Application Note 1944: 2003 Temperature Monitoring Using MAX1253/

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Application Note 1944: 2003 Temperature Monitoring Using MAX1253/

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CONVERSION/SAMPLING CIRCUITS MEASUREMENT CIRCUITS TEMPERATURE SENSORS
Application Note 1944: 2003
Temperature Monitoring Using MAX1253/54 MAX1153/54 MAX1253/54 MAX1153/54 system monitors cost solution monitoring multiple temperatures system. This application note explains measure temperature using remote sensing diodes single-ended differential configuration possible error sources.
Remote temperature monitoring traditionally been accomplished with thermistors, thermocouples temperature sensor ICs. system requires temperature measurements multiple locations, costs quickly MAX1253/1254 MAX1153/1154 provide low-cost solution monitor multiple remote temperatures well single local temperature. addition, these devices offer autonomous alarming functions without interaction required host controller, generating interrupt when user programmable upper lower threshold exceeded, resulting fully stand-alone temperature monitoring. These devices monitor both arbitrary voltages temperature, however this article will focus specifically temperature monitoring therefore intended supplement MAX1253/54 MAX1153/54 data sheet, which covers capabilities devices. Please device data sheet alarming functions, device programming, specifications, etc. Basic Diode Temperature Measurements forward voltage diode diode-connected transistor (VBE) when operating constant current exhibits negative temperature co-efficient approximately 2mV/°C which used measure temperature remote diode. However, vary between diode manufacturers even from device-to-device making absolute temperature measurements unpractical volume production since calibration required each sensor. alternate technique, which compensates variation uses measurements from known currents calculate temperature. This measurement depends ratio, rather than absolute, inherently more accurate. following equation used MAX1253/1254 MAX1153/1154. Temperature (VHIGH VLOW) q/(n ln(IHIGH/ILOW) (°K) Where: VHIGH sensor diode voltage with high current flowing (IHIGH) VLOW sensor diode voltage with current flowing (ILOW) charge electron 1.602. 10-19 Coulombs
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Boltzman's constant 1.38. 10-23 ideality factor make temperature measurements using this method, device performs measurements using separate constant current sources. ratio between IHIGH ILOW trimmed factory. device then calculates temperature using above equation writes result output register Refer Maxim application note: "Compensating Ideality Factor Series Resistance Differences between Thermal Sense Diodes", additional information ideality factor. Using MAX1253/1254 MAX1153/54 temperature measurement process MAX1253/MAX1254 MAX1153/54 fully automated. Switching current sources measurements internally sequenced require user intervention. resulting 12-bit (MAX1253/1254) 10-bit (MAX1153/1153), complement result represents sensor temperature degrees Celsius, with 1LSB 0.125°C (12-bit) 1LSB 0.5°C (10-bit). MAX1253/MAX1254 MAX1153/MAX1154 support both differential temperature measurements combination each. design decision between types measurement depends desired level accuracy type number temperature sensors.
Figure Connecting differential single-ended temperature sensors
Single-ended mode require fewer number connections channel resulting simpler implementation with eight monitored temperatures each device. Implementation easy, just connect anode diode-connected transistor input channel cathode ground. Differential-mode measurement allows higher accuracy providing common-mode rejection, only allows four individual temperatures monitored each device. Connect anode diode-connected transistor even input channel cathode input channel input pair configured differential temperature measurement.
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Figure shows diode connection. Please refer Figures typical temperature error single-ended differential sensor.
Figure Typical A-grade temperature error
Figure Typical B-grade temperature error
Sources Error small signals associated with temperature measurements, proper layout techniques important minimize noise coupled into analog inputs. single-ended measurements, choose ground connections sensors away from high current return paths avoid introduction errors caused voltage drops board/system ground, main drawback single ended measurements. Practical options better
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accuracy performance star configured subsystem ground, signal ground plane. Isolate anode sensor connection trace away from board system noise sources; shield with ground lines ground planes (when available) prevent accuracy degradation temperature measurements caused magnetic/electric noise induction. differential measurements, sensor connection lines parallel each other with minimum spacing. This will improve temperature measurement accuracy minimizing differential noise between lines, since they will have equal exposure most sources noise. further improved noise rejection, shield sensor connections running them between ground planes, when available. should minimize capacitance between AIN(+) AIN(-) terminals much possible. Figure shows effect this capacitance analog inputs when making differential temperature measurement. connection option RG-6 cable, which capacitance 50-75pF/M. This means that device tolerate meters this type cable without significant impact performance.
Figure Effect external sensor capacitance
help further reduce noise, MAX1253/54 MAX1153/54 have on-chip averaging function that enabled. programmed average result from averaging) 2048 times. effect averaging reduces noise approximately factor 1/sqrt(N). Selecting Remote Temperature Sensor Temperature sensing accuracy depends having good-quality, diode-connected small-signal transistor sensor. Accuracy been experimentally verified 2N3904-type devices. transistor must small-signal type with base resistance. Tight specifications forward current gain (+50 +150, example) indicate that manufacturer good process
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controls that devices have consistent characteristics. on-board sensors other ICs' on-board temperature-sensing devices also used. Table Remote Sensor Transistor Manufacturers Manufacturer Central Semiconductor (USA) Fairchild Semiconductor (USA) Motorola (USA) Rohm Semiconductor (Japan) Siemans (Germany) Zetex (England) Model Number CMPT3904 MMBT3904 MMBT3904 SST3904 SMB3904 FMMT3904CTND
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Application Note 1944: 2003 Temperature Monitoring Using MAX1253/

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