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

ADC Sample Rate vs. CircuitNeeds• Minimum Sample Rate Limitation• Using an ADC Below Specified SampleRate• Getting Data Below ADC’s MinimumSample Rate38Sometimes we can not find an ADC that is specified for the sample rate we need for a system.Usually, an ADC that is operated below its specified rate will operate at least as well at lowersampling rates. However, there are ADCs on the market (none from National Semiconductor)that will not function well if not used very close to their specified sample rate.Most high speed ADCs have a sample rate below which they do not perform well. The reasonfor this is that the on-chip capacitors that must hold a charge during the conversion processare very small (to allow for the fast acquisition time required for a high speed ADC). Becausethey are very small, the charge on them can dissipate if the sample rate is too low. Be awareof the minimum sample rate for a converter you are using below its specified sample rate.If you need a conversion rate below the minimum acceptable sample rate of a given ADC,simply clock it above its minimum rate and only look at every 2nd, or 3rd or 10th sample.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray38Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved