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AD597AR Arkusz danych(PDF) 6 Page - Analog Devices |
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AD597AR Arkusz danych(HTML) 6 Page - Analog Devices |
6 / 8 page AD596/AD597 REV. B –6– SINGLE AND DUAL SUPPLY CONNECTIONS In the single supply configuration as used in the setpoint con- troller of Figure 2, any convenient voltage from +5 V to +36 V may be used, with self-heating errors being minimized at lower supply levels. In this configuration, the –VS connection at Pin 5 is tied to ground. Temperatures below zero can be accommo- dated in the single supply setpoint mode, but not in the single supply temperature measuring mode (Figure 1 reconnected for single supply). Temperatures below zero can only be indicated by a negative output voltage, which is impossible in the single supply mode. Common-mode voltages on the thermocouple inputs must remain below the positive supply, and not more than 0.15 V more negative than the minus supply. In addition, a return path for the input bias currents must be provided. If the thermo- couple is not remotely grounded, then the dotted line connec- tions in Figures 1 and 2 are mandatory. STABILITY OVER TEMPERATURE The AD596/AD597 is specified for a maximum error of ±4°C at an ambient temperature of 60 °C and a measuring junction temperature at 175 °C. The ambient temperature stability is specified to be a maximum of 0.05 °C/°C. In other words, for every degree change in the ambient temperature, the output will change no more than 0.05 degrees. So, at 25 °C the maximum deviation from the temperature-voltage characteristic of Table I is ±5.75°C, and at 100°C it is ±6°C maximum (see Figure 5). If the offset error of ±4°C is removed with a single offset adjust- ment, these errors will be reduced to ±1.75°C and ±2°C max. The optional trim circuit shown in Figure 1 demonstrates how the ambient offset error can be adjusted to zero. MAXIMUM MAXIMUM TYPICAL 25 C 100 C 60 C +2.0 C +1.75 C +0.8 C 0 –0.8 C –1.75 C –2.0 C Figure 5. Drift Error vs. Temperature THERMAL ENVIRONMENTAL EFFECTS The inherent low power dissipation of the AD596/AD597 keeps self-heating errors to a minimum. However, device output is capable of delivering ±5 mA to an external load and the alarm circuitry can supply up to 20 mA. Since the typical junction to ambient thermal resistance in free air is 150 °C/W, significant temperature difference between the package pins (where the reference junction is located) and the chip (where the cold junc- tion temperature is measured and then compensated) can exist when the device is operated in a high dissipation mode. These temperature differences will result in a direct error at the out- put. In the temperature proportional mode, the alarm feature will only activate in the event of an open thermocouple or sys- tem transient which causes the device output to saturate. Self-Heating errors will not effect the operation of the alarm but two cases do need to be considered. First, after a fault is cor- rected and the alarm is reset, the AD596/AD597 must be al- lowed to cool before readings can again be accurate. This can take 5 minutes or more depending upon the thermal environ- ment seen by the device. Second, the junction temperature of the part should not be allowed to exceed 150 °C. If the alarm circuit of the AD596/AD597 is made to source or sink 20 mA with 30 V across it, the junction temperature will be 90 °C above ambient causing the die temperature to exceed 150 °C when ambient is above 60 °C. In this case, either the load must be reduced, or a heat sink used to lower the thermal resistance. TEMPERATURE READOUT AND CONTROL Figure 6 shows a complete temperature indication and control system based on the AD596/AD597. Here the AD596/AD597 is being used as a closed-loop thermocouple signal conditioner and an external op amp is used to implement setpoint. This has two important advantages. It provides a high level (10 mV/ °C) output for the A/D panel meter and also preserves the alarm function for open thermocouples. The A/D panel meter can easily be offset and scaled as shown to read directly in degrees Fahrenheit. If a two temperature cali- bration scheme is used, the dominant residual errors will arise from two sources; the ambient temperature rejection (typically ±2°C over a 25°C to 100°C range) and thermocouple nonlin- earity typical +1 °C from 80°C to 550°C for type J and +1°C from –20 °C to 350°C for type K. An external voltage reference is used both to increase the stabil- ity of the A/D converter and supply a stable reference for the setpoint voltage. A traditional requirement for the design of setpoint control thermocouple systems has been to configure the system such that the appropriate action is taken in the event of an open thermocouple. The open thermocouple alarm pin with its flex- ible current-limited output format supports this function when the part operates in the temperature proportional mode. In addition, if the thermocouple is not remotely grounded, it is possible to program the device for either a positive or negative full-scale output in the event of an open thermocouple. This is done by connecting the bias return resistor directly to Pin 1 if a high output voltage is desired to indicate a fault condition. Al- ternately, if the bias return is provided on the thermocouple lead connected to Pin 2, an open circuit will result in an output low reading. Figure 6 shows the ground return connected to Pin 1 so that if the thermocouple fails, the heater will remain off. At the same time, the alarm circuit lights the LED signalling the need to service the thermocouple. Grounding Pin 2 would lead to low output voltage saturation, and in this circuit would result in a potentially dangerous thermal runaway under fault conditions. |
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