High Precision Voltage Reference Microcomputer AD588

Transcription

1 High Precision Voltage Reference Microcomputer FEATURES Low drift:. ppm/ C Low initial error: mv Pin programmable output +0 V, + V, ± V tracking, V, 0 V Flexible output force and sense terminals High impedance ground sense SOIC_W-6 and CERDIP-6 packages MIL-STD--compliant versions available GENERAL DESCRIPTION The represents a major advance in state-of-the-art monolithic voltage references. Low initial error and low temperature drift give the absolute accuracy performance previously not available in monolithic form. The uses a proprietary ion-implanted, buried Zener diode and laser-wafer drift trimming of high stability thin film resistors to provide outstanding performance. The includes the basic reference cell and three additional amplifiers that provide pin programmable output ranges. The amplifiers are laser trimmed for low offset and low drift to maintain the accuracy of the reference. The amplifiers are configured to allow Kelvin connections to the load and/or boosters for driving long lines or high current loads, delivering the full accuracy of the where it is required in the application circuit. The low initial error allows the to be used as a system reference in precision measurement applications requiring -bit absolute accuracy. In such systems, the can provide a known voltage for system calibration in software. The low drift also allows compensation for the drift of other components in a system. Manual system calibration and the cost of periodic recalibration can, therefore, be eliminated. Furthermore, the mechanical instability of a trimming potentiometer and the potential for improper calibration can be eliminated by using the in conjunction with auto calibration software. The is available in seven versions. The JQ and KQ are packaged in a 6-lead CERDIP and are specified for 0 C to +70 C operation. The AQ and BQ are packaged in a 6-lead CERDIP, and the ARWZ is packaged in a 6-lead SOIC, and they are specified for the C to + C industrial temperature range. The ceramic TE and TQ grades are specified for the full military/aerospace temperature range. Protected by Patent Number 4,644,. R GAIN ADJ FUNCTIONAL BLOCK DIAGRAM NOISE REDUCTION 7 A GND SENSE +IN A R GND SENSE IN V HIGH R V LOW PRODUCT HIGHLIGHTS 0 6 A IN 4 BAL ADJ Figure. A OUT SENSE V CT IN A 4 A OUT FORCE OUT SENSE OUT FORCE 6 V S. The offers -bit absolute accuracy without any user adjustments. Optional fine-trim connections are provided for applications requiring higher precision. The fine trimming does not alter the operating conditions of the Zener or the buffer amplifiers, and so does not increase the temperature drift.. Output noise of the is very low, typically 6 μv p-p. A pin is provided for additional noise filtering using an external capacitor.. A precision ± V tracking mode with Kelvin output connections is available with no external components. Tracking error is less than mv, and a fine trim is available for applications requiring exact symmetry between the + V and V outputs. 4. Pin strapping capability allows configuration of a wide variety of outputs: ± V, + V, +0 V, V, and 0 V dual outputs or + V, V, +0 V, and 0 V single outputs Rev. J 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 that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 06, Norwood, MA , U.S.A. Tel: Fax: Analog Devices, Inc. All rights reserved.

2 TABLE OF CONTENTS Features... General Description... Functional Block Diagram... Product Highlights... Revision History... Specifications... Absolute Maximum Ratings... 4 ESD Caution... 4 Pin Configuration and Function Descriptions... Theory of Operation... 6 Applications Information... 7 Calibration... 7 Noise Performance and Reduction... Temperature Performance... 0 Kelvin Connections... Dynamic Performance... Using the with Converters... AD7 4-Bit Digital-to-Analog Converter... AD6 6-Bit Digital-to-Analog Converter... Substituting for Internal References... 6 AD74A -Bit Analog-to-Digital Converter... 6 Resistance Temperature Detector (RTD) Excitation... 6 Boosted Precision Current Source... 7 Bridge Driver Circuits... 7 Outline Dimensions... Ordering Guide... Turn-On Time... 0 REVISION HISTORY 4/0 Rev. I to Rev. J Changes to Calibration Section... /0 Rev. H to Rev. I Changes to Figure 40 and Figure /0 Rev. G to Rev. H Changes to General Description Section... 6/06 Rev. F to Rev. G Changes to Table... 7 Updated Outline Dimensions... /06 Rev. E to Rev. F Replaced Figure... Updated Outline Dimensions... /0 Rev. D to Rev. E Updated Format... Universal Added SOIC Version... Universal Changes to Pin 4 in Figures... Universal Changes to Pin and Pin 0 in Figures... Universal Changes to Specifications Section... Added Table... 4 Added Pin Configuration and Function Descriptions Section.. Added Table 4... Changes to Grade in Reference and in Figure... Updated Outline Dimensions... Changes to Ordering Guide... /0 Rev. C to Rev. D Added KQ Model and Deleted SQ and TQ Models... Universal Changes to General Description... Change to Product Highlights... Changes to Specifications... Change to Ordering Guide... Updated Outline Dimensions... 0/0 Rev. B to Rev. C Changes to General Description... Changes to Specifications... Changes to Ordering Guide... Changes to Table... Deleted Figure 0c... 7 Outline Dimensions Updated... Rev. J Page of 0

3 SPECIFICATIONS C, 0 V output, VS = ± V, unless otherwise noted. Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All minimum and maximum specifications are guaranteed, although only those shown in boldface are tested on all production units. Table. JQ/AQ BQ/KQ ARWZ Parameter Min Typ Max Min Typ Max Min Typ Max Unit OUTPUT VOLTAGE ERROR +0 V, 0 V Outputs ± + + mv + V, V Outputs ± + + mv ± V TRACKING MODE Symmetry Error ±. ±0.7 ±. mv OUTPUT VOLTAGE DRIFT 0 C to 70 C (J, K, B) ± ± ±. ± ± ppm/ C C to + C (A, B) ± ± ± ppm/ C GAIN ADJ AND BAL ADJ Trim Range ±4 ±4 ±4 mv Input Resistance kω LINE REGULATION TMIN to TMAX ±00 ±00 ±00 μv/v LOAD REGULATION TMIN to TMAX +0 V Output, 0 ma < IOUT < 0 ma ±0 ±0 ±0 μv/ma 0 V Output, 0 ma < IOUT < 0 ma ±0 ±0 ±0 μv/ma SUPPLY CURRENT TMIN to TMAX ma Power Dissipation mw OUTPUT NOISE (Any Output) 0. Hz to 0 Hz μv p-p Spectral Density, 00 Hz nv/ Hz LONG-TERM STABILITY (@ C) ppm/000 hr BUFFER AMPLIFIERS Offset Voltage μv Offset Voltage Drift μv/ C Bias Current na Open-Loop Gain db Output Current (A, ) ma Common-Mode Rejection (A, ) VCM = V p-p db Short Circuit Current ma TEMPERATURE RANGE Specified Performance J, K Grades C A, B Grades C Specifications tested using ± V configuration, unless otherwise indicated. See Figure 4 through Figure 6 for output configurations at +0 V, 0 V, + V, V and ± V. Gain and balance adjustments guaranteed capable of trimming output voltage error and symmetry error to zero. For ±0 V output, ±VS can be as low as ± V. See Table for test conditions at various voltages. Rev. J Page of 0

4 ABSOLUTE MAXIMUM RATINGS Table. Parameter Rating +VS to VS 6 V Power Dissipation ( C) 600 mw Storage Temperature Range 6 C to +0 C Lead Temperature (Soldering 0 sec) 00 C Package Thermal Resistance (θja/θjc) 0 C/ C/W Output Protection All outputs safe if shorted to ground Table. Test Conditions Voltage Conditions +0 V Output VS = V, +. V +VS + V 0 V Output V VS. V, +VS = + V ± V Output +VS = + V, VS = V +VS = +0. V, VS = 0. V Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; 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. ESD CAUTION Rev. J Page 4 of 0

5 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS A OUT FORCE A OUT SENSE A IN V S OUT FORCE OUT SENSE IN GAIN ADJ V HIGH NOISE REDUCTION V LOW 6 7 TOP VIEW (Not to Scale) 0 BAL ADJ V CT GND SENSE IN GND SENSE +IN Figure. Pin Configuration Table 4. Pin Function Descriptions Pin No. Mnemonic Description A OUT FORCE Output from Buffering Amplifier with Kelvin Force. Connect to Pin. +VS Positive Power Supply. A OUT SENSE Output from Buffering Amplifier with Kelvin Sense. Connect to Pin. 4 A IN + Input to Amplifier. Connect to VHIGH, Pin 6. GAIN ADJ Reference Gain Adjustment for Calibration. See the Calibration section. 6 VHIGH Unbuffered Reference High Output. 7 NOISE REDUCTION Noise Filtering Pin. Connect external μf capacitor to ground to reduce the output noise (see the Noise Performance and Reduction section). Can be left open. VLOW Unbuffered Reference Low Output. GND SENSE POSITIVE IN + Input to the Ground Sense Amplifier. 0 GND SENSE NEGATIVE IN Input to the Ground Sense Amplifier. VCT Center Tap Voltage used for Calibration. See the Calibration section. BAL ADJ Reference Centering Adjustment for Calibration. See the Calibration section. IN + Input to Amplifier 4. Connect to VLOW, Pin. 4 OUT SENSE Output of Buffering Amplifier 4 with Kelvin Sense. Connect to Pin. OUT FORCE Output of Buffering Amplifier 4 with Kelvin Force. Connect to Pin 4. 6 VS Negative Power Supply. Rev. J Page of 0

6 THEORY OF OPERATION The consists of a buried Zener diode reference, amplifiers used to provide pin programmable output ranges, and associated thin-film resistors, as shown in Figure. The temperature compensation circuitry provides the device with a temperature coefficient of. ppm/ C or less. Amplifier A performs several functions. A primarily acts to amplify the Zener voltage from 6. V to the required 0 V output. In addition, A provides for external adjustment of the 0 V output through Pin, GAIN ADJ. Using the bias compensation resistor between the Zener output and the noninverting input to A, a capacitor can be added at the NOISE REDUCTION pin (Pin 7) to form a low-pass filter and reduce the noise contribution of the Zener to the circuit. Two matched 0 kω nominal thin-film resistors ( and R) divide the 0 V output in half. Pin VCT (Pin ) provides access to the center of the voltage span and BAL ADJ (Pin ) can be used for fine adjustment of this division. Ground sensing for the circuit is provided by Amplifier A. The noninverting input (Pin ) senses the system ground, which is transferred to the point on the circuit where the inverting input (Pin 0) is connected. This can be Pin 6, Pin, or Pin. The output of A drives Pin to the appropriate voltage. Thus, if Pin 0 is connected to Pin, the VLOW pin is the same voltage as the system ground. Alternatively, if Pin 0 is connected to the VCT pin, it is a ground; and Pin 6 and Pin are + V and V, respectively. Amplifier A and Amplifier are internally compensated and are used to buffer the voltages at Pin 6, Pin, and Pin, as well as to provide a full Kelvin output. Thus, the has a full Kelvin capability by providing the means to sense a system ground and provide forced and sensed outputs referenced to that ground. Note that both positive and negative supplies are required for operation of the. NOISE REDUCTION 7 V HIGH 6 A IN 4 A OUT SENSE A A OUT FORCE A R 4 OUT SENSE R OUT FORCE R A 6 V S GAIN ADJ 0 GND SENSE +IN GND SENSE IN V LOW BAL ADJ V CT IN Figure. Functional Block Diagram Rev. J Page 6 of 0

7 APPLICATIONS INFORMATION The can be configured to provide +0 V and 0 V reference outputs, as shown in Figure 4 and Figure 6, respectively. It can also be used to provide + V, V, or a V tracking reference, as shown in Figure. Table details the appropriate pin connections for each output range. In each case, Pin is connected to system ground, and power is applied to Pin and Pin 6. The architecture of the provides ground sense and uncommitted output buffer amplifiers that offer the user a great deal of functional flexibility. The is specified and tested in the configurations shown in Figure 6. The user can choose to take advantage of the many other configuration options available with the. However, performance in these configurations is not guaranteed to meet the extremely stringent data sheet specifications. As indicated in Table, a + V buffered output can be provided using Amplifier in the +0 V configuration (Figure 4). A V buffered output can be provided using Amplifier A in the 0 V configuration (Figure 6). Specifications are not guaranteed for the + V or V outputs in these configurations. Performance is similar to that specified for the +0 V or 0 V outputs. As indicated in Table, unbuffered outputs are available at Pin 6, Pin, and Pin. Loading of these unbuffered outputs impairs circuit performance. Amplifier A and Amplifier can be used interchangeably. However, the is tested (and the specifications are guaranteed) with the amplifiers connected, as indicated in Figure 4 and Table. When either A or is unused, its output force and sense pins should be connected or the input tied to ground. Two outputs of the same voltage can be obtained by connecting both A and to the appropriate unbuffered output on Pin 6, Pin, or Pin. Performance in these dual-output configurations typically meets data sheet specifications. CALIBRATION Generally, the meets the requirements of a precision system without additional adjustment. Initial output voltage error of mv and output noise specs of 0 μv p-p allow for accuracies of bits to 6 bits. However, in applications where an even greater level of accuracy is required, additional calibration may be called for. Provision for trimming has been made through the use of the GAIN ADJ and BAL ADJ pins (Pin and Pin, respectively). The provides a precision 0 V span with a center tap (VCT) that is used with the buffer and ground sense amplifiers to achieve the voltage output configurations in Table. GAIN ADJ and BAL ADJ can be used in any of these configurations to trim the magnitude of the span voltage and the position of the center tap within the span. The gain adjust should be performed first. Although the trims are not interactive within the device, the gain trim moves the balance trim point as it changes the magnitude of the span. Table. Pin Connections Connect Unbuffered Output on Pins Buffered Output Buffered Output on Pins Range Pin 0 to Pin 0 V V 0 V + V +0 V Connections 0 V V 0 V + V +0 V +0 V 6 to, 4 to, 6 to 4, and to V or + V 6 to, 4 to, 6 to 4, and to 0 V 6 6 to, 4 to, to 4, and to + V 6 6 to 4 and to V to and 4 to Unbuffered outputs should not be loaded. Rev. J Page 7 of 0

