SENSOR DESIGN, SIGNAL CONDITIONING, AND INTERFACING PROJECT MAE 534 Mechatronics Design SPRING 1999 Dr. Ramasubramanian DUE: FEBRUARY 24, 1999 WEDNESDAY AT CLASS TIME. PROJECT DESCRIPTION: Design a Beam-based load cell that will have a maximum capacity of 2. Pounds. The complete system consists of the beam type load cell, signal conditioning electronics, and microprocessor interface for readout. You will use the Handyboard for the project. Your group will be provided with a 3/4 wide, 1/8 thick Aluminum bar, 12 inches long, four strain gages (CEA-13-24UZ-12) from Measurements Group, Inc., chemicals and supplies to mount the gages, a breadboard to install the system, and of course, the Handyboard and associated hardware needed to download the program, etc. Further, an instrumentation amplifier IC (INA 125 made by Burr-Brown) and a bipolar power supply will be provided. Miscellaneous resistors and components are available. The data sheet for the amplifier IC is attached. TESTING The beam that you design will be C-clamped to a rigid table, and a cup will be hung from 1-inch from the tip of the beam. Known weights up to 1.5 pounds will be dropped into it. The display should accurately show the results on the LCD. REPORTING AND DEMONSTRATION Write a complete report describing the problem, the solution approach, description of your design, experience gained, testing of your system, and all diagrams and parts list. The report is due on the day of the project demonstration. I will test the system in the laboratory at class time.
INSTRUMENTATION AMPLIFIER With Precision Voltage Reference FEATURES LOW QUIESCENT CURRENT: 46µA PRECISION VOLTAGE REFERENCE: 1.24V, 2.5V, 5V or 1V SLEEP MODE LOW OFFSET VOLTAGE: 25µV max LOW OFFSET DRIFT: 2µV/ C max LOW INPUT BIAS CURRENT: 2nA max HIGH CMR: 1dB min LOW NOISE: 38nV/ Hz at f = 1kHz INPUT PROTECTION TO ±4V WIDE SUPPLY RANGE Single Supply: 2.7V to 36V Dual Supply: ±1.35V to ±18V 16-PIN DIP AND SO-16 SOIC PACKAGES DESCRIPTION The is a low power, high accuracy instrumentation amplifier with a precision voltage reference. It provides complete bridge excitation and precision differential-input amplification on a single integrated circuit. A single external resistor sets any gain from 4 to 1,. The is laser-trimmed for low offset voltage (25µV), low offset drift (2µV/ C), and high common-mode rejection (1dB at G = 1). It operates on single (+2.7V to +36V) or dual (±1.35V to ±18V) supplies. The voltage reference is externally adjustable with pinselectable voltages of 2.5V, 5V, or 1V, allowing use with a variety of transducers. The reference voltage is accurate to ±.5% (max) with ±35ppm/ C drift (max). Sleep mode allows shutdown and duty cycle operation to save power. The is available in 16-pin plastic DIP and SO-16 surface-mount packages and is specified for the 4 C to +85 C industrial temperature range. APPLICATIONS PRESSURE AND TEMPERATURE BRIDGE AMPLIFIERS INDUSTRIAL PROCESS CONTROL FACTORY AUTOMATION MULTI-CHANNEL DATA ACQUISITION BATTERY OPERATED SYSTEMS GENERAL PURPOSE INSTRUMENTATION 1V V REF COM 12 V REF BG V REF 2.5 V REF 5 13 14 15 V REF 1 16 V REF Out V + IN R G 4 6 9 8 7 V IN V+ 1 R R 2R 4R Ref Amp 1kΩ A 1 A 2 Bandgap V REF 3kΩ 1kΩ 3kΩ V 3 SLEEP 2 1 11 Sense + V O = (V IN V IN ) G G = 4 + 6kΩ R G IA REF 5 V O International Airport Industrial Park Mailing Address: PO Box 114, Tucson, AZ 85734 Street Address: 673 S. Tucson Blvd., Tucson, AZ 8576 Tel: (52) 746-1111 Twx: 91-952-1111 Internet: http://www.burr-brown.com/ FAXLine: (8) 548-6133 (US/Canada Only) Cable: BBRCORP Telex: 66-6491 FAX: (52) 889-151 Immediate Product Info: (8) 548-6132 1997 Burr-Brown Corporation PDS-1361B Printed in U.S.A., February, 1998
SPECIFICATIONS: V S = ±15V At T A = +25 C, V S = ±15V, IA common = V, V REF common = V, and R L = 1kΩ, unless otherwise noted. P, U PA, UA PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS INPUT Offset Voltage, RTI Initial ±5 ±25 ±5 µv vs Temperature ±.25 ±2 ±5 µv/ C vs Power Supply V S = ±1.35V to ±18V, G = 4 ±3 ±2 ±5 µv/v Long-Term Stability ±.2 µv/mo Impedance, Differential 1 11 2 Ω pf Common-Mode 1 11 9 Ω pf Safe Input Voltage ±4 V Input Voltage Range See Text Common-Mode Rejection V CM = 1.7V to +1.2V G = 4 78 84 72 db G = 1 86 94 8 db G = 1 1 114 9 db G = 5 1 114 9 db BIAS CURRENT V CM = V 1 25 5 na vs Temperature ±6 pa/ C Offset Current ±.5 ±2.5 ±5 na vs Temperature ±.5 pa/ C NOISE, RTI R S = Ω Voltage Noise, f = 1Hz 4 nv/ Hz f = 1Hz 38 nv/ Hz f = 1kHz 38 nv/ Hz f =.1Hz to 1Hz.8 µvp-p Current Noise, f = 1Hz 17 fa/ Hz f = 1kHz 56 fa/ Hz f =.1Hz to 1Hz 5 pap-p GAIN Gain Equation 4 + 6kΩ/R G V/V Range of Gain 4 1, V/V Gain Error V O = 14V to +13.3V G = 4 ±.1 ±.75 ±.1 % G = 1 ±.3 ±.3 ±.5 % G = 1 ±.5 ±.5 ±1 % G = 5 ±.1 % Gain vs Temperature G = 4 ±1 ±15 ppm/ C G > 4 (1) ±25 ±1 ppm/ C Nonlinearity V O = 14V to +13.3V G = 4 ±.4 ±.2 ±.4 % of FS G = 1 ±.4 ±.2 ±.4 % of FS G = 1 ±.1 ±.1 % of FS G = 5 ±.2 % of FS OUTPUT Voltage: Positive (V+) 1.7 (V+).9 V Negative (V )+1 (V )+.4 V Load Capacitance Stability 1 pf Short-Circuit Current 9/+12 ma VOLTAGE REFERENCE V REF = +2.5V, +5V, +1V Accuracy I L = ±.15 ±.5 ±1 % vs Temperature I L = ±18 ±35 ±1 ppm/ C vs Power Supply, V+ V+ = (V REF + 1.25V) to +36V ±2 ±5 ±1 ppm/v vs Load I L = to 5mA 3 75 ppm/ma Dropout Voltage, (V+) V (2) REF Ref Load = 2kΩ 1.25 1 V Bandgap Voltage Reference 1.24 V Accuracy I L = ±.5 % vs Temperature I L = ±18 ppm/ C The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 2
SPECIFICATIONS: V S = ±15V (CONT) At T A = +25 C, V S = ±15V, IA common = V, V REF common = V, and R L = 1kΩ, unless otherwise noted. P, U PA, UA PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS FREQUENCY RESPONSE Bandwidth, 3dB G = 4 15 khz G = 1 45 khz G = 1 4.5 khz G = 5.9 khz Slew Rate G = 4, 1V Step.2 V/µs Settling Time,.1% G = 4, 1V Step 6 µs G = 1, 1V Step 83 µs G = 1, 1V Step 375 µs G = 5, 1V Step 17 µs Overload Recovery 5% Overdrive 5 µs POWER SUPPLY Specified Operating Voltage ±15 V Specified Voltage Range ±1.35 ±18 V Quiescent Current, Positive I O = I REF = ma 46 525 µa Negative I O = I REF = ma 28 325 µa Reference Ground Current (3) 18 µa Sleep Current (V SLEEP 1mV) R L = 1kΩ, Ref Load = 2kΩ ±1 ±25 µa SLEEP MODE PIN (4) V IH (Logic high input voltage) +2.7 V+ V V IL (Logic low input voltage) +.1 V I IH (Logic high input current) 15 µa I IL (Logic low input current) µa Wake-up Time (5) 15 µs TEMPERATURE RANGE