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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. a ADT45/adt50 one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700 world wide web site: http://www.analog.com fax: 781/326-8703 ? analog devices, inc., 1997 low voltage sot-23 temperature sensors functional block diagram +v s (2.7v to 12v) v out ADT45 adt50 package types available sot-23 3 gnd 1 2 top view (not to scale) +v s v out features low voltage operation (2.7 v to 12 v) calibrated directly in 8 c 10 mv/ 8 c scale factor 6 2 8 c accuracy over temperature (typ) 6 0.5 8 c linearity (typ) stable with large capacitive loads specified C40 8 c to +125 8 c, operation to +150 8 c less than 60 ma quiescent current low self-heating applications environmental control systems thermal protection industrial process control fire alarms power system monitors cpu thermal management general description the ADT45 and adt50 are low voltage, precision centigrade temperature sensors. they provide a voltage output that is lin- early proportional to the celsius (centigrade) temperature. the ADT45/adt50 do not require any external calibration to pro- vide typical accuracies of 1 c at +25 c and 2 c over the C40 c to +125 c temperature range. the low output imped- ance of the ADT45/adt50, linear output and precise calibra- tion simplify interfacing to temperature control circuitry and a/d converters. all three devices are intended for single supply operation from 2.7 v to 12 v maximum. supply current runs well below 60 m a providing very low self-heatingless than 0.1 c in still air. the ADT45/adt50 are functionally and pin compatible with lm45/lm50 respectively. the ADT45 pro- vides a 250 mv output at +25 c and reads temperature from 0 c to +100 c. the adt50 is specified from C40 c to +125 c, provides a 750 mv output at +25 c and operates to +125 c from a single 2.7 v supply. both the ADT45 and adt50 have an output scale factor of +10 mv/ c. operation extends to +150 c with reduced accuracy for all devices when operating from a 12 v supply. the ADT45/adt50 are available in the low cost 3-lead sot-23 surface mount package. free datasheet http://
C2C rev. 0 ADT45/adt50Cspecifications (v s = +2.7 v to +5.5 v, C40 8 c t a +125 8 c unless otherwise noted) 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 ADT45/adt50 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. parameter symbol conditions min typ max units accuracy ADT45 t a = +25 c 1 3 c ADT45 over rated temperature 1.5 4 c adt50 t a = +25 c 1 3 c adt50 over rated temperature 2 4 c scale factor, ADT45 0 c t a +100 c +9.8 +10 +10.2 mv/ c scale factor, adt50 C40 c t a +125 c +9.8 +10 +10.2 mv/ c load regulation t a = +25 c520m c/ m a 0 m a i l 50 m a 20 100 m c/ m a power supply rejection ratio psrr t a = +25 c 30 100 m c/v 3.0 v +v s 5.5 v 50 150 m c/v linearity 0.5 c long-term stability t a = +150 c for 1 khrs 0.4 c output ADT45 output voltage t a = +25 c 250 mv adt50 output voltage t a = +25 c 750 mv output voltage range 100 2,000 mv output load current i l 0 100 m a short-circuit current i sc note 1 250 m a capacitive load driving c l no oscillations 1 1,000 10,000 pf device turn-on time output within 1 c 0.5 1 ms 100 k w i 100 pf load 1 power supply supply range v s 2.7 12 v supply current i sy unloaded 60 m a notes 1 guaranteed but not tested. specifications subject to change without notice. absolute maximum ratings* supply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +13.2 v output pin . . . . . . . . . . . . . . . . . . . . . . . . gnd v out +v s operating temperature range . . . . . . . . . . C55 c to +150 c dice junction temperature . . . . . . . . . . . . . . . . . . . . . +175 c storage temperature range . . . . . . . . . . . . C65 c to +160 c lead temperature (soldering, 60 sec) . . . . . . . . . . . . +300 c * caution 1. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation at or above this specification is not implied. exposure to the above maximum rating conditions for extended periods may affect device reliability. 2. permanent damage may occur on unprotected units from high energy elec- trostatic fields. keep units in conductive foam or packaging at all times until ready to use. use proper antistatic handling procedures. 3. remove power before inserting or removing units from their sockets. package type u ja u jc units sot-23 (rt suffix) 300 1 180 c/w note 1 q ja is specified for device in socket (worst case conditions). warning! esd sensitive device free datasheet http://
