ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

Watch the Time Constant!• Reduced Amplitude With Increasing DataRate• Difficulty Capturing Data– Shortened Capture Window– May Not Cross Logic Threshold34The time constant of the series output resistors and the capacitances after those resistors forma time constant that slows the slew rate of the output. Also, that time constant reduces theoutput amplitude as the output data rate increases. This can make it difficult to capture theoutput data because the capture window is reduced. As the time constant becomes longer orthe output data rate becomes faster we may find that the signal does not even cross the logicthreshold and no data is captured at all.Be careful of this time constant. At very high frequencies it may not be practical to use seriesresistors at the ADC output. When this is the case, it is absolutely essential to have the drivencircuit very close to the ADC output pins and to use a data receiving device with a very lowinput capacitance.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray34Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved