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9.2.2.2 Noise Analysis

Low-noise instrumentation amplifiers such as the INA848 are designed to serve stringent and sensitive

applications, such as surgical tools, microphones or other precision monitoring systems. A througough noise

analysis is a key element in the design process.

TI`s super-beta transistors offer the benefits of low voltage noise and low current noise, thus allowing the INA848

excellent noise performance.

Figure 9-6 shows a simplified noise model including the gain stages of the INA848.

ni1

200

nref

no1

DIFF

REF

OUT

Figure 9-6. Simplified Noise Model

To get the total input-referred noise, eni, consider the source resistance seen by the positive and negative input

pins of the instrumentation amplifier. The key elements that must be considered for a noise analysis in an

instrumenation amplifier are:

• Current noise density ini1of the INA, see Section 7.5

• Voltage noise denisty eni1 of the INA, see Section 7.5

• Voltage noise density caused by source resistance ini× Rin

• Resistor noise from source resistance enRin , given by: √Rin × 4.04 nV/√Hz

• Reference voltage noise enref

The noise sources are uncorrelated (that is, the noise signal is unpredictable). The result of mutliple uncorrelated

noise sources is the square root of the sum of their squares (RSS). Thus, the total RTI noise density, eni, in

nV/√Hz can be derived from the following equation:

eni=

¨e

ni1

2+en:Rin;2+(i

ni1ÂRin)

2+(

en:REF;

)

(5)

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INA848

SBOS946 – SEPTEMBER 2020

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