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TMP17GS Fiches technique(PDF) 7 Page - Analog Devices |
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TMP17GS Fiches technique(HTML) 7 Page - Analog Devices |
7 / 8 page TMP17 REV. 0 –7– A variable temperature controlling thermostat can easily be built using the TMP17 in the circuit of Figure 18. TMP17 10k Ω RHYST RPULL-UP +15V AD790 COMPARATOR (OPTIONAL) C RHIGH 62.7k Ω RSET 10k Ω C TEMP > SETPOINT OUTPUT HIGH TEMP < SETPOINT OUTPUT LOW RLOW 27.3k Ω REF01E 10V Figure 18. Variable Temperature Thermostat R HIGH and RLOW determine the limits of temperature controlled by the potentiometer R SET. The circuit shown operates over the temperature range 25 °C to 105°C. The reference maintains a constant set point voltage and insures that approximately 7 V appears across the sensor. If it is necessary to guardband for extraneous noise, hysteresis can be added by tying a resistor from the output to the ungrounded end of R LOW. Multiple remote temperatures can be measured using several TMP17s with a CMOS multiplexer or a series of 5 V logic gates because of the device’s current-mode output and supply-voltage compliance range. The on-resistance of a FET switch or output impedance of a gate will not affect the accuracy, as long as 4 V is maintained across the transducer. Muxes and logic driving circuits should be chosen to minimize leakage current related errors. Figure 19 illustrates a locally controlled mux switching the signal current from several remote TMP17s. CMOS or TTL gates can also be used to switch the TMP17 supply voltages, with the multiplexed signal being transmitted over a single twisted pair to the load. AD7501 D E C O D E R / D R I V E R T8 T2 T1 REMOTE TMP17s S1 S2 S8 EN TTL DTL TO CMOS I/O CHANNEL SELECT +15V –15V VOUT 10k Ω Figure 19. Remote Temperature Multiplexing To minimize the number of muxes required when a large number of TMP17s are being used, the circuit can be config- ured in a matrix. That is, a decoder can be used to switch the supply voltage to a column of TMP17s while a mux is used to control which row of sensors are being measured. The maxi- mum number of TMP17s which can be used is the product of the number of channels of the decoder and mux. An example circuit controlling 80 TMP17s is shown in Figure 20. A 7-bit digital word is all that is required to select one of the sensors. The enable input of the multiplexer turns all the sensors off for minimum dissipation while idling. +15V COLUMN SELECT 4028 BCD TO DECIMAL DECODER ROW SELECT EN +15V –15V 80 – TMP17s 10k Ω VOUT Figure 20. Matrix Multiplexer To convert the TMP17 output to °C or °F a single inexpensive reference and op amp can be used as shown in Figure 21. Although this circuit is similar to the two temperature trim circuit shown in Figure 11, two important differences exist. First, the gain resistor is fixed alleviating the need for an elevated temperature trim. Acceptable accuracy can be achieved by choosing an inexpensive resistor with the correct tolerance. Second, the TMP17 calibration error can be trimmed out at a known convenient temperature (i.e., room temperature) with a single pot adjustment. This step is independent of the gain selection. R ROFFSET/RGAIN OP196 VOUT = 100mV/( oC OR oF) +5V REF43 V– C F ≈ 9.1kΩ ≈ 9.8kΩ 100k Ω 180k Ω RGAIN ROFFSET TMP17 RCAL 2.5V RGAIN ROFFSET Figure 21. Celsius or Fahrenheit Thermometer |
Numéro de pièce similaire - TMP17GS |
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Description similaire - TMP17GS |
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