Specification Range 4 +85 C Operation Range 55 +125 C Storage Range 55 +125 C Thermal Resistance, θ JA 16-Pin DIP 8 C/W SO-16 Surface-Mount 1 C/W Specification same as P, U. NOTES: (1) Temperature coefficient of the "Internal Resistor" in the gain equation. Does not include TCR of gain-setting resistor, R G. (2) Dropout voltage is the positive supply voltage minus the reference voltage that produces a 1% decrease in reference voltage. (3) V REF COM pin. (4) Voltage measured with respect to Reference Common. Logic low input selects Sleep mode. (5) IA and Reference, see Typical Performance Curves. SPECIFICATIONS: V S = +5V At T A = +25 C, V S = +5V, IA common at V S /2, V REF common = V S /2, V CM = V S /2, and R L = 1kΩ to V S /2, unless otherwise noted. P, U PA, UA PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS INPUT Offset Voltage, RTI Initial ±75 ±5 ±75 µv vs Temperature ±.25 µv/ C vs Power Supply V S = +2.7V to +36V 3 2 5 µv/v Input Voltage Range See Text Common-Mode Rejection V CM = +1.1V to +3.6V G = 4 78 84 72 db G = 1 86 94 8 db G = 1 1 114 9 db G = 5 1 114 9 db GAIN Gain Error V O = +.3V to +3.8V G = 4 ±.1 % OUTPUT Voltage, Positive (V+) 1.2 (V+).8 V Negative (V )+.3 (V )+.15 V POWER SUPPLY Specified Operating Voltage +5 V Operating Voltage Range +2.7 +36 V Quiescent Current I O = I REF = ma 46 525 µa Sleep Current (V SLEEP 1mV) R L = 1kΩ, Ref Load = 2kΩ ±1 ±25 µa Specification same as P, U. 3
PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS (1) Top View 16-Pin DIP, SO-16 Power Supply Voltage, V+ to V... 36V Input Signal Voltage... ±4V Output Short Circuit... Continuous V+ SLEEP V 1 2 3 16 15 14 V REF 1 V REF 5 V REF 2.5 Operating Temperature... 55 C to +125 C Storage Temperature... 55 C to +125 C Lead Temperature (soldering, 1s)... +3 C NOTE: Stresses above these ratings may cause permanent damage. V REF OUT IA REF V+ IN V IN R G 4 5 6 7 8 13 12 11 1 9 V REF BG V REF COM Sense ELECTROSTATIC DISCHARGE SENSITIVITY This integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. V O R G PACKAGE INFORMATION PACKAGE DRAWING PRODUCT PACKAGE NUMBER (1) PA 16-Pin Plastic DIP 18 P 16-Pin Plastic DIP 18 UA SO-16 Surface-Mount 265 U SO-16 Surface-Mount 265 NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. 4
TYPICAL PERFORMANCE CURVES At T A = +25 C and V S = ±15V, unless otherwise noted. 6 G = 5 GAIN vs FREQUENCY 12 COMMON-MODE REJECTION vs FREQUENCY G = 1, 5 Gain (db) 5 4 3 2 1 G = 1 G = 1 G = 4 Common-Mode Rejection (db) 1 8 6 4 2 G = 1 G = 4 G = 5 G = 1 1 1 1 1k 1k 1k 1M Frequency (Hz) 1 1 1 1k 1k 1k 1M Frequency (Hz) 14 POSITIVE POWER SUPPLY REJECTION vs FREQUENCY 12 NEGATIVE POWER SUPPLY REJECTION vs FREQUENCY Power Supply Rejection (db) 12 1 8 6 4 G = 4 G = 5 G = 1 G = 1 Power Supply Rejection (db) 1 8 6 4 2 G = 1 G = 5 G = 1 2 1 1 1 1k 1k 1k 1M Frequency (Hz) G = 4 1 1 1 1k 1k 1k 1M Frequency (Hz) Input Common-Mode Voltage (V) 15 1 5 5 1 INPUT COMMON-MODE VOLTAGE vs OUTPUT VOLTAGE, V S = ±15V Limited by A 2 output swing see text +15V + V D/2 + V O V D/2 IAREF + V CM 15V Limited by A 2 output swing see text 15 15 1 5 5 1 15 Output Voltage (V) Input Common-Mode Voltage (V) 5 4 3 2 1 1 2 3 4 5 IA REF = V INPUT COMMON-MODE VOLTAGE vs OUTPUT VOLTAGE, V S = ±5V Limited by A 2 output swing see text V S = ±5V Limited by A 2 output swing see text 5 4 3 2 1 1 2 3 4 5 Output Voltage (V) V S = +5V 5
TYPICAL PERFORMANCE CURVES (CONT) At T A = +25 C and V S = ±15V, unless otherwise noted. Input-Referred Voltage Noise (nv/ Hz) 1k 1 1 1 INPUT-REFERRED VOLTAGE AND CURRENT NOISE vs FREQUENCY Voltage Noise Current Noise 1 1 1 1k 1k Frequency (Hz) 1k 1 1k 1 1 Input Bias Current Noise (fa/ Hz) Settling Time (µs) 1k 1k 1 1 SETTLING TIME vs GAIN 1 1 1 1k Gain (V/V).1%.1% Offset Voltage Change (µv) 1 8 6 4 2 2 4 6 8 1 INPUT-REFERRED OFFSET VOLTAGE vs SLEEP TURN-ON TIME G = 1 5 1 15 2 25 Time From Turn-On (µs) Quiescent and Sleep Current (µa) QUIESCENT CURRENT AND SLEEP CURRENT vs TEMPERATURE 55 5 45 4 +I Q 35 3 25 2 I Q ±I SLEEP 15 V SLEEP = 1mV +I SLEEP 1 V SLEEP = V 5 I SLEEP 5 75 5 25 25 5 75 1 125 Temperature ( C).3 SLEW RATE vs TEMPERATURE 16 INPUT BIAS AND OFFSET CURRENT vs TEMPERATURE Slew Rate (V/µs).25.2.15.1.5 Input Bias and Offset Current (na) 14 12 1 8 6 4 2 I B I OS 75 5 25 25 5 75 1 125 Temperature ( C) 75 5 25 25 5 75 1 125 Temperature ( C) 6
TYPICAL PERFORMANCE CURVES (CONT) At T A = +25 C and V S = ±15V, unless otherwise noted. SMALL-SIGNAL RESPONSE LARGE-SIGNAL RESPONSE G = 4 G = 4 2mV/div 5V/div G = 1 G = 1 1µs/div 1µs/div INPUT-REFERRED NOISE,.1Hz to 1Hz 2 INPUT BIAS CURRENT vs INPUT OVERLOAD VOLTAGE 2nV/div Input Bias Current (µa) 16 12 8 4 4 8 12 All Gains 16 1µs/div 2 4 4 Overload Voltage (V) Output Voltage (V) V+ (V+) 1 (V+) 2 (V+) 3 (V+) 4 (V+) 5 (V )+5 (V )+4 (V )+3 OUTPUT VOLTAGE SWING vs OUTPUT CURRENT +125 C +75 C 55 C +75 C +25 C 55 C (V )+2 +125 C (V )+1 +25 C V ±2 ±4 ±6 ±8 ±1 Output Current (ma) Delta V OS, RTI (µv) DELTA V OS vs REFERENCE CURRENT 25 2 Sinking 15 1 5 Sourcing 5 8 6 4 2 2 4 6 8 Reference Current (ma) 7
TYPICAL PERFORMANCE CURVES (CONT) At T A = +25 C and V S = ±15V, unless otherwise noted. 3 INPUT-REFERRED OFFSET VOLTAGE PRODUCTION DISTRIBUTION, V S = ±15V 35 INPUT-REFERRED OFFSET VOLTAGE PRODUCTION DISTRIBUTION, V S = +5V Percent of Amplifiers (%) 25 2 15 1 5 Typical production distribution of packaged units..1%.2%.2%.1% Percent of Amplifiers (%) 3 25 2 15 1 5 Typical production distribution of packaged units..1%.2%.1%.5% 5 45 4 35 3 25 2 15 1 5 5 1 15 2 25 3 35 4 45 5 75 675 6 525 45 375 3 225 15 75 75 15 225 3 375 45 525 6 675 75 Input-Referred Offset Voltage (µv) Input-Referred Offset Voltage (µv) Percent of Amplifiers (%) 9 8 7 6 5 4 3 2 1 INPUT-REFERRED OFFSET VOLTAGE DRIFT PRODUCTION DISTRIBUTION Typical production distribution of packaged units. V S = ±15V or +5V Percent of Amplifiers (%) 1 9 8 7 6 5 4 3 2 1 VOLTAGE REFERENCE DRIFT PRODUCTION DISTRIBUTION Typical production distribution of packaged units..3%.2%.5% ±.25 ±.5 ±.75 ±1. ±1.25 ±1.5 ±1.75 ±2. ±2.25 ±2.5 ±2.75 ±3. ±3.25 ±3.5 ±3.75 ±4. 1 2 3 4 5 6 7 8 Voltage Reference Drift (ppm/ C) 9 1 Input-Referred Offset Voltage Drift (µv/ C) Reference Error (%) 15 12 9 6 4 3 6 9 12 15 REFERENCE TURN-ON SETTLING TIME V REF = 1V V REF = 5V V REF = 2.5V 1 2 3 4 5 Time From Power Supply Turn-On (µs) Reference Voltage Deviation (ppm) 5 5 1 15 REFERENCE VOLTAGE DEVIATION vs TEMPERATURE V REF = V BG, 2.5V, 5V, or 1V 2 75 5 25 25 5 75 1 125 Temperature ( C) 8
TYPICAL PERFORMANCE CURVES (CONT) At T A = +25 C and V S = ±15V, unless otherwise noted..1hz to 1Hz REFERENCE NOISE V REF = 2.5V, C L = 1pF REFERENCE TRANSIENT RESPONSE V REF = 2.5V, C L = 1pF 1mA/div +1mA ma 2µV/div 1mA 5mV/div Reference Output 1µs/div 1µs/div Positive AC Line Rejection (db) 12 1 8 6 4 2 POSITIVE REFERENCE AC LINE REJECTION vs FREQUENCY V REF = 2.5V V REF = 5V V REF = 1V Capacitor connected between V REF OUT and V REF COM. C =.1µF C =.1µF Negative AC Line Rejection (db) 12 1 8 6 4 2 NEGATIVE REFERENCE AC LINE REJECTION vs FREQUENCY V REF = 2.5V V REF = 1V V REF = 5V 1 1 1 1k 1k 1k 1M Frequency (Hz) 1 1 1 1k 1k 1k 1M Frequency (Hz) 9
APPLICATION INFORMATION Figure 1 shows the basic connections required for operation of the. Applications with noisy or high impedance power supplies may require decoupling capacitors close to the device pins as shown. The output is referred to the instrumentation amplifier reference (IA REF ) terminal which is normally grounded. This must be a low impedance connection to assure good common-mode rejection. A resistance of 12Ω in series with the IA REF pin will cause a typical device to degrade to approximately 8dB CMR (G = 4). Connecting V REF OUT (pin 4) to one of the four available reference voltage pins (V REF BG, V REF 2.5, V REF 5, or V REF 1) provides an accurate voltage source for bridge applications. For example, in Figure 1 V REF OUT is connected to V REF 1 thus supplying 1V to the bridge. It is recommended that V REF OUT be connected to one of the reference voltage pins even when the reference is not being utilized to avoid saturating the reference amplifier. Driving the SLEEP pin LOW puts the in a shutdown mode. SETTING THE GAIN Gain of the is set by connecting a single external resistor, R G, between pins 8 and 9: (1) G =4+ 6kΩ R G Commonly used gains and R G resistor values are shown in Figure 1. V+.1µF SLEEP (1) DESIRED GAIN R G NEAREST 1% (V/V) (Ω) R G VALUE (Ω) 4 NC NC 5 6k 6.4k 1 1k 1k 2 375 374 5 134 13 1 625 619 2 36 39 5 121 121 1 6 6.4 2 3 3.1 1 6 6.4 NC: No Connection. V REF COM V REF BG V REF 2.5 V REF 5 V REF 1 12 13 14 15 16 1 R (2) R 2R 4R 2 1V V REF Out 4 Ref Amp Bandgap V REF V O = (V + IN V IN ) G G = 4 + 6kΩ V + IN R G 6 A 1 1 9 3kΩ 11 R G 1kΩ Sense + 1kΩ 8 Load V O V IN 7 A 2 3kΩ IA REF 5 NOTE: (1) SLEEP pin should be connected to V+ if shutdown function is not being used. (2) Nominal value of R is 21kΩ, ±25%. 3.1µF V FIGURE 1. Basic Connections. 1
The 6kΩ term in equation 1 comes from the internal metal film resistors which are laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these resistors are included in the gain accuracy and drift specifications of the. The stability and temperature drift of the external gain setting resistor, R G, also affects gain. R G s contribution to gain accuracy and drift can be directly inferred from the gain equation (1). Low resistor values required for high gain can make wiring resistance important. Sockets add to the wiring resistance, which will contribute additional gain error (possibly an unstable gain error) in gains of approximately 1 or greater. OFFSET TRIMMING The is laser trimmed for low offset voltage and offset voltage drift. Most applications require no external offset adjustment. Figure 2 shows an optional circuit for trimming the output offset voltage. The voltage applied to the IA REF terminal is added to the output signal. The op amp buffer is used to provide low impedance at the IA REF terminal to preserve good common-mode rejection. V IN V IN + R G IA REF V O OPA237 ±1mV Adjustment Range V+ 1kΩ 1µA 1/2 REF2 1Ω 1Ω INPUT COMMON-MODE RANGE The input common-mode range of the is shown in the typical performance curves. The common-mode range is limited on the negative side by the output voltage swing of A 2, an internal circuit node that cannot be measured on an external pin. The output voltage of A2 can be expressed as: V 2 = 1.3V IN (V + IN V IN ) (1kΩ/R G ) (voltages referred to IA REF terminal, pin 5) The internal op amp A 2 is identical to A 1. Its output swing is limited to approximately.8v from the positive supply and.25v from the negative supply. When the input common-mode range is exceeded (A 2 s output is saturated), A 1 can still be in linear operation, responding to changes in the non-inverting input voltage. The output voltage, however, will be invalid. PRECISION VOLTAGE REFERENCE The on-board precision voltage reference provides an accurate voltage source for bridge and other transducer applications or ratiometric conversion with analog-to-digital converters. A reference output of 2.5V, 5V or 1V is available by connecting V REF OUT (pin 4) to one of the V REF pins (V REF 2.5, V REF 5, or V REF 1). Reference voltages are lasertrimmed for low inital error and low temperature drift. Connecting V REF OUT to V REF BG (pin 13) produces the bandgap reference voltage (1.24V ±.5%) at the reference output. Positive supply voltage must be 1.25V above the desired reference voltage. For example, with V+ = 2.7V, only the 1.24V reference (V REF BG) can be used. If using dual supplies V REF COM can be connected to V, increasing the V 1µA 1/2 REF2 Microphone, Hydrophone etc. FIGURE 2. Optional Trimming of Output Offset Voltage. 47kΩ 47kΩ INPUT BIAS CURRENT RETURN The input impedance of the is extremely high approximately 1 11 Ω. However, a path must be provided for the input bias current of both inputs. This input bias current flows out of the device and is approximately 1nA. High input impedance means that this input bias current changes very little with varying input voltage. Input circuitry must provide a path for this input bias current for proper operation. Figure 3 shows various provisions for an input bias current path. Without a bias current path, the inputs will float to a potential which exceeds the commonmode range, and the input amplifiers will saturate. If the differential source resistance is low, the bias current return path can be connected to one input (see the thermocouple example in Figure 3). With higher source impedance, using two equal resistors provides a balanced input with possible advantages of lower input offset voltage due to bias current and better high frequency common-mode rejection. 11 Thermocouple 1kΩ Center-tap provides bias current return. FIGURE 3. Providing an Input Common-Mode Current Path.
