THS4561 Datasheet, PDF (27/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

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THS4561
SBOS874 – AUGUST 2017
10.1.3 Noise Analysis
The first step in the output noise analysis is to reduce the application circuit to the simplest form with equal
feedback and gain setting elements to ground. Figure 11 shows the simplest analysis circuit with the FDA and
resistor noise terms to be considered.
e_nRg2
RG
e_nRf2
RF
I_n+2
+
I_n±²
±
e_no2
e_nRg2
RG
e_ni2
RF
e_nRf2
Figure 11. FDA Noise Analysis Circuit
The noise powers are shown in Figure 11 for each term. When the R_F and R_G (or R_I) terms are matched on each
side, the total differential output noise is the root sum squared (RSS) of these separate terms. Using NG ≡ 1 +
R_F / R_G, the total output noise is given by Equation 5. Each resistor noise term is a 4kT × R power (4kT = 1.6E-
20J at 290K).
e_o
e_niNG ² 2 i_NR_F ² 2 4kTR_FNG
(5)
The first term is simply the differential input spot noise times the noise gain, the second term is the input current
noise terms times the feedback resistor (and because there are two uncorrelated current noise terms, the power
is two times one of them), and the last term is the output noise resulting from both the R_F and R_G resistors, at
again twice the value for the output noise power of each side added together. Running a wide sweep of gains
when holding R_F close to 1 kΩ and setting the input up for a 50-Ω match gives the standard values and resulting
noise listed in Table 2.
When the gain increases, the input-referred noise approaches only the gain of the FDA input voltage noise term
at 5 nV/√Hz.
GAIN (V/V)
0.1
1
2
5
10
Table 2. Swept Gain of the Output- and Input-Referred Spot Noise Calculations
R_F
1500
1500
1500
1500
1500
R_G1
15000
1500
750
287
137
R_T
49.9
51.1
52.3
54.9
61.9
R_G2
15000
1500
768
316
165
Z_IN
49.74
49.82
49.98
49.6
50.4
A_V
0.09973
0.994
1.978
5.014
10.08
E_O (nV/√Hz)
9.15
14.03
18.99
33.20
55.05
E_I (nV/√Hz)
91.53
14.03
9.49
6.64
5.51
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