THP210 datasheet(22/45 Pages) TI2

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9.1.3 Noise Analysis

An accurate output-noise calculation allows the designer to compare the performance of alternate FDA solutions.

The combination of differential spot noise at the output pins of the FDA with any passive filtering to the ADC

enables an accurate signal-to-noise ratio (SNR) calculation. This chapter incorporates key elements for an

output 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 9-4 shows the simplest analysis circuit with the FDA. This

circuit considers the thermal resistor noise terms of the external feedback network and the intrinsic input voltage

and current noise terms.

In+

In±

eno

enRg

enRf

enRg

enRf

eni

Figure 9-4. FDA Noise Analysis Circuit

The noise powers are shown in Figure 9-4 for each term. When the RF and RG (or RI) terms are matched on

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

Using NG ≡ 1 + RF / RG as the noise gain, the total output noise density is given by Equation 3. Each resistor

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

e NG

2 i R

2 4kTR NG

(3)

where:

• eni is the differential input spot noise times the noise gain.

• in x RF is the input current noise terms times the feedback resistor.

Because there are two uncorrelated current noise terms, the power is two times one of them.

• enRF is the thermal output noise resulting from both the RF and RG resistors at twice the value for the output

noise power of each side added together.

THP210

SBOS932C – JANUARY 2020 – REVISED MARCH 2021

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THP210 Datasheet(PDF) 22 Page - Texas Instruments