THS4502 Datasheet, PDF (28/49 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4502
THS4503
SLOS352E – APRIL 2002 – REVISED OCTOBER 2011
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 103.
P_OUT
(dBm)
1X
OIP₃
P_O
IMD₃
IIP₃
3X
P_IN
(dBm)
P_S
Figure 104.
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,
just like the rest of the receiver chain. Since intercept
points are given in dBm, this implies an associated
impedance (50 Ω).
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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
amplifiers 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.
ǒ Ǔ OIP₃ + P_O )
Ť_IMD3Ť
2
where
(10)
ǒ Ǔ P_O + 10 log
V^2Pdiff
2R_L 0.001
(11)
NOTE
P_o is the output power of a
single tone, R_L is the
differential load resistance, 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.
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