8 Figure shows gain and balance trims in a + V and V tracking configuration. A 00 kω, 0-turn potentiometer is used for each trim. The potentiometer for gain trim is connected between Pin 6 (VHIGH) and Pin (VLOW) with the wiper connected to Pin (GAIN ADJ). The potentiometer is adjusted to produce exactly 0 V between Pin and Pin, the amplifier outputs. The balance potentiometer, also connected between Pin 6 and Pin with the wiper to Pin (BAL ADJ), is then adjusted to center the span from + V to V. Trimming in other configurations works in exactly the same manner. When producing +0 V and + V, GAIN ADJ is used to trim +0 V and BAL ADJ is used to trim + V. In the 0 V and V configuration, GAIN ADJ is again used to trim the magnitude of the span, 0 V, while BAL ADJ is used to trim the center tap, V. Trimming the introduces no additional errors over temperature, so precision potentiometers are not required. For single-output voltage ranges, or in cases when balance adjust is not required, Pin should be connected to Pin. If gain adjust is not required, Pin should be left floating. In single output configurations, GAIN ADJ is used to trim outputs utilizing the full span (+0 V or 0 V), while BAL ADJ is used to trim outputs using half the span (+ V or V). Input impedance on both the GAIN ADJ and BAL ADJ pins is approximately 0 kω. The GAIN ADJ trim network effectively attenuates the 0 V across the trim potentiometer by a factor of about 00 to provide a trim range of. mv to +7. mv with a resolution of approximately 0 μv/turn (0-turn potentiometer). The BAL ADJ trim network attenuates the trim voltage by a factor of about 400, providing a trim range of ±4. mv with resolution of 40 μv/turn. R 7 A A R 0 6 R 4 A 4 V S 6 0.µF 0.µF +0V +V +V SYSTEM GROUND V R 00kΩ 0T GAIN ADJUST R µf A SYSTEM GROUND 0.µF 0.µF A R 00kΩ 0T BALANCE ADJUST NOISE REDUCTION A SYSTEM GROUND R kω A V S Figure. + V and V Outputs NOISE REDUCTION 7 6 A R 0 R 4 A Figure 6. 0 V Output 4 V S 6 +V 0.µF 0.µF +V V +V SYSTEM GROUND V V 0V V 0.µF SYSTEM GROUND 0.µF V SYSTEM GROUND Figure V Output Rev. J Page of 0

