AD595AD

a Monolithic Thermocouple Amplifiers with Cold Junction Compensation AD594/AD595 FEATURES Pretrimmed for Type J (AD594) or Type K (AD595) Thermocouples Can Be Used with Type T Thermocouple Inputs Low Impedance Voltage Output: 10 mV/ⴗC Built-In Ice Point Compensation Wide Power Supply Range: +5 V to ⴞ15 V Low Power: T < SETPOINT HIGH = > T > SETPOINT TEMPERATURE COMPARATOR OUT 0 258C –0.68C 508C TEMPERATURE OF AD594C/AD595C Figure 8. Drift Error vs. Temperature –6– REV. C AD594/AD595 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 connecting 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. The alarm can be used with both single and dual supplies. It can be operated above or below ground. The collector and emitter 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. +10V CONSTANTAN (ALUMEL) 14 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 Figure 10 this signal is compatible with the input of a TTL gate which can be used as a buffer and/or inverter. 13 12 11 10 9 8 AD594/ AD595 +A G G +TC IRON (CHROMEL) 1 2 3 4 5 ICE POINT –TC COMP. 6 7 +5V 20kV 13 14 12 11 9 10 GND ALARM TTL GATE ALARM OUT CONSTANTAN (ALUMEL) 10mV/8C OVERLOAD DETECT ALARM RELAY 10mV/8C 8 –12V OVERLOAD DETECT AD594/ AD595 Figure 12. –ALM Driving A Negative Referenced Load +A G G +TC IRON (CHROMEL) 1 2 3 4 5 ICE POINT –TC COMP. 6 7 GND 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. V+ LED CONSTANTAN (ALUMEL) 13 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 270V 14 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. 10mV/8C 12 11 10 9 8 The AD594/AD595 may be configured as a stand-alone Celsius thermometer as shown in Figure 13. OVERLOAD DETECT AD594/ AD595 14 G G +TC IRON (CHROMEL) 1 2 +5V TO +15V +A 3 4 5 12 11 9 8 AD594/ AD595 +A 7 G G COMMON +TC 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 compensation 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. REV. C 10 OVERLOAD DETECT ICE POINT –TC COMP. 6 13 OUTPUT 10mV/8C 1 2 3 4 5 ICE POINT –TC COMP. 6 7 GND 0 TO –15V 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 compensation voltage and hence will indicate the AD594/AD595 temperature with a scale factor of 10 mV/°C. In this three terminal, voltage output, temperature sensing mode, the AD594/ AD595 will operate over the full military –55°C to +125°C temperature range. –7– AD594/AD595 and to arrange its output voltage so that it corresponds to a thermocouple referred to 0°C. This voltage is simply added to the thermocouple voltage and the sum then corresponds to the standard voltage tabulated for an ice-point referenced thermocouple. Thermocouples are economical and rugged; they have reasonably good long-term stability. Because of their small size, they respond quickly and are good choices where fast response is important. They function over temperature ranges from cryogenics to jet-engine exhaust and have reasonable linearity and accuracy. V1' Because the number of free electrons in a piece of metal depends on both temperature and composition of the metal, two pieces of dissimilar metal in isothermal and contact will exhibit a potential difference that is a repeatable function of temperature, as shown in Figure 14. The resulting voltage depends on the temperatures, T1 and T2, in a repeatable way. Cu CONSTANTAN V1' = V1 FOR PROPERLY SCALED V3' = f(T3) V1 C731g–0–11/99 THERMOCOUPLE BASICS Cu CuNi– V2 T3 V3' T1 V1 IRON Cu CONSTANTAN Figure 15. Substitution of Measured Reference Temperature for Ice Point Reference Cu CONSTANTAN T2 T1 The temperature sensitivity of silicon integrated circuit transistors is quite predictable and repeatable. This sensitivity is exploited in the AD594/AD595 to produce a temperature related voltage to compensate the reference of “cold” junction of a thermocouple as shown in Figure 16. IRON ICE POINT REFERENCE UNKNOWN TEMPERATURE Figure 14. Thermocouple Voltage with 0°C Reference Since the thermocouple is basically a differential rather than absolute measuring device, a know reference temperature is required for one of the junctions if the temperature of the other is to be inferred from the output voltage. Thermocouples made of specially selected materials have been exhaustively characterized in terms of voltage versus temperature compared to primary temperature standards. Most notably the water-ice point of 0°C is used for tables of standard thermocouple performance. T3 CONSTANTAN T1 An alternative measurement technique, illustrated in Figure 15, is used in most practical applications where accuracy requirements do not warrant maintenance of primary standards. The reference junction temperature is allowed to change with the environment of the measurement system, but it is carefully measured by some type of absolute thermometer. A measurement of the thermocouple voltage combined with a knowledge of the reference temperature can be used to calculate the measurement junction temperature. Usual practice, however, is to use a convenient thermoelectric method to measure the reference temperature Cu IRON Cu Figure 16. Connecting Isothermal Junctions Since the compensation is at the reference junction temperature, it is often convenient to form the reference “junction” by connecting directly to the circuit wiring. So long as these connections and the compensation are at the same temperature no error will result. OUTLINE DIMENSIONS Dimensions shown in inches and (mm). Cerdip (Q) Package 0.77 ±0.015 (19.55 ±0.39) 0.430 (10.92) 0.040 (1.02) R 14 8 0.265 0.290 ±0.010 (6.73) (7.37 ±0.25) 1 14 8 0.260 ±0.020 (6.6 ±0.51) 0.310 (7.87) 7 1 PIN 1 0.31 ±0.01 (7.87 ±0.25) 0.700 ±0.010 (17.78 ±0.25) 0.035 ±0.010 (0.89 ±0.25) ( 7 PIN 1 0.095 (2.41) 0.085 (2.16) 0.125 (3.18) MIN 0.047 ±0.007 +0.003 0.100 (1.19 ±0.18) 0.017 –0.002 (2.54) BSC 0.43 +0.08 –0.05 PRINTED IN U.S.A. TO-116 (D) Package 0.180 ±0.030 (4.57 ±0.76) 0.035 ±0.010 (0.889 ±0.254) 0.032 (0.812) 0.30 (7.62) REF 0.018 (0.457) 0.600 (15.24) BSC ( –8– 0.148 ±0.015 (3.76 ±0.38) 0.180 ±0.030 (4.57 ±0.76) 0.125 3.175) MIN 0.01 ±0.002 (0.25 ±0.05) 0.300 (7.62) REF SEATING PLANE 0.100 (2.54) BSC 15° 0° 0.010 ±0.001 (0.254 ±0.025) REV. C