OPA607 datasheet(29/52 Pages) TI2

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(

)

I R

4kTR

(8)

Substituting large resistor values into Equation 8 can quickly dominate the total equivalent input referred noise.

A source impedance on the noninverting input of 2-kΩ adds a Johnson voltage noise term similar to that of the

amplifier (6.3 nV/√ Hz).

Table 8-1 compares the noise contributions from the various terms when the OPA810 is configured in a

noninverting gain of 5 V/V as Figure 8-11 shows. Two cases are considered where the resistor values in case

2 are 10x the resistor values in case 1. The total output noise in case 1 is 34 nV/√ Hz while the noise in case

2 is 51.5 nV/√ Hz. The large value resistors in case 2 dilute the benefits of selecting a low noise amplifier like

the OPA810. To minimize total system noise, reduce the size of the resistor values. This increases the amplifiers

output load and results in a degradation of distortion performance. The increased loading increases the dynamic

power consumption of the amplifier. The circuit designer must make the appropriate tradeoffs to maximize the

overall performance of the amplifier to match the system requirements.

Case1: 200

Case2: 2 k

S+ = 5V

S- = -5V

Case1: 250

Case2: 2.5 k

Case1: 1 k

Case2: 10 k

Figure 8-11. Comparing Noise Contributors for Two Cases with the Amplifier in a Noninverting Gain of

5 V/V

Table 8-1. Comparing Noise Contributions for the Circuit in Figure 8-11

NOISE

SOURCE

OUTPUT

NOISE

EQUATION

CASE 1

CASE 2

NOISE

SOURCE

VALUE

VOLTAGE

NOISE

CONTRIBUTIO

N (nV/√ Hz)

NOISE

POWER

CONTRIBUTIO

N (nV2/Hz)

CONTRIBUTIO

N (%)

NOISE

SOURCE

VALUE

VOLTAGE

NOISE

CONTRIBUTIO

N (nV/√ Hz)

NOISE

POWER

CONTRIBUTIO

N (nV2/Hz)

CONTRIBUTIO

N (%)

Source resistor,

ERS (1 +

RF /RG)

1.82 nV/√

9.1

82.81

7.15

5.76 nV/√

28.8

829.44

31.29

Gain resistor,

ERG (RF /

RG)

2.04 nV/√

8.16

66.59

5.75

6.44 nV/√

25.76

663.58

25.03

Feedback

resistor, RF

ERF

4.07 nV/√

4.07

16.57

1.43

12.87 nV/√

12.87

165.64

6.25

Amplifier

voltage noise,

ENI

ENI (1 + RF /

RG)

6.3 nV/√

31.5

992.25

85.67

6.3 nV/√

31.5

992.25

37.43

Inverting

current noise,

IBI

IBI (RF || RG)

5 fA/√ Hz

5.0E-3

—

5 fA/√ Hz

50E-3

—

Noninverting

current noise,

IBN

IBNRS (1 +

RF/ RG)

5 fA/√ Hz

1.0E-3

—

5 fA/√ Hz

10E-3

—

www.ti.com

OPA810

SBOS799E – AUGUST 2019 – REVISED AUGUST 2024

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