9 NOISE PERFORMANCE AND REDUCTION The noise generated by the is typically less than 6 μv p-p over the 0. Hz to 0 Hz band. Noise in a MHz bandwidth is approximately 600 μv p-p. The dominant source of this noise is the buried Zener, which contributes approximately 00 nv/ Hz. In comparison, the op amp s contribution is negligible. Figure 7 shows the 0. Hz to 0 Hz noise of a typical. µv If further noise reduction is desired, an optional capacitor, CN, can be added between the NOISE REDUCTION pin and ground, as shown in Figure. This forms a low-pass filter with the 4 kω RB on the output of the Zener cell. A μf capacitor has a db point at 40 Hz and reduces the high frequency noise (to MHz) to about 00 μv p-p. Figure shows the MHz noise of a typical both with and without a μf capacitor. Figure Hz to 0 Hz Noise (0. Hz to 0 Hz BPF with Gain of 000 Applied) Note that a second capacitor is needed in order to implement the noise reduction feature when using the in the 0 V mode (Figure 6). The noise reduction capacitor is limited to 0. μf maximum in this mode. C N = mf NO C N Figure. Effect of μf Noise Reduction Capacitor on Broadband Noise Rev. J Page of 0

10 TURN-ON TIME Upon application of power (cold start), the time required for the output voltage to reach its final value within a specified error band is the turn-on settling time. Two components normally associated with this are the time for active circuits to settle and the time for thermal gradients on the chip to stabilize. Figure and Figure 0 show the turn-on characteristics of the. The settling is about 600 μs. Note the absence of any thermal tails when the horizontal scale is expanded to ms/cm in Figure 0. Output turn-on time is modified when an external noise reduction capacitor is used. When present, this capacitor presents an additional load to the internal Zener diode current source, resulting in a somewhat longer turn-on time. In the case of a μf capacitor, the initial turn-on time is approximately 60 ms (see Figure ). Note that if the noise reduction feature is used in the ± V configuration, a kω resistor between Pin 6 and Pin is required for proper startup. V S Figure. Electrical Turn-On Figure. Turn-On with CN = μf 00-0 TEMPERATURE PERFORMANCE The is designed for precision reference applications where temperature performance is critical. Extensive temperature testing ensures that the device s high level of performance is maintained over the operating temperature range. Figure 0. Extended Time Scale Turn-On Figure shows typical output temperature drift for the BQ and illustrates the test methodology. The box in Figure is bounded on the sides by the operating temperature extremes and on top and bottom by the maximum and minimum output voltages measured over the operating temperature range. The slope of the diagonal drawn from the lower left corner of the box determines the performance grade of the device. V MAX V MIN SLOPE = T.C. = (T MAX T MIN ) V 0.000V 0.00 ( C C) = 0.ppm/ C OUTPUT (Volts) V MAX 0.00 V MIN TEMPERATURE ( C) V MIN V MAX 00-0 Figure. Typical BQ Temperature Drift Rev. J Page 0 of 0

11 Each A and B grade unit is tested at C, 0 C, + C, +0 C, +70 C, and + C. This approach ensures that the variations of output voltage that occur as the temperature changes within the specified range is contained within a box whose diagonal has a slope equal to the maximum specified drift. The position of the box on the vertical scale changes from device to device as initial error and the shape of the curve vary. Maximum height of the box for the appropriate temperature range is shown in Figure. Duplication of these results requires a combination of high accuracy and stable temperature control in a test system. Evaluation of the produces a curve similar to that in Figure, but output readings may vary, depending on the test methods and equipment utilized. DEVICE MAXIMUM OUTPUT CHANGE (mv) GRADE 0 C TO +70 C C TO + C C TO + C JQ JQ JQ JQ (TYP) JQ JQ Figure. Maximum Output Change mv KELVIN CONNECTIONS Force and sense connections, also referred to as Kelvin connections, offer a convenient method of eliminating the effects of voltage drops in circuit wires. As seen in Figure 4, the load current and wire resistance produce an error (VERROR = R IL) at the load. The Kelvin connection of Figure 4 overcomes the problem by including the wire resistance within the forcing loop of the amplifier and sensing the load voltage. The amplifier corrects for any errors in the load voltage. In the circuit shown, the output of the amplifier would actually be at 0 V + VERROR, and the voltage at the load would be the desired 0 V The has three amplifiers that can be used to implement Kelvin connections. Amplifier A is dedicated to the ground force-sense function, while uncommitted Amplifier A and Amplifier are free for other force-sense chores. + 0V R I = 0 R V = 0V RI L R I L I R L LOAD I = 0 V = 0V RI L Figure 4. Advantage of Kelvin Connection V = 0V R LOAD In some single-output applications, one amplifier can be unused. In such cases, the unused amplifier should be connected as a unity-gain follower (force and sense pin tied together), and the input should be connected to ground. An unused amplifier section can be used for other circuit functions, as well. Figure through Figure show the typical performance of A and. OPEN-LOOP GAIN (db) GAIN PHASE k 0k 00k M 0M FREQUENCY (Hz) Figure. Open-Loop Frequency Response (A, ) PHASE (Degrees) 00-0 Rev. J Page of 0

