Datasheet
40
30
20
15
0 10 20 30 40 50 60
50
55
f - Frequency - MHz
THIRD-ORDER OUTPUT INTERCEPT POINT
vs
FREQUENCY
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
N
i
N
A
R
g
R
f
e
g
e
f
e
s
R
s
e
n
N
o
i
ni
i
ii
R
t
e
t
N
i
S
i
N
o
S
o
+
-
fully-diff
amp
R
g
R
f
e
g
e
f
ȧ
ȡ
Ȣ
R
g
R
f
)
R
g
R
g
)
R
s
R
t
2
ǒ
R
s
)R
t
Ǔ
ȧ
ȣ
Ȥ
2
(e
ni
)
2
(i
ni
)
2
(i
ii
)
2
4kTR
t
ȧ
ȡ
Ȣ
2R
s
R
G
R
s
)2R
g
R
t
)
2R
s
R
g
R
s
)2R
g
ȧ
ȣ
Ȥ
2
4kTR
f
2
ǒ
R
g
R
f
Ǔ
2
4kTR
g
2
ȧ
ȧ
ȡ
Ȣ
R
g
R
g
)
R
s
R
t
2
ǒ
R
s
)R
t
Ǔ
ȧ
ȧ
ȣ
Ȥ
2
N
A
: Fully Differential Amplifier
Noise
Source
Scale Factor
(12)
(13)
(14)
(15)
(16)
(17)
R
g
2
R
g
2
THS4502
THS4503
www.ti.com
SLOS352E –APRIL 2002–REVISED OCTOBER 2011
Figure 105.
Figure 106. Noise Sources in a Fully Differential
Amplifier Circuit
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 NOICE 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.
Figure 107. Scaling Factors for Individual Noise
Sources Assuming a Finite Value Termination
Resistor
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