THS4504 Datasheet, PDF (26/45 Pages) National Semiconductor (TI) – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4504
THS4505
SLOS363D – AUGUST 2002 – REVISED MAY 2008 ......................................................................................................................................................... www.ti.com
P_OUT
(dBm)
OIP₃
P_O
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
1X
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
THS4504.
IMD₃
IIP₃
3X
P_IN
(dBm)
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
P_S
Figure 86. Graphical Representation of 2-Tone
and 3rd-Order Intercept Point
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.
OIP₃ = P_O +
|IMD₃|
2
(10)
V²
P_O = 10 log
Pdiff
2R_L x 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.
20 _{Normalized to 200 Ω}
10
R_L = 800 Ω
0
0
20
40
60
80 100
f − Frequency − MHz
Figure 87. Equivalent 3rd-Order Intercept Point for
the THS4504
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 Equation 23 through Equation 27.
26
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