Transistor junctions monitor temperature zones

Matt Smith, Analog Devices, Limerick, Ireland -- 12/21/2000

A simple circuit uses transistor junctions to monitor multiple temperature zones (Figure 1). The temperature sensors are ordinary, general-purpose, low-cost, diode-connected transistors. The well-known diode equation V_BE =(kT/q)´ln(I_C /I_S ) shows that there is a temperature dependency of approximately 2.2 mV/°C for a base-emitter junction. By forcing a two-level current through the base-emitter junction and measuring the resultant voltage, you can accurately determine the junction temperature, a technique known as DV_BE sensing. To prevent self-heating with this technique, current levels must be low. IC₂ uses this approach and supplies a low-level switched current source on its D+ and D– pins. An on-chip ADC converts the voltage information on D+ and D– into digital data that IC₂ stores in a register.

To monitor multiple-channel temperatures, you need to multiplex the measurement channels. A four-channel differential multiplexer, IC₁ , selects the transistor junction that the circuit measures. The differential multiplexer ensures that D+ and D– remain as differential signals to preserve noise immunity. By cycling through the A₀ and A₁ address lines of the multiplexer, the µC or µP can poll each channel in sequence. If extra channels are necessary, you can add multiplexer channels.

The on-resistance of the multiplexer channel results in a voltage drop across the channel. Therefore, you initially need to calibrate the circuit to remove this error. Fortunately, the error is constant because the channel resistance remains constant. You can use an offset register in IC₂ to store and automatically subtract the offset.

The remote-sensing transistors connect via a twisted-pair cable, and the cable can be as long as 50 ft. In extremely noisy environments, using a shielded twisted pair prevents the noise from interfering with the sensitive measurement. The circuit features a standard two-wire SMBus or I² C interface, enabling communication with a µC or µP.

The circuit can accommodate a theoretical temperature range of –128 to +128°C. However, the practical range is more limited than these temperatures because moisture causes leakage currents and, hence, temperature errors. IC₂ also contains high- and low-limit registers and has an alert output. Thus, you can use the circuit to ensure that temperatures remain within an allowable band. Any deviation outside the limits, either high or low, results in activation of the alert output. The alert line drives an interrupt line on the µC or µP. The circuit can also detect fault conditions, such as open or short circuits, on the sensing elements. Fault conditions generate an alert signal. An interrupt-service routine can then interrogate the status register to provide fault identification and initiate corrective action.

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