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| THS4503-EP_17 Datasheet, PDF (28/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS | |||
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THS4503âEP
SGLS291A â APRIL 2005 â JANUARY 2012
Ni
NA
eg
Rg
Rf
ef
Si
en
Ni
No
Rs
+
Rt
ini
fully-diff
amp
So
No
es
â
et
iii
eg
Rg
Rf
ef
Figure 107. Noise Sources in a Fully
Differential Amplifier Circuit
NA: Fully Differential Amplifier
Noise
Source Scale Factor
(eni)2
2
ȡ ȣ Rg
ȧȢ ȧȤ Rf
)
Rg
)
Rg
RsRt
2ÇRs)RtÇ
(12)
(ini)2
(iii)2
4kTRt
Rg2
Rg2
2RsRG
2
ȡ ȣ Rs)2Rg
ȧȢ ȧȤ Rt
)
2RsRg
Rs)2Rg
(13)
(14)
(15)
4kTRf
Ç Ç2
2
Rg
Rf
(16)
È¡
Rg
È£2
4kTRg
ȧȢ ȧȤ 2
Rg
)
RsRt
2ÇRs)RtÇ
(17)
Figure 108. Scaling Factors for Individual
Noise Sources Assuming a Finite Value
Termination Resistor
www.ti.com
NA: Fully Differential Amplifier; termination = 2Rg
Noise
Source Scale Factor
(eni)2
2
ȡ ȣ Rg
ȧȢ ȧȤ Rf
)
Rg
Rg
)
Rs
2
(ini)2
Rg2
(iii)2
4kTRf
Rg2
Ç Ç2
2
Rg
Rf
4kTRg
2
ȡ ȣ Rg
ȧȢ ȧȤ 2
Rg
)
Rs
2
(18)
(19)
(20)
(21)
(22)
Figure 109. Scaling Factors for Individual Noise
Sources Assuming No termination Resistance is
Used (e.g., RT is open)
È¡
È£2
ȧȧȧȢ ȧȧȧȤ Ni + 4kTRs
2RtRg
Rt)2Rg
Rs)R2t)Rt2RRg g
(23)
Figure 110. Input Noise With a
Termination Resistor
Ç Ç2
Ni + 4kTRs
2Rg
Rs ) 2Rg
(24)
Figure 111. Input Noise Assuming No Termination
Resistor
Noise Factor and Noise Figure Calculations
NA + SÇNoise Source Scale FactorÇ
(25)
F
+
1
)
NA
NI
(26)
NF + 10 log (F)
(27)
28
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