a 1.2 V Micropower, Precision Shunt Voltage Reference AD1580 PIN CONFIGURATION SOT-23 Package FEATURES Wide Operating Range: 50 mA–10 mA Initial Accuracy: 60.1% max Temperature Drift: 650 ppm/8C max Output Impedance: 0.5 V max Wideband Noise (10 Hz–10 kHz): 20 mV rms Operating Temperature Range: –408C to +858C High ESD Rating 4 kV Human Body Model 400 V Machine Model Compact, Surface-Mount, SOT-23 Package V+ 1 3 NC (OR V–) V– 2 TOP VIEW NC = NO CONNECT 50 45 GENERAL DESCRIPTION The AD1580’s superior accuracy and stability is made possible by the precise matching and thermal tracking of on-chip components. Proprietary curvature correction design techniques have been used to minimize the nonlinearities in the voltage output temperature characteristics. The AD1580 is stable with any value of capacitive load. The low minimum operating current makes the AD1580 ideal for use in battery powered 3 V or 5 V systems. However, the wide operating current range means that the AD1580 is extremely versatile and suitable for use in a wide variety of high current applications. 40 35 QUANTITY The AD1580 is a low cost, two-terminal (shunt), precision bandgap reference. It provides an accurate 1.225 V output for input currents between 50 µA and 10 mA. 30 25 20 15 10 5 0 –40 –30 –20 –10 10 20 0 TEMPERATURE DRIFT – ppm/°C 30 40 Reverse Voltage Temperature Drift Distribution The AD1580 is available in two grades, A and B, both of which are provided in an SOT-23 package, the smallest surface mount package available on the market. Both grades are specified over the industrial temperature range of –40°C to +85°C. 300 250 1. Portable, Battery-Powered Equipment: Cellular Phones, Notebook Computers, PDAs, GPS and DMM. 2. Computer Workstations Suitable for use with a wide range of video RAMDACs. QUANTITY 200 TARGET APPLICATIONS 150 100 50 3. Smart Industrial Transmitters 4. PCMCIA Cards. 5. Automotive. 6. 3 V/5 V 8–12-Bit Data Converters. 0 –10 –8 –6 –4 –2 0 2 4 6 8 10 OUTPUT ERROR – mV Reverse Voltage Error Distribution REV. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. © Analog Devices, Inc., 1995 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 Fax: 617/326-8703 AD1580–SPECIFICATIONS (@ T = +258C, I A Model Min Reverse Voltage Output 1.215 IN = 100 mA, unless otherwise noted) AD1580A Typ Max 1.225 1.235 Min AD1580B Typ Max 1.224 1.225 Units 1.226 V Reverse Voltage Temperature Drift –40°C to +85°C 100 50 ppm/°C Minimum Operating Current, TMIN to TMAX 50 50 µA Reverse Voltage Change with Reverse Current 50 µA < IIN < 10 mA, TMIN to TMAX 50 µA < IIN < 1 mA, TMIN to TMAX 2.5 0.5 5 2.5 0.5 5 mV mV Dynamic Output Impedance (∆VR/∆IR) IIN = 1 mA ± 100 µA (f = 120 Hz) 0.4 1 0.4 0.5 Ω OUTPUT NOISE RMS Noise Voltage: 10 Hz to 10 kHz Low Frequency Noise Voltage: 0.1 Hz to 10 Hz 20 5 20 5 µV rms µV p-p Turn-On Settling Time to 0.1%1 5 5 µs 80 80 µV Output Voltage Hysteresis 2 Temperature Range Specified Performance, TMIN to TMAX Operating Range3 –40 –55 +85 +125 –40 –55 +85 +125 °C °C NOTES 1 Measured with no load capacitor. 2 Output hysteresis is defined as the change in the +25°C output voltage after a temperature excursion to +85°C and then to –40°C. 3 The operating temperature range is defined as the temperature extremes at which the device will continue to function. Parts may deviate from their specified performance. Specifications subject to change without notice. ABSOLUTE MAXIMUM RATINGS 1 Reverse Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 mA Forward Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 mA Internal Power Dissipation2 SOT-23 (RT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 Watts Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C Operating Temperature Range AD1580/RT . . . . . . . . . . . . . . . . . . . . . . . – 55°C to +125°C Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . . +215°C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . . +220°C ESD Susceptibility3 Human Body Model . . . . . . . . . . . . . . . . . . . . . . . . . . 