ADT45/adt50 C3C rev. 0 functional description an equivalent circuit for the ADT45/adt50 of micropower centi- grade temperature sensors is shown in figure 1. at the heart of the temperature sensor is a bandgap core comprised of transistors q1 and q2, biased by q3 to approximately 8 m a. the bandgap core operates both q1 and q2 at the same collector current level; however, since the emitter area of q1 is 10 times that of q2, q1s v be and q2s v be are not equal by the following relationship: d v be = v t ln a e , q 1 a e , q 2 ? ? ? ? +v s v out q4 q1 10x q2 1x 25 m a 3x 2x r1 r2 r3 7.5 m a 2x q3 6x gnd figure 1. temperature sensor simplified equivalent circuit ordering guide accuracy linear operating at +25 8 c temperature package model ( 8 c max) range option* ADT45grt 6 3.0 0 c to +100 c rt-3 adt50grt 6 3.0 C40 c to +125 c rt-3 *rt = surface mount. pin configuration 3 gnd 1 2 top view (not to scale) +v s v out resistors r1 and r2 are used to scale this result to produce the output voltage transfer characteristic of each temperature sensor and, simultaneously, r2 and r3 are used to scale q1s v be as an offset term in v out . table i summarizes the differences between the two temperature sensors output characteristics: table i. ADT45/adt50 output characteristics offset output voltage output voltage sensor voltage (v) scaling (mv/ 8 c) @ +25 8 c ADT45 0 10 250 mv adt50 0.5 10 750 mv the output voltage of the temperature sensor is available at the emitter of q4, which buffers the bandgap core and provides load current drive. q4s current gain, working with the available base current drive from the previous stage, sets the short-circuit current limit of these devices to 250 m a. free datasheet http://
ADT45/adt50 C4C rev. 0 Ctypical performance characteristics a. ADT45 b. adt50 v s = 3v temperature C 8 c 1.4 0 1.2 1 0.8 0.6 0.4 0.2 1.6 1.8 2 2 50 2 25 0 25 50 75 100 125 output voltage C volts a b figure 2. output voltage vs. temperature a. maximum limit b. typical accuracy error c. minimum limit temperature C 8 c 2 2 5 1 0 2 1 2 2 2 3 2 4 3 4 5 0 20 40 60 80 100 120 140 a c accuracy error C 8 c b figure 3. ADT45 accuracy error vs. temperature temperature C 8 c 5 C5 C40 140 C20 0 20 40 60 80 100 120 4 0 C1 C2 C3 3 accuracy error C 8 c 1 a. maximum limit b. typical accuracy error c. minimum limit a b c 2 C4 figure 4. adt50 accuracy error vs. temperature temperature C 8 c 0.4 0.3 0 2 50 125 2 25 0 25 50 75 100 0.2 0.1 power supply rejection C 8 c/v v 1 = 1 3v to 1 5.5v, no load figure 5. power supply rejection vs. temperature frequency C hz 100 0.01 20 100k 100 1k 10k 31.6 10 3.16 1 0.32 0.1 0.032 power supply rejection C 8 c/v figure 6. power supply rejection vs. frequency temperature C 8 c 4 3 0 2 1 5 2 50 125 2 25 0 25 50 75 100 minimum supply voltage required to meet data sheet specification minimum supply voltage C volts no load ADT45/ adt50 figure 7. minimum supply voltage vs. temperature free datasheet http://
ADT45/adt50 C5C rev. 0 temperature C 8 c 50 40 10 30 20 60 2 50 125 2 25 0 25 50 75 100 b a supply current C m a a. v 1 = 5v b. v 1 = 3v no load figure 8. supply current vs. temperature supply voltage C volts 40 30 0 20 10 50 07 123456 8 supply current C m a t a = 1 25 8 c, no load figure 9. supply current vs. supply voltage temperature C 8 c 400 300 0 200 100 2 50 125 2 25 0255075 100 = v 1 pin low-to-high ( 0v to 3v ) v out settles within 1 8 c = v 1 pin high-to-low ( 3v to 0v ) response time C m s figure 10. v out response time for v+ power-up/power- down vs. temperature time C m s 0 1.0 0.8 0.6 0.4 0.2 2 50 250 0 100 50 150 200 300 350 400 450 output voltage C volts v 1 signal = t a = 25 8 c figure 11. v out response time to power-on vs. time time C sec 120 0 0 300 50 change C % 100 150 200 250 100 80 60 40 20 sot-23 soldered to 0.338" 3 0.307" cu pcb v 1 = 2.7v to 5v, no load figure 12. thermal response time in still air air velocity C fpm time constant C sec 35 0 0 700 100 200 300 400 500 600 30 20 15 10 5 25 v 1 = 2.7v to 5v, no load sot-23 soldered to 0.338" 3 0.307" cu pcb figure 13. thermal response time in forced air free datasheet http://