12 POWER SUPPLY REJECTION (db) SUPPLY SUPPLY V S = ±V WITH V p-p SINE WAVE CMRR (db) k 0k 00k M 0M FREQUENCY (Hz) Figure 6. Power Supply Rejection vs. Frequency (A, ) k 0k 00k M 0M FREQUENCY (Hz) Figure. Common-Mode Rejection vs. Frequency (A, ) Figure 7. Unity-Gain Follower Pulse Response (Large Signal) V S = ±V V CM = V p-p + C NOISE SPECTRAL DENSITY (nv/ Hz) k 0k FREQUENCY (Hz) Figure 0. Input Noise Voltage Spectral Density Figure. Unity-Gain Follower Pulse Response (Small Signal) Rev. J Page of 0

13 DYNAMIC PERFORMANCE The output buffer amplifiers (A and ) are designed to provide the with static and dynamic load regulation superior to less complete references. Many analog-to-digital and digital-to-analog converters present transient current loads to the reference, and poor reference response can degrade the converter s performance. Figure and Figure display the characteristics of the output amplifier driving a 0 ma to 0 ma load. A OR In some applications, a varying load can be both resistive and capacitive in nature or can be connected to the by a long capacitive cable. Figure and Figure 6 display the output amplifier characteristics driving a 000 pf, 0 ma to 0 ma load. 0V A OR C L 000pF kω 0V 0V Figure. Capacitive Load Transient Response Test Circuit V L V I L V L kω 0V 0V Figure. Transient Load Test Circuit 00-0 C L = 0 C L = 000pF V L Figure 6. Output Response with Capacitive Load V L Figure. Large-Scale Transient Response Figure and Figure 4 display the output amplifier characteristics driving a ma to 0 ma load, a common situation found when the reference is shared among multiple converters or is used to provide a bipolar offset current. + 0V A OR I L V L kω 0V 0V kω Figure. Transient and Constant Load Test Circuit Figure 7 and Figure display the crosstalk between output amplifiers. The top trace shows the output of, dc-coupled and offset by 0 V, while the output of A is subjected to a 0 ma to 0 ma load current step. The transient at settles in about μs, and the load-induced offset is about 00 μv. A + kω + 0V V L 0V 0V Figure 7. Load Crosstalk Test Circuit 0V mv/cm 00mV/CM V L Figure. Load Crosstalk 00-0 V L Figure 4. Transient Response ma to0 ma Load Rev. J Page of 0

14 Attempts to drive a large capacitive load (in excess of 000 pf) can result in ringing or oscillation, as shown in the step response photo (Figure ). This is due to the additional pole formed by the load capacitance and the output impedance of the amplifier, which consumes phase margin. The recommended method of driving capacitive loads of this magnitude is shown in Figure 0. The 0 Ω resistor isolates the capacitive load from the output stage, while the 0 kω resistor provides a dc feedback path and preserves the output accuracy. The μf capacitor provides a high frequency feedback loop. The performance of this circuit is shown in Figure. V IN V IN Figure. Output Amplifier Step Response, CL = μf 00-0 Figure. Output Amplifier Step Response Using Figure 0 Compensation kΩ µf + V IN 0Ω C L µf Figure 0. Compensation for Capacitive Loads Rev. J Page 4 of 0