4 kV Machine Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 V ORDERING GUIDE Model Initial Output Error Temperature Coefficient Package Option AD1580ART AD1580ART-REEL1 AD1580ART-REEL72 AD1580BRT AD1580BRT-REEL1 AD1580BRT-REEL72 10 mV 10 mV 10 mV 1 mV 1 mV 1 mV 100 ppm/°C 100 ppm/°C 100 ppm/°C 50 ppm/°C 50 ppm/°C 50 ppm/°C RT RT RT RT RT RT NOTES 1 Provided on a 13-inch reel containing 7,000 pieces. 2 Provided on a 7-inch reel containing 2,000 pieces. NOTES 1 Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 Specification is for device in free air at +25°C: SOT-23 Package: θJA = 300°C/Watt. 3 The human body model is a 100 pF capacitor discharged through 1.5 kΩ. For the machine model, a 200 pF capacitor is discharged directly into the device. PACKAGE BRANDING INFORMATION Four marking fields identify the device generic, grade, and date of processing. The first field is the product identifier. A “0” identifies the generic as the AD1580. The second field indicates the device grade; “A” or “B.” In the third field a numeral or letter indicates a calendar year; “5” for 1995, “A” for 2001. In the fourth field, letters A-Z represent a two week window within the calendar year; starting with “A” for the first two weeks of January. CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD1580 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. –2– WARNING! ESD SENSITIVE DEVICE REV. 0 Typical Performance Characteristics–AD1580 600 500 NOISE VOLTAGE – nV/√Hz REVERSE VOLTAGE CHANGE – ppm 1000 0 –500 ~20ppm/°C –1000 400 200 –1500 –2000 –55 –35 –15 5 25 65 45 TEMPERATURE – °C 85 105 1.0 125 100 3 80 REVERSE CURRENT – µA REVERSE VOLTAGE CHANGE – mV 4 TA = 125°C 1 TA = –40°C – +85°C 0 100 1k 10k FREQUENCY – Hz 100k 1M Figure 3. Noise Spectral Density Figure 1. Output Drift for Different Temperature Characteristics 2 10 60 40 +85°C 20 +25°C –40°C –1 0.01 0.1 1 REVERSE CURRENT – mA 0 10 0 0.2 0.4 0.6 0.8 1.0 REVERSE VOLTAGE – V 1.0 +25°C –40°C FORWARD VOLTAGE – V 0.8 +85°C 0.6 0.4 0.2 0.1 1 10 FORWARD CURRENT – mA 100 Figure 5. Forward Voltage vs. Forward Current REV. 0 1.4 Figure 4. Reverse Current vs. Reverse Voltage Figure 2. Output Voltage Error vs. Reverse Current 0 0.01 1.2 –3– AD1580 THEORY OF OPERATION V+ V1 ∆VBE VBE +5V(+3V) ±10% VS The AD1580 uses the “bandgap” concept to produce a stable, low temperature coefficient voltage reference suitable for high accuracy data acquisition components and systems. The device makes use of the underlying physical nature of a silicon transistor base-emitter voltage in the forward-biased operating region. All such transistors have approximately a –2 mV/°C temperature coefficient, unsuitable for use directly as a low TC reference; however, extrapolation of the temperature characteristic of any one of these devices to absolute zero (with collector current proportional to absolute temperature) reveals that its VBE will go to approximately the silicon bandgap voltage. Thus, if a voltage could be developed with an opposing temperature coefficient to sum with VBE, a zero TC reference would result. The AD1580 circuit in Figure 6, provides such a compensating voltage, V1 by driving two transistors at different current densities and amplifying the resultant VBE difference (∆VBE— which has a positive TC). The sum of VBE and V1 provide a stable voltage reference. V– Figure 6. Schematic Diagram APPLYING THE AD1580 The AD1580 is simple to use in virtually all applications. To operate the AD1580 as a conventional shunt regulator (Figure 7a), an external series resistor is connected between the supply voltage and the AD1580. For a given supply voltage the series resistor, RS, determines the reverse current flowing through the AD1580. The value of RS must be chosen to accommodate the expected variations of the supply voltage, VS, load