amount of supply voltage headroom available to the reference. Approximately 18µA flows out of the V REF COM terminal, therefore, it is recommended that it be connected through a low impedance path to sensor common to avoid possible ground loop problems. Reference noise is proportional to the reference voltage selected. With V REF = 2.5V,.1Hz to 1Hz peak-to-peak noise is approximately 9µVp-p. Noise increases to 36µVp-p for the 1V reference. Output drive capability of the voltage reference is improved by connecting a transistor as shown in Figure 4. The external transistor also serves to remove power from the. Internal resistors that set the voltage reference output are ratio-trimmed for accurate output voltages (±.5% max). The absolute resistance values, however, may vary ±25%. Adjustment of the reference output voltage with an external resistor is not recommended because the required resistor value is uncertain. A transition region exists when V SLEEP is between 4mV and 2.7V (with respect to V REF COM) where the output is unpredictable. Operation in this region is not recommended. The achieves high accuracy quickly following wakeup (V SLEEP 2.7V). See the typical performance curve Input-Referred Offset Voltage vs Sleep Turn-on Time. If shutdown is not being used, connect the SLEEP pin to V+. LOW VOLTAGE OPERATION The can be operated on power supplies as low as ±1.35V. Performance remains excellent with power supplies ranging from ±1.35V to ±18V. Most parameters vary only slightly throughout this supply voltage range see typical performance curves. Operation at very low supply voltage requires careful attention to ensure that the common-mode voltage remains within its linear range. See Input Common-Mode Voltage Range. As previously mentioned, when using the on-board reference with low supply voltages, it may be necessary to connect V REF COM to V to ensure V S V REF 1.25V. TIP29C 1V V+ V REF COM V REF BG V REF 2.5 V REF 5 V REF 1 V REF Out to load (transducer) 12 13 14 15 16 4 Ref Amp Bandgap V REF SINGLE SUPPLY OPERATION The can be used on single power supplies of +2.7V to +36V. Figure 5 shows a basic single supply circuit. The IA REF, V REF COM, and V terminals are connected to ground. Zero differential input voltage will demand an output voltage of V (ground). When the load is referred to ground as shown, actual output voltage swing is limited to approximately 15mV above ground. The typical performance curve Output Voltage Swing vs Output Current shows how the output swing varies with output current. With single supply operation, careful attention should be paid to input common-mode range, output voltage swing of both op amps, and the voltage applied to the IA REF terminal. V IN+ and V IN must both be 1V above ground for linear operation. You cannot, for instance, connect the inverting input to ground and measure a voltage connected to the noninverting input. FIGURE 4. Reference Current Boost. +3V 1.5V V +3V SHUTDOWN The has a shutdown option. When the SLEEP pin is LOW (1mV or less), the supply current drops to approximately 1µA and output impedance becomes approximately 8kΩ. Best performance is achieved with CMOS logic. To maintain low sleep current at high temperatures, V SLEEP should be as close to V as possible. This should not be a problem if using CMOS logic unless the CMOS gate is driving other currents. Refer to the typical performance curve, Sleep Current vs Temperature. R 1Ω G V O 12 5 R 1.5V + V 3 L FIGURE 5. Single Supply Bridge Amplifier. 12
INPUT PROTECTION The inputs of the are individually protected for voltage up to ±4V. For example, a condition of 4V on one input and +4V on the other input will not cause damage. Internal circuitry on each input provides low series impedance under normal signal conditions. To provide equivalent protection, series input resistors would contribute excessive noise. If the input is overloaded, the protection circuitry limits the input current to a safe value of approximately 12µA to 19µA. The typical performance curve Input Bias Current vs Input Overload Voltage shows this input current limit behavior. The inputs are protected even if the power supplies are disconnected or turned off. +5V SLEEP 1 2 V REF COM 12 V REF BG 13 V REF 2.5 14 V REF 5 15 2.5V V REF 1 16 4 Ref Amp Bandgap V REF V IN + 6 A 1 1 9 R G 1kΩ 3kΩ 11 Sense + 1kΩ 8 Load + 6kΩ V O = +2.5V + [(V IN V IN ) (4 + )] R G V IN 7 A 2 3kΩ IA REF 5 3 2.5V (1) (Psuedoground) NOTE: (1) Psuedoground is at +2.5V above actual ground. This provides a precision reference voltage for succeeding single-supply op amp stages. FIGURE 6. Psuedoground Bridge Measurement, 5V Single Supply. 13