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AD595CD Scheda tecnica(PDF) 6 Page - Analog Devices |
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AD595CD Scheda tecnica(HTML) 6 Page - Analog Devices |
6 / 8 page AD594/AD595 REV. C –6– of R3 should be approximately 280 k Ω. The final connection diagram is shown in Figure 7. An approximate verification of the effectiveness of recalibration is to measure the differential gain to the output. For type E it should be 164.2. AD594/ AD595 –T +IN –IN +T COM 1 14 4 2 3 –C 5 6 FB VO 8 9 R3 +C R1 R2 Figure 7. Type E Recalibration When implementing a similar recalibration procedure for the AD595 the values for R1, R2, R3 and r will be approximately 650 Ω, 84 kΩ, 93 kΩ and 1.51, respectively. Power consump- tion will increase by about 50% when using the AD595 with type E inputs. Note that during this procedure it is crucial to maintain the AD594/AD595 at a stable temperature because it is used as the temperature reference. Contact with fingers or any tools not at ambient temperature will quickly produce errors. Radiational heating from a change in lighting or approach of a soldering iron must also be guarded against. USING TYPE T THERMOCOUPLES WITH THE AD595 Because of the similarity of thermal EMFs in the 0 °C to +50°C range between type K and type T thermocouples, the AD595 can be directly used with both types of inputs. Within this ambi- ent temperature range the AD595 should exhibit no more than an additional 0.2 °C output calibration error when used with type T inputs. The error arises because the ice point compensa- tor is trimmed to type K characteristics at 25 °C. To calculate the AD595 output values over the recommended –200 °C to +350 °C range for type T thermocouples, simply use the ANSI thermocouple voltages referred to 0 °C and the output equation given on page 2 for the AD595. Because of the relatively large nonlinearities associated with type T thermocouples the output will deviate widely from the nominal 10 mV/ °C. However, cold junction compensation over the rated 0 °C to +50°C ambient will remain accurate. STABILITY OVER TEMPERATURE Each AD594/AD595 is tested for error over temperature with the measuring thermocouple at 0 °C. The combined effects of cold junction compensation error, amplifier offset drift and gain error determine the stability of the AD594/AD595 output over the rated ambient temperature range. Figure 8 shows an AD594/ AD595 drift error envelope. The slope of this figure has units of °C/°C. TEMPERATURE OF AD594C/AD595C –0.6 C 50 C 25 C 0 +0.6 C Figure 8. Drift Error vs. Temperature THERMAL ENVIRONMENT EFFECTS The inherent low power dissipation of the AD594/AD595 and the low thermal resistance of the package make self-heating errors almost negligible. For example, in still air the chip to am- bient thermal resistance is about 80 °C/watt (for the D package). At the nominal dissipation of 800 µW the self-heating in free air is less than 0.065 °C. Submerged in fluorinert liquid (unstirred) the thermal resistance is about 40 °C/watt, resulting in a self- heating error of about 0.032 °C. SETPOINT CONTROLLER The AD594/AD595 can readily be connected as a setpoint controller as shown in Figure 9. CONSTANTAN (ALUMEL) IRON (CHROMEL) +5V COMMON HEATER 20M (OPTIONAL) FOR HYSTERESIS SETPOINT VOLTAGE INPUT TEMPERATURE CONTROLLED REGION LOW = > T < SETPOINT HIGH = > T > SETPOINT TEMPERATURE COMPARATOR OUT HEATER DRIVER OVERLOAD DETECT G –TC +TC 1 2 3 4 567 13 12 11 10 AD594/ AD595 14 ICE POINT COMP. +A 98 G Figure 9. Setpoint Controller The thermocouple is used to sense the unknown temperature and provide a thermal EMF to the input of the AD594/AD595. The signal is cold junction compensated, amplified to 10 mV/ °C and compared to an external setpoint voltage applied by the user to the feedback at Pin 8. Table I lists the correspondence between setpoint voltage and temperature, accounting for the nonlinearity of the measurement thermocouple. If the setpoint temperature range is within the operating range (–55 °C to +125 °C) of the AD594/AD595, the chip can be used as the transducer for the circuit by shorting the inputs together and utilizing the nominal calibration of 10 mV/ °C. This is the centi- grade thermometer configuration as shown in Figure 13. In operation if the setpoint voltage is above the voltage corre- sponding to the temperature being measured the output swings low to approximately zero volts. Conversely, when the tempera- ture rises above the setpoint voltage the output switches to the positive limit of about 4 volts with a +5 V supply. Figure 9 shows the setpoint comparator configuration complete with a heater element driver circuit being controlled by the AD594/ AD595 toggled output. Hysteresis can be introduced by inject- ing a current into the positive input of the feedback amplifier when the output is toggled high. With an AD594 about 200 nA into the +T terminal provides 1 °C of hysteresis. When using a single 5 V supply with an AD594, a 20 M Ω resistor from V O to +T will supply the 200 nA of current when the output is forced high (about 4 V). To widen the hysteresis band decrease the resistance connected from VO to +T. |
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