current, IL, and the AD1580 reverse voltage, VR, while maintaining an acceptable reverse current, IR, through the AD1580. I R + IL RS RS IL VR 2.94kΩ (1.30kΩ) VR IR VOUT VOUT (a) (b) Figure 7. Typical Connection Diagram TEMPERATURE PERFORMANCE The AD1580 is designed for reference applications where stable temperature performance is important. Extensive temperature testing and characterization ensures that the device’s performance is maintained over the specified temperature range. Some confusion exists in the area of defining and specifying reference voltage error over temperature. Historically, references have been characterized using a maximum deviation per degree centigrade, i.e., 50 ppm/°C. However, because of nonlinearities in temperature characteristics which originated in standard Zener references (such as “S” type characteristics), most manufacturers now use a maximum limit error band approach to specify devices. This technique involves the measurement of the output at three or more different temperatures to guarantee that the voltage will fall within the given error band. The proprietary curvature correction design techniques used to minimize the AD1580 nonlinearities allow the temperature performance to be guaranteed using the maximum deviation method. This method is of more use to a designer than the one which simply guarantees the maximum error band over the entire temperature change. Figure 8 shows a typical output voltage drift for the AD1580 and illustrates the methodology. The maximum slope of the two diagonals drawn from the initial output value at 25°C to the output values at 85°C and –40°C determines the performance grade of the device. For a given grade of the AD1580 the designer can easily determine the maximum total error from the initial tolerance plus temperature variation. For example, the AD1580BRT initial tolerance is ± 1 mV, a ± 50 ppm/°C temperature coefficient corresponds to an error band of ± 4 mV 1.2258 1.2256 (VMAX – VO) SLOPE = TC = ———————————–––– (85°C – 25°C) x 1.225 x 10–6 VMAX 1.2254 OUTPUT VOLTAGE – V The minimum value for RS should be chosen when VS is at its minimum, and IL and VR are at their maximum while maintaining the minimum acceptable reverse current. The value of RS should be large enough to limit IR to 10 mA when VS is at its maximum, and IL and VR are at their minimum. The equation for selecting RS is as follows: 1.2252 1.2250 VO 1.2248 1.2246 1.2244 (VMIN – VO) SLOPE = TC = ———————————––––– (–40°C – 25°C) x 1.225 x 10–6 1.2242 RS = (VS – VR )/(IR + IL ) 1.2240 Figure 7b shows a typical connection with the AD1580BRT operating at a minimum of 100 µA that can provide ± 1 mA to its load, while accommodating ± 10% power supply variations. 1.2238 –55 VMIN –35 –15 45 5 25 65 TEMPERATURE – °C 85 105 125 Figure 8. Output Voltage vs. Temperature –4– REV. 0 AD1580 (50 × 10–6 × 1.225 V × 65°C) thus, the unit is guaranteed to be 1.225 V ± 5 mV over the operating temperature range. OUTPUT IMPEDANCE VERSUS FREQUENCY Understanding the effect of the reverse dynamic output impedance in a practical application may be important to successfully apply the AD1580. A voltage divider is formed by the AD1580’s output impedance and the external source impedance. When using an external source resistor of about 30 kΩ (IR = 100 µA), 1% of the noise from a 100 kHz switching power supply is developed at the output of the AD1580. Figure 11 shows how a 1 µF load capacitor connected directly across the AD1580 reduces the affect of power supply noise to less than 0.01%. Duplication of these results requires a combination of high accuracy and stable temperature control in a test system. Evaluation of the AD1580 will produce a curve similar to that in Figures 1 and 8. VOLTAGE OUTPUT NONLINEARITY VERSUS TEMPERATURE When using a reference with data converters it is important to understand how temperature drift affects the overall converter