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.

To get the total input-referred noise, e_ni, 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 i_ni1of the INA, see Section 7.5
• Voltage noise denisty e_ni1 of the INA, see Section 7.5
• Voltage noise density caused by source resistance i_ni× R_in
• Resistor noise from source resistance e_nRin , given by: √Rin × 4.04 nV/√Hz
• Reference voltage noise e_nref

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

Equation 5.