ADT45/adt50 C6C rev. 0 time C sec change C % 120 60 0 010 123456789 100 80 40 20 sot-23 soldered to 0.338" 3 0.307" cu pcb v 1 = 2.7v to 5v, no load figure 14. thermal response time in stirred oil bath time/division 0% 10 90 100 1ms 10mv figure 15. temperature sensor wideband output noise voltage. gain = 100, bw = 157 khz frequency C hz 2400 1000 0 10 10k 100 1k voltage noise density C nv / hz 2200 2000 1600 1800 1400 1200 800 600 400 200 ADT45/adt50 figure 16. voltage noise spectral density vs. frequency applications section mounting considerations if the ADT45/adt50 temperature sensors are thermally at- tached and protected, they can be used in any temperature mea- surement application where the maximum temperature range of the medium is between C40 c to +125 c. properly cemented or glued to the surface of the medium, these sensors will be within 0.01 c of the surface temperature. caution should be exercised as any wiring to the device can act as heat pipes, introducing errors if the surrounding air-surface interface is not isothermal. avoiding this condition is easily achieved by dabbing the leads of the temperature sensor and the hookup wires with a bead of thermally conductive epoxy. this will ensure that the ADT45/ adt50 die temperature is not affected by the surrounding air temperature. these temperature sensors, as well as any associated circuitry, should be kept insulated and dry to avoid leakage and corrosion. in wet or corrosive environments, any electrically isolated metal or ceramic well can be used to shield the temperature sensors. condensation at very cold temperatures can cause errors and should be avoided by sealing the device using electrically non- conductive epoxy paints or dips, or any one of many printed circuit board coatings and varnishes. thermal environment effects the thermal environment in which the ADT45/adt50 sensors are used determines two important characteristics: self-heating effects and thermal response time. illustrated in figure 17 is a thermal model of the ADT45/adt50 temperature sensors, which is useful in understanding these characteristics. + C c ch c c t a p d t j u jc t c u ca figure 17. ADT45/adt50 thermal circuit model in the sot-23 package, the thermal resistance junction-to-case, q jc , is 180 c/w. the thermal resistance case-to-ambient, q ca , is the difference between q ja and q jc , and is determined by the characteristics of the thermal connection. the temperature sensors power dissipation, represented by p d , is the product of the total voltage across the device and its total supply current (including any current delivered to the load). the rise in die temperature above the mediums ambient temperature is given by: t j = p d q jc +q ca () + t a thus, the die temperature rise of an ADT45 rt package mounted into a socket in still air at 25 c and driven from a +5 v supply is less than 0.04 c. the transient response of the ADT45/adt50 sensors to a step change in the temperature is determined by the thermal resistances and the thermal capacities of the die, c ch , and the case, c c . the thermal capacity of the case, c c , varies with the measurement medium since it includes anything that is in direct contact with the package. in all practical cases, the thermal capacity of the case is the limiting factor in the thermal response time of the sensor and can be represented by a single-pole rc- time constant response. free datasheet http://
ADT45/adt50 C7C rev. 0 figures 11 to 13 illustrate the thermal response time of the ADT45/adt50 sensors under various conditions. the thermal time constant of a temperature sensor is defined to be the time required for the sensor to reach 63.2% of the final value for a step change in the temperature. basic temperature sensor connections the circuit in figure 18 illustrates the basic circuit configura- tion for the ADT45/adt50 temperature sensors. 