However, with a fully differential 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 a fully differential amplifier. The
THS4500 series of devices yields optimum distortion
performance when loaded with 200 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.
This 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 10 gives the definition
of an intercept point, relative to the intermodulation
distortion.
THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 2004
ǒ Ǔ OIP₃ + P_O )
Ť_IMD3Ť
2
where
(10)
ǒ Ǔ P_O + 10 log
V^2Pdiff
2R_L 0.001
(11)
#IMPLIED. NOTE: P_o is the output power of a
single tone, R_L is the differential load resist-
ance, and V_P(diff) is the differential peak
voltage for a single tone.
As can be seen in the equation, when a higher
impedance is used, the same level of intermodulation
distortion performance results in a lower intercept
point. Therefore, it is important to comprehend the
impedance seen by the output of the fully differential
amplifier when selecting a minimum intercept point.
The graphic below shows the relationship between
the strict definition of an intercept point with a
normalized, or equivalent, intercept point for the
THS4502.
60
Gain = 1
R_f = 499 Ω
50
V_O = 2 V_PP
V_S = ± 5 V
40
200 kHz Tone Spacing
Normalized to 50 Ω
30
20 _{Normalized to 200 Ω}
10
R_L = 800 Ω
0
0
20
40
60
80 100
f − Frequency − MHz
Figure 87.
Comparing specifications between different device
types becomes easier when a common impedance
level is assumed. For this reason, the intercept points
on the THS4500 family of devices are reported
normalized to a 50-Ω load impedance.
AN ANALYSIS OF NOISE IN FULLY
DIFFERENTIAL AMPLIFIERS
Noise analysis in fully differential amplifiers is anal-
ogous to noise analysis in single-ended amplifiers.
The same concepts apply. Below, a generic circuit
diagram consisting of a voltage source, a termination
resistor, two gain setting resistors, two feedback
resistors, and a fully differential amplifier is shown,
including all the relevant noise sources. From this
circuit, the noise factor (F) and noise figure (NF) are
calculated. The figures indicate the appropriate
scaling factor for each of the noise sources in two
different cases. The first case includes the termin-
ation resistor, and the second, simplified case as-
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