OPA210 datasheet(19/38 Pages) TI

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8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification, and TI

does not warrant its accuracy or completeness. TI’s customers are responsible for determining

suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

The OPAx210 are unity-gain stable, precision operational amplifiers with very low noise. Applications with noisy

or high-impedance power supplies require decoupling capacitors close to the device pins. In most cases, 0.1-µF

capacitors are adequate.

8.1.1 Basic Noise Calculations

Low-noise circuit design requires careful analysis of all noise sources. External noise sources can dominate in

many cases; consider the effect of source resistance on overall op amp noise performance. Total noise of the

circuit is the root-sum-square combination of all noise components.

The resistive portion of the source impedance produces thermal noise proportional to the square root of the

resistance. This function is plotted in Figure 7-2. The source impedance is usually fixed; consequently, select the

op amp and the feedback resistors to minimize the respective contributions to the total noise.

Figure 8-1 illustrates both noninverting (A) and inverting (B) op amp circuit configurations with gain. In circuit

configurations with gain, the feedback network resistors also contribute noise. In general, the current noise of the

op amp reacts with the feedback resistors to create additional noise components. However, the extremely low

current noise of the OPAx210 means that the device current noise contribution can be neglected.

Generally, the feedback resistor values are chosen to make these noise sources negligible. Low impedance

feedback resistors load the output of the amplifier. The equations for total noise are shown for both

configurations.

(A) Noise in Noninverting Gain Configuration

GND

Source

GND

Source

(B) Noise in Inverting Gain Configuration

:1; '

1 =

l1+42

p„¨:A

;2 +:A

;2 +

41æ42

o2+:E

„ 45

;2 +

„

d41„42

41 + 42

hp2 >8

4/5

:2; A

5 =

¥4„G$„6(-)„45 d 8

¾*V

:3; A

41æ42 =

¨4„G

„ 6(-) „

d41„42

41 + 42

h d 8

¾*V

:4; G

$ = 1.38065

„ 10

F23

d ,

:5; 6(-) = 237.15+6(°%) >-?

Noise at the output is given as EO, where

:6; '

1 =

l1+ 42

45 + 41

p„¨:A

;2 +

41+45 æ42

o2+FE

„

H:45 +41;„42

45 + 41 + 42 IG

4/5

:7; A

41+45 æ42 =

¨4„G

„ 6(-) „

H:45 +41;„42

45 + 41 + 42 I

d 8

¾*V

:8; G

$ = 1.38065

„ 10

F23

d ,

:9; 6(-) = 237.15+6(°%) >-?

Thermal noise of RS

Thermal noise of R1 || R2

Boltzmann Constant

Temperature in kelvins

Boltzmann Constant

Temperature in kelvins

Thermal noise of (R1 + RS) || R2

Where eN is the voltage noise of the amplifier. For the OPAx210 op amp, eN = 2.2 nV/√Hz at 1 kHz.

Where iN is the current noise of the amplifier. For the OPAx210 op amp, iN = 400 fA/√Hz at 1 kHz.

NOTE: For additional resources on noise calculations visit the TI Precision Labs Series.

Figure 8-1. Noise Calculation in Gain Configurations

www.ti.com

OPA210, OPA2210

SBOS924F – SEPTEMBER 2018 – REVISED JANUARY 2021

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