0.1 m f v out 2.7v < v s < 12v v s gnd ADT45/50 figure 18. b asic temperature sensor circuit configuration note the 0.1 m f bypass capacitor on the input. this capacitor should be a ceramic type, have very short leads (surface mount would be preferable), and located as close a physical proximity to the temperature sensor supply pin as practical. since these temperature sensors operate on very little supply current and could be exposed to very hostile electrical environments, it is important to minimize the effects of rfi (radio-frequency interference) on these devices. the effect of rfi on these tem- perature sensors in specific and analog ics in general is mani- fested as abnormal dc shifts in the output voltage due to the rectification of the high frequency ambient noise by the ic. in those cases where the devices are operated in the presence of high frequency radiated or conducted noise, a large value tanta- lum capacitor (>2.2 m f) placed across the 0.1 m f ceramic may offer additional noise immunity. fahrenheit thermometers although the ADT45/adt50 temperature sensors are centi- grade temperature sensors, a few components can be used to convert the output voltage and transfer characteristics to read fahrenheit temperatures directly. shown in figure 19a is an example of a simple fahrenheit thermometer using the ADT45. this circuit can be used to sense temperatures from 41 f to 257 f with an output transfer characteristic of 1 mv/ f using the ADT45. this particular approach does not lend itself well to the adt50 because of its inherent 0.5 v output offset. the circuit is constructed with an ad589, a 1.23 v voltage refer- ence, and four resistors whose values are shown in the figure table. the scaling of the output resistance levels was to ensure minimum output loading on the temperature sensors. a general- ized expression for the circuits transfer equation is given by: v out = r 1 r 1 + r 2 ? ? ? ? (ADT45) + r 3 r 3 + r 4 ? ? ? ? (ad589) where ADT45 = output voltage of the ADT45 at the measure- ment temperature, t m , and ad589 = output voltage of the reference = 1.23 v. note that the output voltage of this circuit in not referenced to the circuits common. if this output voltage were to be directly applied to the input of an adc, the adcs common should be adjusted accordingly. ADT45 v s v out gnd 0.1 m f v s ad589 1.23v r2 r3 r1 r4 + e v out sensor tcv out r1 (k v ) r2 (k v ) r3 (k v ) r4 (k v ) ADT45 1mv/ 8 f 45.3 10 10 374 figure 19a. ADT45 fahrenheit thermometers the same circuit principles can be applied to the adt50, but because of the adt50s inherent offset, the circuit uses two fewer resistors as shown in figure 19b. in this circuit, the out- put voltage transfer characteristic is 1 mv/ f, but is referenced to the circuits common; however, there is a 58 mv (58 f) offset in the output voltage. for example, the output voltage of the circuit would read 18 mv, if the adt50 is placed in C40 f ambient environment, and 315 mv at 257 f. adt50 v s v out gnd 0.1 m f v s r2 10k v r1 45.3k v v out @ 1mv/ 8 f = measured 8 f v out @ 2 40 8 f = 18mv v out @ 1 257 8 f = 315mv + C figure 19b. adt50 fahrenheit thermometer version 1 at the expense of additional circuitry, the offset produced by the circuit in figure 19b can be avoided by using the circuit in figure 19c. in this circuit, the output of the adt50 is condi- tioned by a single-supply, micropower op amp, the op193. although the entire circuit operates from a single +3 v supply, the output voltage of the circuit reads the temperature directly with a transfer characteristic of 1 mv/ f, without offset. this is accom- plished through the use of an adm660, a supply voltage inverter. the +3 v supply is inverted and applied to the op193s vC terminal. thus, for a temperature range between C40 f and +257 f, the output of the circuit reads C40 mv to +257 mv. a general expres- sion for the circuits transfer equation is given by: v out = r 6 r 5 + r 6 ? ? ? ? 1 + r 4 r 3 ? ? ? ? ( adt 50) r 4 r 3 ? ? ? ? v s 2 ? ? ? ? free datasheet http://
ADT45/adt50 C8C rev. 0 average and differential temperature measurement in many commercial and industrial environments, temperature sensors are often used to measure the average temperature in a building, or the difference in temperature between two locations on a factory floor or in an industrial process. the circuits in figures 20a and 20b demonstrate an inexpensive approach to average and differential temperature measurement. in figure 20a, an op193 is used to amplify the stepped down averaged output of three temperature sensors to produce an output volt- age scaled to 10 mv/ c, which represents the average tempera- ture at three locations. the circuit can be extended to as many temperature sensors as