THS4503-EP_17 Datasheet, PDF (27/40 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 105
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).
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, 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.
THS4503−EP
SGLS291A − APRIL 2005 − JANUARY 2012
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 THS4503.
THIRD-ORDER OUTPUT INTERCEPT POINT
vs
FREQUENCY
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 106
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. 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 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’s 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 equations 23 through 27.
27