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By Paul Lee, Analog Devices, Inc.

Introduction

When evaluating an amplifier's performance for a low noise application, one factor to consider is noise, both internal and external. This article briefly discusses the fundamentals of both internal and external noise and identifies the tradeoffs associated in selecting the optimal amplifier for low noise design.

External Noise

There is a distinction between internal and external noise sources. External noise can include any external influence, such as external components and electrical/electromagnetic interference. Interference is any unwanted signals arriving as either voltage or current, at any of the amplifier's terminals or induced in its associated circuitry. It can appear as spikes, steps, sine waves, or random noise. Interference can come from anywhere: machinery, nearby power lines, RF transmitters or receivers, computers, or even circuitry within the same equipment (that is, digital circuits or switching-type power supplies). If all interference could be eliminated by careful design and/or layout of the board, there would still be random noise associated with the amplifier and its circuit components.

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•Application Note: Low-Noise Amplifier Selection For Optimal Noise Performance