Precision voltage references - TI.com

1 1 Analog Applications JournalNovember 1999 Analog and Mixed-Signal ProductsPrecision voltage referencesIntroductionOne reason why designing a data conversion system issuch a challenge is the fact that the system accuracy verymuch depends on the accuracy of the voltage establishedby the internal or external DC voltage reference . Thevoltage reference is used to produce a precise value ofoutput voltage for setting the full-scale input of the dataconversion system. In an analog-to-digital converter(ADC), the DC voltage reference together with the analoginput signal is used to generate the digitized output in a digital-to-analog converter (DAC), the DACselects and produces an analog output from the DC refer-ence voltage according to the digital input signal presentedat the input of the DAC.

2 Any errors in the reference volt-age over the operating temperature range will adverselyaffect the linearity and spurious free dynamic range(SFDR) of the ADC/DAC. Practically all voltage refer-ences vary with time or environmental factors such ashumidity, pressure, and temperature. As a result mostCMOS ADCs/DACs have internal references suitable onlyfor applications demanding 12-bit resolution eventhough the converter may be capable of higher CMOS converters operate from or 5-V sup-plies, which limits the on-chip voltage reference to aband-gap reference . By way of the external reference pinsprovided on the chip, an external Precision reference canalso be connected to a CMOS ADC or DAC.

3 A precisionexternal voltage reference has a much lower temperaturecoefficient, thermal hysteresis, and long-term drift thanan on-chip band-gap voltage reference ; therefore, inapplications demanding high accuracy (14-bit or 16-bitADCs/DACs), an external Precision voltage reference isoften voltage references are available with varyingdegrees of Precision and initial accuracy over some operating temperature range. But often what is not obvi-ous when reading a manufacturer s data sheet is how theinitial accuracy of the device is affected by other key deviceparameters such as line regulation, load regulation, initialvoltage error, output voltage temperature coefficient (TC),output voltage noise, turn-on settling time, thermal hyste-resis, quiescent supply current, and long-term design originsModern voltage references are constructed using theenergy-band-gap voltage of integrated transistors, buriedzener diodes, and junction field-effect transistors.

4 Eachtechnology offers inherent performance characteristicsthat can be enhanced with compensation networks oradditional active circuitry. The basis topologies for theband-gap, buried zener, and XFET references are shownin Figures 1, 2, and 3, Instruments IncorporatedData AcquisitionBy Perry Miller, Application Specialist Data Converters, Texas Instruments, Dallas,and Doug Moore, Managing Director, Thaler Corp., Tucson, +VVOUTR1R2R3R4+ IR1R2R3 VINI1I2 VOUTIPTAT+ I2 I1 VCCV = VBG OUTVPTATVBE2 VBE1 VCC + Figure 1. Band-gap reference circuitContinued on next pageFigure 2. Buried zener reference circuitFigure 3.

5 XFET reference circuitVZTexas Instruments IncorporatedData Acquisition2 Analog Applications JournalAnalog and Mixed-Signal ProductsNovember referenceAt its simplest, a band-gap reference is simply two tran-sistors with different emitter areas used for generating avoltage proportional to absolute temperature. VBE1andVBE2have opposite temperature coefficients. The voltageVCCis converted to a current I1and I2that are mirrored tothe output branch. The output equation is(1)where is the scale factor, VBE1is the base-emitter volt-age of the larger of the two transistors, and VBE2is thebase-emitter voltage of the second band-gap references are widely used in ADC/DACconverters as well as for external reference source becausethey are fairly inexpensive.

6 Generally, they are used insystem designs where a maximum accuracy of 10 bits isrequired. Band-gap references typically have an initialerror of and a TC of 25 50 ppm/ C. The outputvoltage noise is typically 15 30 Vp-p( 10 Hz) with along-term stability of 20 30 ppm/1000 referenceThe zener voltage reference and feedback amplifier shownin Figure 2 are used to provide a very stable output. Acurrent source is used to bias a zener diode. Thezener voltage is divided by the resistor network R1 and voltage is applied to the non-inverting input of theoperational amplifier, which amplifies the voltage to therequired output voltage . The amplifier gain is determinedby the resistor networks R3 and R4, where G = 1 + R4 zener diode is used because it is the most stablezener diode over time and output equation is(2)Buried zener diode references are more expensive thanband-gap references but provide a higher performancelevel.

7 They typically have an initial error of , aTC of 1 10 ppm/ C, and less than 10- Vp-p( to 10-Hz)),VV(VVBE2BE1BE1O +=noise. The long-term stability is typically 6 15 ppm/1000hrs. Buried zener-based references are frequently usedfor 12-bit, 14-bit, and higher resolution systems becausethe performance of the buried zener-based references canbe extended by incorporating nonlinear temperature com-pensationnetworks into the design. The compensationnetwork is trimmed at several temperatures to optimizethe electrical performance over the operating tempera-ture referenceThe XFET reference is a new reference technique thatconsists of two junction field-effect transistors, one ofwhich has an extra channel implant to raise the pinch-offvoltage.

8 The two JFETs are run at the same drain difference in pinch-off voltage is amplified and usedto form a voltage reference . The general equation is(3)where VPis the difference in pinch-off voltage betweenthe two FETs and IPTATis the positive temperature coefficient correction simplified schematic for the XFET reference isshown in Figure XFET references are relatively new and provide aperformance level between band-gap and zener initial error is typically , a TC of 10 ppm/ C,and 15- Vp-p( to 10-Hz) noise. The long-term stabilityis ppm/1000 selection for a 14-bit converterSpecified parameters for voltage references include lineregulation, load regulation, initial voltage error, outputvoltage temperature coefficient (TC), output voltagenoise, turn-on settling time, thermal hysterisis, quiescentsupply current, and long term most important parameters for data acquisitionsystems design are initial error, output voltage tempera-ture coefficient (TC), thermal hysteresis, noise, and long-term stability of the voltage reference 1 summarizes the major error sources for thethree references that are compared in this applicationnote.

9 The data represents the highest grade for each)(R3),(IR1R3R2R1 VVPTATPO+ ++ =THALER CORP. VRE3050 MAXIM MAX6250 ANALOG DEVICES ADR293 PARAMETERTEMPERATURE RANGETEMPERATURE RANGETEMPERATURE RANGE 40 C to +85 C 40 C to +85 C 40 C to +85 COutput VInitial ppm/ C ppm/ ppm/ CNoise ( 10 Hz) Vp-pThermal hysteresis2 ppm 20 ppm15 ppm25 C 50 C 25 CLong-term ppm/1000 ppm/1000 ppm/1000 V 36 V 36 V 15 VTurn-on settling time10 s10 s<10 sLine regulation (8 V VIN 10 V)25 ppm/VLoad regulation (source 0 mA IO 15 mA)5 ppm/mA7 ppm/mA100 ppm/mAPSRR (10 Hz 900 Hz)95 dB90 dB40 dBTable 1. voltage reference major error sources (all information is based on published data sheets)Continued from previous page V.

10 ZR3R41R2R1R2VO ++=Texas Instruments IncorporatedData Acquisition3 Analog Applications JournalNovember 1999 Analog and Mixed-Signal model in the 8-pin plastic DIP package overthe industrial temperature range ( 40 C to +85 C). Thepoorest-performing references are band-gap type and arenot included in this summary. Buried zener diodes havebetter overall performance than band-gap devices and theXFET references . The buried zener reference with a third-ordertemperature compensation network (VRE3050) isthe best performer with respect to initial error, TC, andthermal of parametersInitial error The output voltage tolerance of a refer-ence after the device is turned on and warmed up.