required as long as the circuits transfer equation is maintained. in this application, it is recommended that one temperature sensor type be used throughout the circuit; otherwise, the output voltage of the circuit will not produce an accurate reading of the various ambient conditions. r1 300k v ADT45/50 ADT45/50 ADT45/50 r2 300k v r3 300k v r1 3 r5 = r4 7.5k v v node r5 100k v r6 7.5k v op193 2.7v < +v s < 12v 0.1 m f v temp(avg) @ 10mv/ 8 c for ADT45/50 v node < 0.1v v temp(avg) = (ADT45/50 (1) + ADT45/50 (2) + ADT45/50 (3) ) 1 3 for r1 = r2 = r3; r4 = r6 figure 20a. configuring multiple sensors for average temperature measurements the circuit in figure 20b illustrates how a pair of adt50 sensors can be used with an op193 configured as a difference amplifier to read the difference in temperature between two locations. in these element adt50 r3 r4 r5 r6 258.6k v 10k v 47.7k v 10k v adt50 adm660 gnd v out v s 10 m f/0.1 m f r1 50k v r2 50k v +3v c1 10 m f r3 r5 r6 r4 0.1 m f C3v 10 m f nc nc 10 m f v out @ 1mv/ 8 f C40 8 f # t a # 257 8 f op193 + + + + figure 19c. adt50 fahrenheit thermometer version 2 applications, it is always possible that one temperature sensor would be reading a temperature below that of the other sensor. to accommodate this condition, the output of the op193 is offset to a voltage at one-half the supply via r5 and r6. thus, the output voltage of the circuit is measured relative to this point, as shown in the figure. using the adt50, the output voltage of the circuit is scaled by 10 mv/ c. to minimize error in the difference between the two measured temperatures, a common, readily available thin-film resistor network is used for r1Cr4. op193 0.1 m f 1 m f 2.7v < v s < 12v 0.1 m f 0.1 m f adt50 @ t1 adt50 @ t2 r1* r2* r3* r8 25k v r9 25k v r4* r5 100k v r6 100k v v out r7 100k v 0 8 c # t a # 125 8 c v out = t2Ct1 @ 10mv/ 8 c centered at v s 2 *r1-r4, caddock t914C100k v C100, or equivalent figure 20b. configuring multiple sensors for differential temperature measurements microprocessor interrupt generator these inexpensive temperature sensors can be used with a volt- age reference and an analog comparator to configure an inter- rupt generator useful in microprocessor applications. with the popularity of fast 486 and pentium laptop computers, the need free datasheet http://
ADT45/adt50 C9C rev. 0 to indicate that a microprocessor over-temperature condition has grown tremendously. the circuit illustrated in figure 21 demonstrates one way to generate an interrupt using an ADT45, a cmp402 analog comparator, and a ref191, a 2 v precision voltage reference. the circuit has been designed to produce a logic high interrupt signal if the microprocessor temperature exceeds +80 c. this 80 c trip point was chosen arbitrarily (final value set by the microprocessor thermal reference design) and is set using an r3-r4 voltage divider of the ref191s output voltage. since the output of the ADT45 is scaled by 10 mv/ c, the voltage at the cmp402s inverting terminal is set to 0.8 v. since temperature is a slowly moving quantity, the possibility for comparator chatter exists. to avoid this condition, hysteresis is used around the comparator. in this application, a hysteresis of 5 c about the trip point was arbitrarily chosen; the ultimate value for hysteresis should be determined by the end applica- tion. the output logic voltage swing of the comparator with r1 and r2 determine the amount of comparator hysteresis. using a 3.3 v supply, the output logic voltage swing of the cmp402 is 2.6 v; thus, for a hysteresis of +5 c (50 mv @ 10 mv/ c), r1 is set to 20 k w and r2 is set to 1 m w . an expression for this circuits hysteresis is given by: pentium is a trademark of intel corporation. ADT45 ref191 0.1 m f v s gnd v out r5 100k v 3.3v 0.1 m f 0.1 m f 0.1 m f r1a 10k v r1b 10k v c l 1000pf 1 m f + r4 10k v v ref r2 1m v <80 8 c >80 8 c interrupt r3 16k v 1/4 cmp402 figure 21. pentium? over temperature interrupt generator v hys = r 1 r 2 ? ? ? ? ( v logic swing , cmp 02 ) since the likelihood that this circuit would be used in close proximity to high speed digital circuits, r1 is split into equal values and a 1000 pf is used to form a low-pass filter on the output of the ADT45. furthermore, to prevent high frequency noise from contaminating the comparator trip