performance. The nonlinearity of the reference output drift represents additional error that is not easily calibrated out of the system. This characteristic (Figure 9) is generated by normalizing the measured drift characteristic to the end point average drift. The residual drift error of approximately 500 ppm shows that the AD1580 is compatible with systems that require 10-bit accurate temperature performance. OUTPUT IMPEDANCE – Ω 1k RESIDUAL DRIFT ERROR – ppm 600 500 100 CL = 0 10 ∆IR = 0.1IR IR = 100µA IR = 1mA 400 0.1 10 300 200 100 1k 10k FREQUENCY – Hz 100k NOISE PERFORMANCE AND REDUCTION –35 –15 5 65 25 45 TEMPERATURE – °C 85 105 The noise generated by the AD1580 is typically less than 5 µV p-p over the 0.1 Hz to 10 Hz band. Figure 12 shows the 0.1 Hz to 10 Hz noise of a typical AD1580. Noise in a 10 Hz–10 kHz bandwidth is approximately 20 µ V rms (Figure 13a). If further noise reduction is desired, a 1-pole low-pass filter may be added between the output pin and ground. A time constant of 0.2 ms will have a –3 dB point at about 800 Hz, and will reduce the high frequency noise to about 6.5 µV rms, (Figure 13b). A time constant of 960 ms will have a –3 dB point at 165 Hz, and will reduce the high frequency noise to about 2.9 µV rms (Figure 13c). 125 Figure 9. Residual Drift Error REVERSE VOLTAGE HYSTERESIS A major requirement for high performance industrial equipment manufacturers is a consistent output voltage at nominal temperature following operation over the operating temperature range. This characteristic is generated by measuring the difference between the output voltage at +25°C after operation at +85°C, and the output, also at +25°C after operation at –40°C. Figure 10 displays the hysteresis associated with AD1580. This characteristic exists in all references and has been minimized in the AD1580. 4.5µV p-p 40 35 QUANTITY 30 25 20 15 1µV/DIV 10 5 0 –400 1s/DIV Figure 12. 0.1 Hz–10 Hz Voltage Noise –300 –200 –100 0 100 200 300 400 HYSTERESIS VOLTAGE – µV Figure 10. Reverse Voltage Hysteresis Distribution REV. 0 1M Figure 11. Output Impedance vs. Frequency 100 0 –55 CL = 1µF 1 –5– AD1580 Output turn-on time is modified when an external noise reduction filter is used. When present, the time constant of the filter will dominate overall settling. 21µV rms 40µV/DIV (a) 2.4V 6.5µV rms τ = 0.2ms 20µV/DIV VIN (b) 0V OUTPUT ERROR 1mV/DIV 2 µs/DIV 2.9µV rms τ = 960ms 10µV/DIV (c) 10ms/DIV Figure 13. Total RMS Noise OUTPUT 0.5mV/DIV 2 ms/DIV TURN-ON TIME Many low power instrument manufacturers are becoming increasingly concerned with the turn-on characteristics of components being used in their systems. Fast turn-on components often enable the end user to keep power off when not needed, and yet respond quickly when the power is turned on for operation. Figure 14a displays the turn-on characteristic of the AD1580. Upon application of power (cold start), the time required for the output voltage to reach its final value within a specified error is the turn-on settling time. Two components normally associated with this are: time for active circuits to settle and time for thermal gradients on the chip to stabilize. This characteristic is generated from cold-start operation and represents the true turn-on waveform after power up. Figure 15 shows both the coarse and fine turn-on settling characteristics of the device; the total settling time to within 1.0 mV is about 6 us, and there is no long thermal tail when the horizontal scale is expanded to 2 ms/div. Figure 15. Turn-On Settling TRANSIENT RESPONSE Many A/D and D/A converters present transient current loads to the reference, and poor reference response can degrade the converter’s performance. Figure 16 displays both the coarse and fine settling characteristics of the device to load transients of ±50 µA. 20mV/DIV 1mV/DIV IR = 100µA + 50µA STEP (a) 2.4V (b) IR = 100µA – 50µA STEP VIN 0V CL = 200pF 20mV/DIV 