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| THS4500-EP Datasheet, PDF (29/38 Pages) National Semiconductor (TI) – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIER | |||
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THS4500-EP
www.ti.com
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 112. Noise Sources in a Fully Differential
Amplifier Circuit
Scaling Factors for Individual Noise Sources
Assuming a Finite Value Termination Resistor
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
Rg2
(13)
(iii)2
Rg2
(14)
ȡ ȣ 2RsRG
2
4kTRt
Rs)2Rg
ȧȢ ȧȤ Rt
)
2RsRg
Rs)2R
g
(15)
4kTRf
Ç Ç 2
Rg 2
Rf
(16)
È¡
Rg
È£2
4kTRg
ȧȢ ȧȤ 2
Rg
)
RsRt
2ÇRs)RtÇ
(17)
SLOS832 â JUNE 2013
Scaling Factors for Individual Noise Sources
Asseming No Termination Resistance is Used
(that is, RT is Open)
NA: Fully Differential Amplifier; termination = 2Rg
Noise
Source Scale Factor
(eni)2
2
ȡ ȣ Rg
ȧȢ ȧȤ Rf
)
Rg
Rg
)
Rs
2
(18)
(ini)2
Rg2
(19)
(iii)2
Rg2
(20)
4kTRf
Ç Ç 2
Rg 2
Rf
(21)
4kTRg
2
ȡ ȣ Rg
ȧȢ ȧȤ 2
Rg
)
Rs
2
(22)
Input Noise With a Termination Resistor
È¡
È£2
ȧȧȧȢ ȧȧȧȤ Ni + 4kTRs
2RtRg
Rt)2Rg
Rs)R2t)Rt2RRg g
(23)
Input Noise Assuming No Termination Resistor
Ni = 4kTRS
2RG
2
RS + 2RG
(24)
Noise Factor and Noise Figure Calculations
NA = S(Noise Source ´ Scale Factor)
(25)
F = 1 + NA
NI
(26)
NF + 10 log (F)
(27)
Copyright © 2013, Texas Instruments Incorporated
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