Amplifier and Data Converter Selection Guide (Rev. B

DACs by ArchitectureDelta-Sigma (∆Σ) DACs69➔Delta-sigma (∆Σ) DACs are the converseof delta-sigma ADCs with a digitalmodulator and analog filter. ∆Σ DACsinclude a serial interface, controlregisters, modulator, switched capacitorfilter and a clock for the modulator and filter.∆Σ DACs have high resolution and lowpower making them ideal for closed-loopcontrol in industrial control applications,high-resolution test and measurementequipment, remote applications, batterypoweredinstruments and isolated systems.∆Σ DACs Selection GuideRes. Settling Time Number of Output Output Linearity Monotonicity PowerDevice (Bits) (ms) DACs Interface (V) V REF (%) (Bits) (mW) Package Price *DAC1220 20 15 1 Serial, SPI 5 Ext 0.0015 20 2.5 SSOP-16 $6.33DAC1221 16 2 1 Serial, SPI 2.5 Ext 0.0015 16 1.2 SSOP-16 $5.01* Suggested resale price in U.S. dollars in quantities of 1,000.➔High-Accuracy and General Purpose DACsResistor "String" and R-2R DACs consistof three major elements: logic circuitry; sometype of resistor network means of switchingeither a reference voltage or current to theproper input terminals of the network as afunction of the digital value of each digitalinput bit, and a reference voltage.Technical InformationR-2R DACs—are used to achieve the bestintegral linearity performance. In an R-2RDAC, a current is generated by a referencevoltage, which flows through the R-2Rresistor network based on the digital input,which divides the current by two at eachR-2R node. The advantage of a R-2R typeDAC is that it relies on the matching of theR and 2R resistor segments and not theabsolute value of the resistors thus allowingtrim techniques to be used to adjust theintegral linearity and differential linearity.Voltage Segment DACs (String DACs)—aresimply a string of resistors, each of value R.The code loaded into the DAC registerdetermines at which node on the string thevoltage is tapped off to be fed into theoutput amplifier by closing one of theswitches connecting the string to theamplifier. The DAC is monotonic, because itis a string of resistors. In higher resolution12- and 16-bit DACs, two resistor strings areused to minimize the number of switches inthe design. In a two-resistor stringconfiguration, the most significant bits drivea decoder tree, which selects the voltagesfrom two adjacent taps of the first resistorstring and applies them to the inputs of twobuffers. These buffers then force thesevoltages across the endpoints of the secondresistor string. The least significant data bitsdrive a second decoder tree, which selectsthe voltage at one of the switch outputs anddirects it to the output buffer.REFADJREFOUT REFINVREFROFFSETVREFRFB2R+10V InternalReferenceBufferR/4R/2 R/2 R/4RFB1SJDATADACLatchOutputBufferVOUT2R 2R 2R 2R 2R 2R 2R 2R 2RR/4VOUTCLOCK/WEControlandInterfaceVREFAGNDCSBufferVoltage segment DAC.Segmented R-2R DAC.Texas Instruments 3Q 2007Amplifier and Data Converter Selection Guide