Datasheet

IMD
3
OIP
3
IIP
3
3X
P
IN
(dBm)
1X
P
OUT
(dBm)
P
O
P
S
10
20
30
40
50
60
0 20 40 60 80 100
OIP − Third-Order Output Intersept Point − dBm
f − Frequency − MHz
0
Normalized to 50
Gain = 1
R
f
= 499
V
O
= 2 V
PP
V
S
= ± 5 V
200 kHz Tone Spacing
R
L
= 800
Normalized to 200
3
AN ANALYSIS OF NOISE IN FULLY
OIP =P +
3 O
|IMD |
3
2
(10)
P =10log
O
2
V
Pdiff
2R x0.001
L
(11)
THS4504
THS4505
SLOS363D AUGUST 2002 REVISED MAY 2008 .........................................................................................................................................................
www.ti.com
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
THS4504.
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
Figure 87. Equivalent 3rd-Order Intercept Point for
would. Because closed-loop amplifiers deliver signals
the THS4504
to their outputs regardless of the impedance present,
it is important to comprehend this when evaluating
Comparing specifications between different device
the intercept point of a fully differential amplifier. The
types becomes easier when a common impedance
THS4500 series of devices yields optimum distortion
level is assumed. For this reason, the intercept points
performance when loaded with 200 to 1 k , very
on the THS4500 family of devices are reported
similar to the input impedance of an analog-to-digital
normalized to a 50- load impedance.
converter over its input frequency band. As a result,
terminating the input of the ADC to 50 can actually
be detrimental to system performance.
DIFFERENTIAL AMPLIFIERS
This discontinuity between open-loop, class-A
Noise analysis in fully differential amplifiers is
amplifiers and closed-loop, class-AB amplifiers
analogous to noise analysis in single-ended
becomes apparent when comparing the intercept
amplifiers. The same concepts apply. Below, a
points of the two types of devices. Equation 10 gives
generic circuit diagram consisting of a voltage source,
the definition of an intercept point, relative to the
a termination resistor, two gain setting resistors, two
intermodulation distortion.
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
NOTE: P
o
is the output power of a single tone, R
L
is
assumes that the voltage source is properly
the differential load resistance, and V
P(diff)
is the
terminated by the gain-setting resistors. With these
differential peak voltage for a single tone.
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 .
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