Datasheet

40
30
20
15
0 10 20 30 40 50 60
50
55
f − Frequency − MHz
60
70 80 90 100
45
35
25
Normalized to 200 Ω
Gain = 1
R
f
= 392 Ω
V
S
= ± 5 V
Tone Spacing = 200 kHz
OIP
3
R
L
= 800 Ω
Normalized to 50 Ω
− Third-Order Output Intercept Point − dBm
OIP
3
OIP
3
+ P
O
)
ǒ
Ť
IMD
3
Ť
2
Ǔ
where
P
O
+ 10 log
ǒ
V
2
Pdiff
2R
L
0.001
Ǔ
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.
THS4500
THS4501
www.ti.com SLOS350F –APRIL 2002–REVISED OCTOBER 2011
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
Figure 110. Equivalent 3rd-Order Intercept Point
an analog-to-digital converter over its input frequency
for the THS4500
band. As a result, terminating the input of the ADC to
50 Ω can actually be detrimental to system
Comparing specifications between different device
performance.
types becomes easier when a common impedance
This discontinuity between open-loop, class-A
level is assumed. For this reason, the intercept points
amplifiers and closed-loop, class-AB amplifiers
on the THS4500 family of devices are reported
becomes apparent when comparing the intercept
normalized to a 50-Ω load impedance.
points of the two types of devices. Equation 10 gives
the definition of an intercept point, relative to the
AN ANALYSIS OF NOISE IN FULLY
intermodulation distortion.
DIFFERENTIAL AMPLIFIERS
Noise analysis in fully differential amplifiers is
(10)
analogous to noise analysis in single-ended
amplifiers; the same concepts apply. Figure 111
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
(11)
figures indicate the appropriate scaling factor for each
As can be seen in the equations, when a higher
of the noise sources in two different cases. The first
impedance is used, the same level of intermodulation
case includes the termination resistor, and the
distortion performance results in a lower intercept
second, simplified case assumes that the voltage
point. Therefore, it is important to understand the
source is properly terminated by the gain-setting
impedance seen by the output of the fully differential
resistors. With these scaling factors, the amplifier
amplifier when selecting a minimum intercept point.
input noise power (N
A
) can be calculated by summing
Figure 110 shows the relationship between the strict
each individual noise source with its scaling factor.
definition of an intercept point with a normalized, or
The noise delivered to the amplifier by the source (N
I
)
equivalent, intercept point for the THS4500.
and input noise power are used to calculate the noise
factor and noise figure as shown in Equation 23
through Equation 27.
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