THS4500-EP Datasheet, PDF (28/38 Pages) National Semiconductor (TI) – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIER

THS4500-EP
SLOS832 – JUNE 2013
Due to the intercept point 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).
However, with a fully differential amplifier, the output
does not require termination as an RF amplifier
would. Because closed-loop amplifiers deliver signals
to the outputs regardless of the impedance present, it
is important to comprehend this feature 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
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.
(11)
As can be seen in the equations, when a higher
impedance is used, the same level of intermodulation
distortion performance results in a lower intercept
point. Therefore, it is important to understand the
impedance seen by the output of the fully differential
amplifier when selecting a minimum intercept point.
Figure 111 shows the relationship between the strict
definition of an intercept point with a normalized, or
equivalent, intercept point for the THS4500.
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60
55
Normalized to 200 Ω
Normalized to 50 Ω
50
45
40
35
OIP₃ R_L= 800 Ω
30
Gain = 1
25 _{Rf = 392 Ω}
20
V_S = ± 5 V
Tone Spacing = 200 kHz
15
0 10 20 30 40 50 60 70 80 90 100
f − Frequency − MHz
Figure 111. Equivalent 3rd-Order Intercept Point
for the THS4500
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
analogous to noise analysis in single-ended
amplifiers; the same concepts apply. Figure 112
shows 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 termination resistor, and the
second, simplified case assumes that the voltage
source is properly terminated by the gain-setting
resistors. With these scaling factors, the amplifier
input noise power (N_A) can be calculated by summing
each individual noise source with its scaling factor.
The noise delivered to the amplifier by the source (N_I)
and input noise power are used to calculate the noise
factor and noise figure as shown in Equation 23
through Equation 27.
28
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