point, a 0.1 m f capacitor is used across r4. thermocouple signal conditioning with cold-junction compensation the circuit in figure 22 conditions the output of a type k thermocouple, while providing cold-junction compensation, for temperatures between 0 c and 250 c. the circuit operates from single +3.3 v to +12 v supplies and has been designed to produce an output voltage transfer characteristic of 10 mv/ c. a type k thermocouple exhibits a seebeck coefficient of approximately 41 m v/ c; therefore, at the cold junction, the ADT45 with a temperature coefficient of 10 mv/ c is used with r1 and r2 to introduce an opposing cold-junction tempera- ture coefficient of C41 m v/ c. this prevents the isothermal, cold- junction connection between the circuits pcb tracks and the thermocouples wires from introducing an error in the mea- sured temperature. this compensation works extremely well for ADT45 v out 0C2.5v r6 100k v 5% r2* 102 v r1* 24.9k v op193 cu cu r5* 1.21m v p1 50k v 0.1 m f r4 4.99k v r3 10m v 5% 3.3v < v s < 12v chromel alumel type k thermo 2 couple 0.1 m f cold junction v s v out gnd isothermal block 0 8 c # t # 250 8 c + C note: all resistors 1% unless otherwise noted by * figure 22. a single supply, type k thermocouple signal condition circuit with cold-junction compensation free datasheet http://
ADT45/adt50 C10C rev. 0 circuit ambient temperatures in the range of +20 c to +50 c. over a 250 c measurement temperature range, the thermocouple produces an output voltage change of 10.151 mv. since the required circuits output full-scale voltage is 2.5 v, the gain of the circuit is set to 246.3. choosing r4 equal to 4.99 k w sets r5 equal to 1.22 m w . since the closest 1% value for r5 is 1.21 m w , a 50 k w potentiometer is used with r5 for fine trim of the full-scale output voltage. although the op193 is a superior single-supply, micropower operational amplifier, its output stage is not rail-to- rail; as such, the 0 c output voltage level is 0.1 v. if this circuit were to be digitized by a single-supply adc, the adcs common should be adjusted accordingly to 0.1 v. using ADT45/adt50 sensors in remote locations in many industrial environments, sensors are required to oper- ate in the presence of high ambient noise. these noise sources take on many forms; for example, scr transients, relays, radio transmitters, arc welders, ac motors, et cetera. furthermore, they may be used at considerable distance from the signal condi- tioning circuitry. these high noise environments are very typi- cally in the form of electric fields, so the voltage output of the temperature sensor can be susceptible to contamination from these noise sources. illustrated in figure 23 is a way to convert the output voltage of an ADT45/adt50 sensor into a current to be transmitted down a long twisted-pair shielded cable to a ground referenced receiver. the temperature sensors do not possess the capability of high output current operation; thus, a garden variety pnp transistor is used to boost the output current drive of the circuit. as shown in the table, the values of r2 and r3 were chosen to produce an arbitrary full-scale output current of 2 ma. lower values for the full-scale current are not recommended because the minimum- scale output current produced by the circuit could be contami- nated by nearby ambient magnetic fields operating in the vicinity of the circuit/cable pair. because of the use of an external tran- sistor, the minimum recommended operating voltage for this circuit is +5 v. note, to minimize the effects of emi (or rfi) both the circuits and the temperature sensors supply pins are bypassed with good quality, ceramic capacitors. twisted pair belden type 9502 or equivalent ADT45/50 2n2907 0.1 m f r1 4.7k v 0.01 m f +5v v out r3 v out r2 gnd sensor r2 r3 ADT45 adt50 634 887 634 887 figure 23. a remote, two-wire boosted output current temperature sensor a temperature to 4 maC20 ma loop transmitter in many process control applications, two-wire transmitters are used to convey analog signals through noisy ambient environ- ments. these current transmitters use a zero-scale signal current of 4 ma that can be used to power the transmitters signal conditioning circuitry. the full-scale output signal in these transmitters is 20 ma. a circuit that transmits temperature