15 USING THE WITH CONVERTERS The is an ideal reference for a wide variety of analog-todigital and digital-to-analog converters. Several representative examples follow. AD7 4-BIT DIGITAL-TO-ANALOG CONVERTER High resolution CMOS digital-to-analog converters require a reference voltage of high precision to maintain rated accuracy. The combination of the and AD7 takes advantage of the initial accuracy, drift, and full Kelvin output capability of the, as well as the resolution, monotonicity, and accuracy of the AD7 to produce a subsystem with outstanding characteristics (see Figure ). AD6 6-BIT DIGITAL-TO-ANALOG CONVERTER Another application that fully utilizes the capabilities of the is supplying a reference for the AD6, as shown in Figure. Amplifier A senses system common and forces VCT to assume this value, producing + V and V at Pin 6 and Pin, respectively. Amplifier A and Amplifier buffer these voltages out to the appropriate reference force-sense pins of the AD6. The full Kelvin scheme eliminates the effect of the circuit traces or wires and the wire bonds of the and AD6 themselves, which would otherwise degrade system performance. N.C. V DD V REFS 6 R FS BIT DAC 4 I OUT R A A R R A +0V V REF 4 AGNDS AGNDF 6 6 V S MS INPUT REGISTER 4 DAC REGISTER LS INPUT REGISTER AD7 4 LDAC CSLSB CSMSB 0 WR Figure. /AD7 Connections 7 7 DB DB0 DGND V SS V V V H A + IN A OUT A A IN +V A 0kΩ V CT 0kΩ A IN 0 A A + IN A 4 IN V 4 A 4 OUT +V REF FORCE +V REF SENSE V REF SENSE 6 V REF FORCE V S S E G M E N T S E L E C T O R T A P S E L E C T O R AD6 7 V TO +V V L A 4 + IN GND MSBs LSBs LATCHES CS LDAC HBE LBE DB DB0 Figure. High Accuracy ± V Tracking Reference for AD6 Rev. J Page of

16 SUBSTITUTING FOR INTERNAL REFERENCES Many converters include built-in references. Unfortunately, such references are the major source of drift in these converters. By using a more stable external reference like the, drift performance can be improved dramatically. AD74A -BIT ANALOG-TO-DIGITAL CONVERTER The AD74A is specified for gain drift from 0 ppm/ C to 0 ppm/ C (depending on grade), using its on-chip reference. The reference contributes typically 7% of this drift. Using an as a reference source can improve the total drift by a factor of to 4. RESISTANCE TEMPERATURE DETECTOR (RTD) EXCITATION The RTD is a circuit element whose resistance is characterized by a positive temperature coefficient. A measurement of resistance indicates the measured temperature. Unfortunately, the resistance of the wires leading to the RTD often adds error to this measurement. The 4-wire ohms measurement overcomes this problem. This method uses two wires to bring an excitation current to the RTD and two additional wires to tap off the resulting RTD voltage. If these additional two wires go to a high input impedance measurement circuit, the effect of their resistance is negligible. They therefore transmit the true RTD voltage. Using this combination can result in apparent increases in full-scale error due to the difference between the on-board reference, by which the device is laser-trimmed, and the external reference, with which the device is actually applied. The on-board reference is specified to be 0 V ± 00 mv, while the external reference is specified to be 0 V ± mv. This may result in up to 0 mv of apparent full-scale error beyond the ± mv specified AD74 gain error. External Resistor and Resistor R allow this error to be nulled. Their contribution to full-scale drift is negligible. The high output drive capability allows the to drive up to six converters in a multiconverter system. All converters have gain errors that track to better than ± ppm/ C. I EXC I = 0 R R + RTD α R RTD R R I = 0 Figure 4. 4-Wire Ohms Measurement STS 4 CS AO HIGH BITS A R R A 4 R 0Ω R 00Ω 0 TURN 6.Ω V IN 0V R/C 6 CE AD74A 0 REF IN REF OUT BIPP OFF 0V IN 4 0V IN MIDDLE BITS LOW BITS +V +V V R A 6 V S ANA COM DIG COM Figure. /AD74A Connections Rev. J Page 6 of 0

17 A practical consideration when using the 4-wire ohms technique with an RTD is the self-heating effect that the excitation current has on the temperature of the RTD. The designer must choose the smallest practical excitation current that still gives the desired resolution. RTD manufacturers usually specify the self-heating effect of each of their models or types of RTDs. Figure 6 shows an providing the precision excitation current for a 00 Ω RTD. The small excitation current of ma dissipates a mere 0. mw of power in the RTD. BOOSTED PRECISION CURRENT SOURCE In the RTD current-source application, the load current is limited to ±0 ma by the output drive capability of Amplifier A. In the event that more drive current is needed, a series-pass transistor can be inserted inside the feedback loop to provide higher current. Accuracy and drift performance are unaffected by the pass transistor. BRIDGE DRIVER CIRCUITS The Wheatstone bridge is a common transducer. In its simplest form, a bridge consists of four two-terminal elements connected to form a quadrilateral, a source of excitation connected along one of the diagonals and a detector comprising the other diagonal. Figure shows a simple bridge driven from a unipolar excitation supply. EO, a differential voltage, is proportional to the deviation of the element from the initial bridge values. Unfortunately, this bridge output voltage is riding on a common-mode voltage equal to approximately VIN/. Further processing of this signal may necessarily be limited to high common-mode rejection techniques, such as instrumentation or isolation amplifiers. Figure shows the same bridge transducer, this time driven from a pair of bipolar supplies. This configuration ideally eliminates the common-mode voltage and relaxes the restrictions on any processing elements that follow A R C VISHAY S0C OR SIMILAR A 0Ω V CC Q R A A R R V S R C = 0kΩ.0mA 0.0% 00Ω V OR GROUND + R A A 0 R R 4 6 V S I L = 0V R C LIMITED BY Q AND R C POWER DISSIPATION RTD = Ω K4 0.4 C/mW SELF-HEATING LOAD Figure 6. Precision Current Source for RTD Figure 7. Boosted Precision Current Source + V IN R + E O Figure. Bridge Transducer Excitation Unipolar Drive R V R + E O + R V Figure. Bridge Transducer Excitation Bipolar Drive 00-0 Rev. J Page 7 of 0

