THS4541IRUNR.B datasheet(30/66 Pages) TI2

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7.5 Noise Analysis

Figure 7-10 shows that 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, with the FDA and resistor noise terms to

be considered.

In+

In±

eno

enRg

enRf

enRg

enRf

eni

Figure 7-10. FDA Noise-Analysis Circuit

The noise powers are shown for each term. When the Rf and Rg terms are matched on each side, the total

differential output noise is the RSS of these separate terms. Equation 6 shows the total output noise using NG ≡

1 + Rf / Rg. Each resistor noise term is a 4kTR power.

e NG

2 i Rf

2 4k RfNG

(6)

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 terms, the power is two times one of the

terms). The last term is the output noise resulting from both the Rf and Rg resistors, again times two, for the

output noise power of each side added together. Using the exact values for a 50-Ω, matched, single-ended to

differential gain, sweep with a fixed Rf = 402 Ω (see Table 8-1) and the intrinsic noise eni = 2.2 nV and In = 1.9

pA for the THS4541, gives an output spot noise from Equation 6. Then, dividing by the signal gain (Av) gives the

input-referred, spot-noise voltage (ei) shown in Table 7-5.

Table 7-5. Swept Gain Output and Input-Referred, Spot-Noise Calculations(1)

Rt, EXACT (Ω)

Rg1, EXACT (Ω)

Rg2, EXACT (Ω)

NOISE GAIN

eno (nV/√ Hz)

ei (nV/√ Hz)

55.2

399

425

1.94

6.64

60.1

191

218

2.85

8.71

4.36

65.6

124

153

3.63

10.7

3.56

89.7

119

4.37

12.1

3.03

79.7

67.8

98.3

5.09

13.7

2.74

89.1

54.2

86.5

5.65

15.4

2.56

101

43.2

76.6

6.25

16.7

2.39

117

35.2

70.1

6.74

17.3

2.16

138

29.0

65.8

7.11

18.6

2.06

170

23.6

62.5

7.44

18.9

1.89

220

18.7

59.3

7.78

19.6

1.78

313

14.6

57.7

7.97

20.0

1.66

545

10.8

56.6

8.11

20.3

1.56

2209

7.26

56.1

8.16

21.1

1.50

(1)

Rf = 402 Ω.

Notice that the input-referred ei is less than 2.2 nV/√Hz for just the THS4541 above a gain of 7 V/V. This result is

because NG is less than Av when the source impedance is included in the NG calculation.

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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