information in this fashion is illustrated in figure 25. using an ADT45/adt50 as the tem- perature sensor, the output current is linearly proportional to the temperature of the medium. the entire circuit operates from the ref193s +3 v output. the ref193 requires no external trim- ming for two reasons: (1) the ref193s tight initial output voltage tolerance and (2) the low supply current of the ADT45/ adt50, the op193 and the ref193. the entire circuit con- sumes less than 3 ma from a total budget of 4 ma. the op193 regulates the output current to satisfy the current summation at the noninverting node of the op193. a generalized expression for the kcl equation at the op193s pin 3 is given by: i out = 1 r 7 ADT45 / adt 50 r 3 r 1 + v ref r 3 r 2 ? ? ? ? for each of the three temperature sensors, table ii provides the values for each of the components, p1, p2 and r1Cr4: table ii. circuit element values for loop transmitter sensor r1( v ) p1( v ) r2( v ) p2( v ) r3( v ) r4( v ) ADT45 97.6 k 5 k 1.58 m 100 k 140 k 56.2 k adt50 97.6 k 5 k 931 k 50 k 97.6 k 47 k the 4 ma offset trim is provided by p2, and p1 provides the circuits full-scale gain trim at 20 ma. these two trims do not interact because the noninverting input of the op193 is held at a virtual ground. the zero-scale and full-scale output currents of the circuit are adjusted according to the operating tempera- ture range of each temperature sensor. the schottky diode, d1, is required in this circuit to prevent loop supply power- on transients from pulling the noninverting input of the op193 more than 300 mv below its inverting input. without this di- ode, such transients could cause phase reversal of the opera- tional amplifier and possible latchup of the transmitter. the loop supply voltage compliance of the circuit is limited by the maximum applied input voltage to the ref193 and is from +9 v to +18 v. a temperature-to-frequency converter another common method of transmitting analog information from a remote location is to convert a voltage to an equivalent in the frequency domain. this is readily done with any of the low cost, monolithic voltage-to-frequency converters (vfcs) avail- able. these vfcs feature a robust, open-collector output tran- sistor for easy interfacing to digital circuitry. the digital signal produced by the vfc is less susceptible to contamination from external noise sources and line voltage drops because the only important information is the frequency of the digital signal. as long as the conversions between temperature and frequency are done accurately, the temperature data from the sensors can be reliably transmitted. the circuit in figure 24 illustrates a method by which the out- puts of these temperature sensors can be converted to a fre- quency using the ad654. the output signal of the ad654 is a square wave that is proportional to the dc input voltage across pins 4 and 3. the transfer equation of the circuit is given by: f out = v tmp v offset 10 ( r t c t ) free datasheet http://
ADT45/adt50 C11C rev. 0 ad654 sensor r t (r1+p1) c t ADT45 adt50 11.8k v + 500 v 16.2k v + 500 v 1.7nf 1.8nf +5v 0.1 m f v s c t v out gnd ADT45/50 r pu 5k v f out 10 m f/0.1 m f r1 p1 r t p2 100k v +5v r off1 470 v f out offset r off2 10 v nb: att a (min), f out = 0hz figure 24. a temperature-to-frequency converter an offset trim network (f out offset) is included with this circuit to set f out at 0 hz when the temperature sensors mini- mum output voltage is reached. potentiometer p1 is required to calibrate the absolute accuracy of the ad654. the table in the figure illustrates the circuit element values for each of the three sensors. the nominal offset voltage required for 0 hz output from the ADT45 is 50 mv; for the adt50, the offset voltage required is 100 mv. in all cases for the circuit values shown, the output frequency transfer characteristic of the circuit was set at 50 hz/ c. at the receiving end, a frequency-to-voltage converter (fvc) can be used to convert the frequency back to a dc voltage for further processing. one such fvc is the ad650. for complete information on the ad650 and ad654, please consult the individual data sheets for those devices. driving long cables or heavy capacitive loads although the ADT45/adt50 temperature sensors are capable of driving capacitive loads up to 