1mV/DIV 1µs/DIV Figure 16. Transient Settling 250mV/DIV Figure 16a shows the settling characteristics of the device for an increased reverse current of 50 µA. Figure 16b shows the response when the reverse current is decreased by 50 µA. The transients settle to 1 mV in about 3 µs. 5µs/DIV Figure 14a. Response Time RL RS = 11.5kΩ VIN VR CL VOUT Attempts to drive a large capacitive load (in excess of 1,000 pF) may result in ringing, as shown in the step response photo (Figure 17). This is due to the additional poles formed by the load capacitance and the output impedance of the reference. A recommended method of driving capacitive loads of this magnitude is shown in Figure 14b. A resistor isolates the capacitive load from the output stage, while the capacitor provides a single pole lowpass filter and lowers the output noise. Figure 14b. Turn-On, Settling, and Transient Test Circuit –6– REV. 0 AD1580 One family of ADCs that the AD1580 is well suited for is the AD7714-3 and AD7715-3. The AD7714/AD7715 are chargebalancing (sigma-delta) A/D converters with on-chip digital filtering intended for the measurement of wide dynamic range, low frequency signals such as those representing chemical, physical or biological processes. Figure 19 shows the AD1580 connected to the AD7714/AD7715 for 3 V operation. 2.0V VIN 1.8V 3V CL = 0.01µF 34.8kΩ REFIN(+) AD1580 50µs/DIV 10mV/DIV RSW 5kΩ (TYP) AD7714/15–3 HIGH IMPEDANCE >1GΩ REFIN(–) CREF (3–8pF) SWITCHING FREQUENCY DEPENDS ON FCLKIN Figure 17. Transient Response with Capacitive Load PRECISION MICROPOWER LOW DROPOUT REFERENCE Figure 19. Reference Circuit for the AD7714/AD7715–3 The circuit in Figure 18 provides an ideal solution for making a stable voltage reference with low standby power consumption, low input/output dropout capability, and minimum noise output. The amplifier both buffers and optionally scales up the AD1580 output voltage, VR. Output voltages as high as 2.1 V can supply 1 mA of load current. A one-pole filter connected between the AD1580 and the OP193 input may be used to achieve low output noise. The nominal quiescent power consumption is a mere 200 µW. The AD1580 is ideal for creating the reference level to use with 12-bit multiplying DACs such as the AD7943, AD7945, and AD7948. In the single supply bias mode (Figure 20), the impedance seen looking into the IOUT2 terminal changes with DAC code. If the AD1580 drives IOUT2 and AGND directly, less than 0.2 LSBs of additional linearity error will result. The buffer amp eliminates any linearity degradation that could result from variations in the reference level . +3.3V 3V 34.8kΩ 205Ω OP193 4.7µF VDD VOUT = +1.225V OR VOUT = +1.225 (1+R2/R3) RBF C1 IOUT1 VIN VREF DAC IOUT2 AD7943/45/48 AGND A1 VOUT AD1580 R3 R2 DGND A1: OP295 AD822 OP2283 +3.3V 41.2kΩ Figure 18. Micropower Buffered Reference A1 AD1580 USING THE AD1580 WITH 3 V DATA CONVERTERS SIGNAL GROUND The AD1580’s low output drift (50 ppm/°C) and compact subminiature SOT-23 package makes it ideally suited for today’s high performance converters in space critical applications. REV. 0 Figure 20. Single Supply System –7– AD1580 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). SOT-23 0.1200 (3.048) 0.550 (1.397) 0.1040 (2.642) 0.0470 (1.194) 0.0827 (2.101) C2081–18–10/95 0.1102 (2.799) PIN 1 0.0413 (1.049) 0.0236 (0.599) 0.0374 (0.950) 0.0177 (0.450) 0.0807 (2.050) 0.0701 (1.781) 0.0059 (0.150) 0.0034 (0.086) 0.0440 (1.118) 0.0320 (0.813) 0.0040 (0.102) 0.0005 (0.013) SEATING PLANE 0.0210 (0.533) 0.0100 (0.254) 0.0146 (0.371) 0.0050 (0.127) 0.027 (0.686) REF TAPE AND REEL DIMENSIONS Dimensions shown in millimeters. 14.4 MAX 1.8 ± 0.1 0.30 ± 0.05 4.0 ± 0.10 1.75 ± 0.10 1.5 MIN 2.0 ± 0.05 180 (7") OR 330 (13") 2.7 ± 0.1 8.0 ± 0.30 3.5 ± 0.05 +0.05 1.5 –0.00 20.2 MIN 13.0 ± 0.2 50 (7") MIN OR 100 (13") MIN 3.1 ± 0.1 1.0 MIN 8.4 0.75 MIN +1.5 –0.0 PRINTED IN U.S.A. DIRECTION OF UNREELING –8– REV. 0