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AD595 bảng dữ liệu(PDF) 7 Page - Analog Devices |
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AD595 bảng dữ liệu(HTML) 7 Page - Analog Devices |
7 / 8 page AD594/AD595 REV. C –7– ALARM CIRCUIT In all applications of the AD594/AD595 the –ALM connection, Pin 13, should be constrained so that it is not more positive than (V+) – 4 V. This can be most easily achieved by connect- ing Pin 13 to either common at Pin 4 or V– at Pin 7. For most applications that use the alarm signal, Pin 13 will be grounded and the signal will be taken from +ALM on Pin 12. A typical application is shown in Figure 10. In this configuration the alarm transistor will be off in normal operation and the 20 k pull up will cause the +ALM output on Pin 12 to go high. If one or both of the thermocouple leads are interrupted, the +ALM pin will be driven low. As shown in Fig- ure 10 this signal is compatible with the input of a TTL gate which can be used as a buffer and/or inverter. CONSTANTAN (ALUMEL) IRON (CHROMEL) +5V ALARM TTL GATE 20k ALARM OUT GND OVERLOAD DETECT G –TC +TC 1 234 5 6 7 13 12 11 10 AD594/ AD595 14 ICE POINT COMP. +A 98 G 10mV/ C Figure 10. Using the Alarm to Drive a TTL Gate (“Grounded’’ Emitter Configuration) Since the alarm is a high level output it may be used to directly drive an LED or other indicator as shown in Figure 11. CONSTANTAN (ALUMEL) IRON (CHROMEL) COMMON LED 270 V+ OVERLOAD DETECT G –TC +TC 12 3 4 5 67 13 12 11 10 AD594/ AD595 14 ICE POINT COMP. +A 98 G 10mV/ C Figure 11. Alarm Directly Drives LED A 270 Ω series resistor will limit current in the LED to 10 mA, but may be omitted since the alarm output transistor is current limited at about 20 mA. The transistor, however, will operate in a high dissipation mode and the temperature of the circuit will rise well above ambient. Note that the cold junction compensa- tion will be affected whenever the alarm circuit is activated. The time required for the chip to return to ambient temperature will depend on the power dissipation of the alarm circuit, the nature of the thermal path to the environment and the alarm duration. The alarm can be used with both single and dual supplies. It can be operated above or below ground. The collector and emit- ter of the output transistor can be used in any normal switch configuration. As an example a negative referenced load can be driven from –ALM as shown in Figure 12. CONSTANTAN (ALUMEL) IRON (CHROMEL) +10V GND ALARM RELAY –12V OVERLOAD DETECT G –TC +TC 1 234 5 6 7 13 12 11 10 AD594/ AD595 14 ICE POINT COMP. +A 98 G 10mV/ C Figure 12. –ALM Driving A Negative Referenced Load The collector (+ALM) should not be allowed to become more positive than (V–) +36 V, however, it may be permitted to be more positive than V+. The emitter voltage (–ALM) should be constrained so that it does not become more positive than 4 volts below the V+ applied to the circuit. Additionally, the AD594/AD595 can be configured to produce an extreme upscale or downscale output in applications where an extra signal line for an alarm is inappropriate. By tying either of the thermocouple inputs to common most runaway control conditions can be automatically avoided. A +IN to common connection creates a downscale output if the thermocouple opens, while connecting –IN to common provides an upscale output. CELSIUS THERMOMETER The AD594/AD595 may be configured as a stand-alone Celsius thermometer as shown in Figure 13. +5V TO +15V OUTPUT 10mV/ C GND 0 TO –15V OVERLOAD DETECT G –TC +TC 12 3 45 6 7 13 12 11 10 AD594/ AD595 14 ICE POINT COMP. +A 98 G Figure 13. AD594/AD595 as a Stand-Alone Celsius Thermometer Simply omit the thermocouple and connect the inputs (Pins 1 and 14) to common. The output now will reflect the compensa- tion voltage and hence will indicate the AD594/AD595 temperature with a scale factor of 10 mV/ °C. In this three termi- nal, voltage output, temperature sensing mode, the AD594/ AD595 will operate over the full military –55 °C to +125°C tem- perature range. |
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