10,000 pf without oscillation, output voltage transient response times can be improved with the use of a small resistor in series with the output of the tem- perature sensor, as shown in figure 26. as an added benefit, this resistor forms a low-pass filter with the cables capacitance, which helps to reduce bandwidth noise. furthermore, since the temperature sensor is likely to be used in environments where the ambient noise level can be very high, this resistor helps to prevent rectification by the devices of the high frequency noise. the combination of this resistor and the supply bypass capacitor offers the best protection. ADT45/50 gnd +v s 0.1 m f v out 750 v long cable or heavy capactive loads figure 26. driving long cables or heavy capacitive loads commentary on long-term stability the concept of long-term stability has been used for many years to describe by what amount an ics parameter would shift dur- ing its lifetime. this is a concept that has been typically applied to both voltage references and monolithic temperature sensors. unfortunately, integrated circuits cannot be evaluated at room temperature (+25 c) for 10 years or so to determine this shift. as a result, manufacturers very typically perform accelerated lifetime testing of integrated circuits by operating ics at elevated temperatures (between +125 c and +150 c) over a shorter period of time (typically, between 500 and 1000 hours). ref193 + +3v 1 m f r2* p2* 4ma adjust r1* v out v s ADT45/50 p1* 20ma adjust gnd r3* d1 r3* r5 100k v r6 100k v r7 100 v q1 2n1711 d1: hp5082C2810 i l op193 *see text for values v out r l 250 v v loop +9v to +18v figure 25. a temperature to 4 ma-to-20 ma loop transmitter free datasheet http://
ADT45/adt50 C12C rev. 0 outline dimensions dimensions shown in inches and (mm). c3175C8C10/97 printed in u.s.a. 3-lead plastic surface mount package rt-3 (sot-23) 0.1200 (3.048) 0.1102 (2.799) pin 1 0.0550 (1.397) 0.0470 (1.194) 0.0236 (0.599) 0.0177 (0.450) 0.1040 (2.642) 0.0827 (2.101) 0.0413 (1.049) 0.0374 (0.950) 0.0807 (2.050) 0.0701 (1.781) 1 2 3 seating plane 0.0440 (1.118) 0.0320 (0.813) 0.0040 (0.102) 0.0005 (0.013) 0.0210 (0.533) 0.0146 (0.371) 0.027 (0.686) ref 0.0059 (0.150) 0.0034 (0.086) 0.0100 (0.254) 0.0050 (0.127) as a result of this operation, the lifetime of an integrated circuit is significantly accelerated due to the increase in rates of reac- tion within the semiconductor material. a well-understood, and universal, model used by the semiconductor industry is the arrhenius model, which relates the change in rates of reaction to a change in elevated temperatures. from the arrhenius model, an acceleration factor can be calculated and applied to the specified parameter. for example, this acceleration factor can be used to reduce a temperature sensors long-term stability (e.g., 0.4 c after 1000 hours at t j = +150 c) to an observed shift in that parameter at +25 c. for any semiconductor de- vice, the acceleration factor is expressed as: f = exp e a k ? ? ? ? 1 t 1 1 t 2 ? ? ? ? ? ? where f = calculated acceleration factor; e a = activation energy in ev = 0.7 ev; k = boltzmanns constant = 8.63 10 C5 ev/k; t 1 = test temperature in kelvin, t j = +150 c = 423.15k; and t 2 = desired operating temperature in kelvin, t j = +25 c = 298.15k for example, if the desired operating temperature of an ic is +25 c and has been subjected to test temperature of +150 c, the acceleration factor is: f = 3.23 10 C4 with this background information, the ADT45/adt50s long- term stability can be mapped to what its equivalent observed shift would be at t a = +25 c. as quoted in the data sheet, the long-term stability of these temperature sensors after 1000 hours at +150 c is 0.4 c. this shift is equivalent to 0.01 c/day at t j = +150 c. to determine what the observed shift would be at t a = +25 c is a matter of applying the acceleration factor calcu- lated above to this result: 0.01 c/day 3.23 10 C4 = 0.003 m c/day @ +25 c thus, if any of the ADT45/adt50 devices were to be used at 25 c, then the observed shift would be no more than 0.003 m c per day, or 0.1 m c per month. calculating the observed shift for any other operating temperature is simply a matter of calcu- lating a new acceleration factor. free datasheet http://

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