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Noise Analysis using Analog Devices Operational Amplifiers
Publish Date: Oct 06, 2013
Overview
Analog designers rely on datasheets to define the specification of their circuit designs. Although datasheets include the specifications of a component and remain an indispensable resource, they lack
information of how parts will behave within different configurations. This is why circuit simulation is a critical tool in the design flow to complement datasheets to provide the insight needed to improve
design performance and reduce iterations. In this series of application notes, we review how NI Multisim and devices from semiconductor industry partners can use simulation to improve design
performance.
This article provides an overview of the theory behind the Noise Analysis feature in NI Multisim and describes how to use it. It also provides a guideline for verifying the inclusion of noise effects in an
operational-amplifier macro-model and uses a practical example to illustrate the benefit of adopting simulation in the design flow.
Table of Contents
Noise Analysis overview
Setting Noise Analysis Parameters
Noise in op-amps and in other models
Practical problem
Resources
1. Noise Analysis overview
Noise Analysis is a small signal analysis which is carried out at discrete frequencies using a linearized version of the circuit. The mechanics are very similar to those of an AC analysis. The key
difference is that in AC analysis the signal generators are AC signal sources explicitly defined by the user whereas in noise analysis the signal generators are invisible noise sources attached to
every noise-generating element in the circuit, such as to the resistor shown below:
Figure 1: Noise model of a simple resistor
Additionally, in AC analysis the AC signal source amplitude and direction are set explicitly by the user while in noise analysis the amplitude of the noise source is set through a formula representing
the physical noise phenomena of the noise-generating element. The noise generators within the circuit are completely uncorrelated and therefore have no direction; their additive effect on any circuit
node is calculated in an RMS-manner.
Note that unless explicitly modeled by the user, noise originating externally to the circuit (e.g from a previous stage in a signal chain) is not injected or modeled by Noise Analysis. For example, AC
sources do not inject any noise into the circuit; all voltage sources are shorted and all current sources are open-circuited.
Noise generators
Every resistor and every semiconductor device in the circuit has at least one noise generator associated with it. While plain resistors only generate thermal noise, semiconductors may generate
thermal, shot, and Flicker noise. Op-amp and precision reference components typically use macro-modeling techniques to synthesize white and 1/f noise (this is discussed later).
2. Setting Noise Analysis Parameters
Analysis parameters tab