AN1841| Application Note
Maxim/Dallas > App Notes > A/D and D/A CONVERSION/SAMPLING CIRCUITS Keywords: noise, power supply coupling, capacitive crosstalk, grounding currents, digital crosstalk, FFT, random noise, harmonic distortion, ADC, DAC, clock, jitter, analog to digital converter, converters
Dec 27, 2002
APPLICATION NOTE 1841
Effects of Digital Crosstalk in Data Converters Part 2: Crosstalk on the Clock
Part 2 of a series of articles explaining the effects of digital crosstalk in data converters. This article talks about noise on the clock source (jitter) and the effects on the analog-to-digital converter (ADC). This is Part 2 of a 3-part series:
q q
Part 1: Effects of Digital Crosstalk in Data Converters Part 3: Digital Data Signal Crosstalk on the Clock
What is the effect of crosstalk from the digital data signals into the clock signal of a data-conversion system? This is a more obscure problem than crosstalk from digital bits into the analog signal path, which was the subject discussed in the previous article: Part 1 of this series, Effects of Digital Crosstalk in Data Converters. Data sheets for data converters frequently mention that it is important to minimize crosstalk onto the data converter clock. When asked what happens when it is present, many engineers have the insight that "it creates noise". While this is generally a true statement, there is value to understanding this at a deeper level, so that a design engineer can more efficiently understand and troubleshoot a circuit with this type of problem. After reading this article, the reader will understand how harmonic distortion and other signal-dependant-error problems can be caused by crosstalk from digital data signals onto the clock. These can take a long time to diagnose and fix if the mechanis
ms are not understood. Understanding the concepts from the first article in this series will be helpful to understand before reading this article.
How Noise Couples onto a Clock Signal
Before we discuss what digital-bit crosstalk can do to a data converter system, it is important to understand what general effects clock noise has on a data converter system. A data converter clock signal seems like a digital signal. How can it be susceptible to crosstalk? The clock signal marks a point in time - the point where it crosses a digital signal threshold. If the rise/fall/ transition time were zero, then the sampling instant would be precisely defined, despite any noise in the environment. But a real-world clock has slope in its rise and fall. During this transition period, the clock signal is analog. If there is any voltage crosstalk during the slew period near the threshold crossing, it can alter the point in time that the threshold is crossed. This creates a noise on the clock, which is commonly referred to as jitter.
AN1841 Application Note maxim Download PDF
Add this permalink to your bookmarks for future download of AN1841 ApplicationNote
Permalink: http://application.emcelettronica.com/maxim/AN1841
