Amplifier and Data Converter Selection Guide (Rev. B

52 ADCs by Architecture➔Delta-Sigma (∆Σ) ADCsDelta-sigma converters are capable of veryhigh resolution, and are ideal for convertingsignals over a very wide range of frequenciesfrom DC to several megahertz. In a deltasigmaADC, the input signal is oversampled bya modulator, then filtered and decimated by adigital filter producing a high-resolution datastream at a lower sampling rate.The delta-sigma architecture approachallows resolution to be traded for speed andboth to be traded for power. This nearlycontinuous relationship between data rate,resolution and power consumption makesdelta-sigma converters extraordinarily flexible.In many delta-sigma converters, thisrelationship is programmable, allowing asingle device to handle multiplemeasurement requirements.Because delta-sigma converters oversampletheir inputs, they can perform mostanti-aliasing filtering in the digital domain.Modern VLSI design techniques havebrought the cost of complex digital filters farbelow the cost of their analog equivalents.Formerly unusual functions, such assimultaneous 50Hz and 60Hz notch filtering,are now built into many delta-sigma ADCs.Typical high-resolution applications fordelta-sigma ADCs include audio, industrialprocess control, analytical and testinstrumentation and medical instrumentation.Recent innovations in ADC architectureshave led to a new class of ADC architecturewhich uses both the pipeline and theoversampling principle.These, very high-speedconverters push the data rates into the MSPSrange, while maintaining resolutions of16-bits and higher.These speeds enable ahost of new wide bandwidth signal processingapplications such as communications andmedical imaging.Most delta-sigma ADCs have inherentlydifferential inputs. They measure the actualdifference between two voltages, instead ofthe difference between one voltage andground. The differential input structure of adelta-sigma makes it ideal for measuringdifferential sources such as bridge sensorsand thermocouples. Frequently, no inputamplifiers are required for theseapplications.Delta-sigma converters work differently thanSAR converters. A SAR takes a "snapshot" ofan input voltage and analyzes it to determinethe corresponding digital code. A delta-sigmameasures the input signal for a certainperiod of time and outputs a digital codecorresponding to the signal's average overthat time. It is important to remember theway delta-sigma converters operate,particularly for designs incorporatingmultiplexing and synchronization.It is very easy to synchronize delta-sigmaconverters together, so that they sample atthe same time but it's more difficult tosynchronize a delta-sigma converter to anexternal event. Delta-sigma converters arehighly resistant to system clock jitter. Theaction of oversampling effectively averagesthe jitter, reducing its impact on noise.AnalogInputDifferentialAmplifierDACModulatorIntegratorMany delta-sigma converters include inputbuffers and programmable gain amplifiers(PGA). An input buffer increases the inputimpedance to allow direct connection to highsource impedance signals. A PGA increasesthe converter’s resolution when measuringsmall signals. Bridge sensors are an exampleof a signal source that can take advantage ofthe PGA within the converter.Every ADC requires a reference, and forhigh-resolution converters, low-noise,low-drift references are critical. Mostdelta-sigma converters have differentialreference inputs.The following pages provide a broad rangeof delta-sigma ADCs available fromTI for a wide range of applications.To help facilitate the selection process,an interactive online data converterparametric search engine is available atdataconverter.ti.com with links to alldata converter specifications.ComparatorDigitalFilterDelta-sigma ADCs consist of a delta-sigma modulator followed by a digitaldecimation filter. The modulator incorporates a comparator and integrator in afeedback loop with a DAC. The loop is synchronized by a clock.DigitalOutputAmplifier and Data Converter Selection Guide Texas Instruments 3Q 2007