18 As shown in Figure 40, the is an excellent choice for the control element in a bipolar bridge driver scheme. Transistor Q and Transistor Q serve as series-pass elements to boost the current drive capability to the ma required by a typical 0 Ω bridge. A differential gain stage can still be required if the bridge balance is not perfect. Such gain stages can be expensive. Additional common-mode voltage reduction is realized by using the circuit illustrated in Figure 4. A, the ground sense amplifier, serves the supplies on the bridge to maintain a virtual ground at one center tap. The voltage that appears on the opposite center tap is now single-ended (referenced to ground) and can be amplified by a less expensive circuit. R 7 A A R 6 4 R A 4 6 V S 0Ω 0Ω +V + E O V Q = N04 Q = N Figure 40. Bipolar Bridge Drive +V 0Ω Q = N A R R A 4 0Ω Q = N06 AD OP-07 R + R A 6 V S V 0 Figure 4. Floating Bipolar Bridge Drive with Minimum CMV Rev. J Page of 0

19 OUTLINE DIMENSIONS 0.0 (0.44) 0.0 (0.76) (0.) 7.40 (0.) 0.6 (0.4) 0.00 (0.7) 0.0 (0.0) 0.0 (0.00) COPLANARITY.7 (0.000) BSC.6 (0.04). (0.0) (0.00) SEATING PLANE 0. (0.00) 0. (0.0) 0.0 (0.007) (0.0) 0. (0.00) 4.7 (0.000) 0.40 (0.07) COMPLIANT TO JEDEC STANDARDS MS-0-AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure 4. 6-Lead Standard Small Outline Package [SOIC_W] Wide Body (RW-6) Dimensions shown in millimeters and (inches) 0707-B 0.00 (0.) MIN 0.0 (.4) MAX (7.7) 0.0 (.) PIN 0.00 (.4) BSC 0.40 (.4) MAX 0.00 (.0) MAX 0.00 (.0) 0. (.) 0.0 (0.) 0.04 (0.6) (.7) 0.00 (0.76) (.) 0.0 (0.) 0.0 (.) MIN SEATING PLANE 0.0 (.) 0.0 (7.7) (0.) 0.00 (0.0) CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure 4. 6-Lead Ceramic Dual In-Line Package [CERDIP] (Q-6) Dimensions shown in inches and (millimeters) ORDERING GUIDE Temperature Coefficient Temperature Range ( C) Package Option Model, Initial Error (mv) Package Description ARWZ ppm/ C to + 6-Lead Standard Small Outline Package [SOIC-W] RW-6 AQ ppm/ C to + 6-Lead Ceramic Dual In-Line Package [CERDIP] Q-6 BQ. ppm/ C to + 6-Lead Ceramic Dual In-Line Package [CERDIP] Q-6 JQ ppm/ C 0 to 70 6-Lead Ceramic Dual In-Line Package [CERDIP] Q-6 KQ. ppm/ C 0 to 70 6-Lead Ceramic Dual In-Line Package [CERDIP] Q-6 For details on grade and package offerings screened in accordance with MIL-STD-, refer to the Analog Devices Military Products Databook or current /B data sheet. Z = RoHS Compliant Part. Temperature coefficient specified from 0 C to 70 C. Rev. J Page of 0

20 NOTES 00 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D00-0-4/0(J) Rev. J Page 0 of 0