OPA828 datasheet(21/47 Pages) TI2

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7.3.5 Noise Performance

Figure 7-7 shows the total circuit noise for varying source impedances with the operational amplifier in a

unity-gain configuration (with no feedback resistor network and therefore no additional noise contributions). The

OPA828 and OPA211 are shown with total circuit noise calculated. The op amp contributes both a voltage

noise component and a current noise component. The voltage noise is commonly modeled as a time-varying

component of the offset voltage. The current noise is modeled as the time-varying component of the input bias

current and reacts with the source resistance to create a voltage component of noise. Therefore, the lowest

noise op amp for a given application depends on the source impedance. For low source impedance, current

noise is negligible, and voltage noise generally dominates. The OPAx828 devices have both low voltage noise

and extremely low current noise because of the FET input of the op amps. As a result, the current noise

contribution of the OPAx828 is negligible for any practical source impedance, which makes the OPAx828 an

excellent choice for applications with high source impedance.

Equation 2 provides a simple calculation for total noise, EO, of a unity gain buffer op amp circuit:

EO = eN2 + iN x RS2 + 4kTRS

(2)

where

• eN = voltage noise

• iN = current noise

• RS = source impedance

• k = Boltzmann's constant = 1.38 × 10–23 J/K

• T = temperature in kelvins (K)

Source resistance, RS (:)

0.1n

10n

100n

10P

100

10k

100k

Resistor Noise

OPA828

OPA211

Figure 7-7. Noise Performance of the OPA828 and OPA211 in Unity-Gain Buffer Configuration

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OPA828, OPA2828

SBOS671D – SEPTEMBER 2018 – REVISED DECEMBER 2022

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OPA828 Datasheet(PDF) 21 Page - Texas Instruments