2

2

o

ni

N

F

F

e

e NG

2 i R

2 4kTR NG

+

±

2

2

2

2

2

2

2

2

13

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Application Information (continued)

8.1.3 Noise Performance

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 13 shows the simplest analysis circuit with the FDA and

resistor noise terms to be considered.

Figure 13. FDA Noise Analysis Circuit

side, the total differential output noise is the root sum squared (RSS) of these separate terms.

Using NG

Each resistor noise term is a 4kT × R power (4kT = 1.6E-20 J at 290 K).

(1)

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

again twice the value for the output noise power of each side added together. Running a wide sweep of gains

When the gain increases, the input-referred noise approaches only the gain of the FDA input voltage noise term

at 3.7 nV/

√Hz.

Table 1. Swept Gain of the Output- and Input-Referred Spot Noise Calculations

GAIN (V/V)

0.1

2000

20000

0.1

9.4

93.9

1

2000

2000

1

13.6

13.6

2

2000

1000

2

17.8

8.9

5

2000

402

4.98

29.5

5.9

10

2000

200

10

48.6

4.9