Measure temperature in remote locations

Helen Stapleton, Analog Devices, Limerick, Ireland -- EDN, 10/12/2000

The compact and low-power temperature-to-frequency converter in Figure 1 is ideally suited for measuring temperature in remote locations. An AD22100A temperature sensor generates a voltage proportional to the ambient temperature. This voltage then drives an AD7740 VFC (voltage-to-frequency converter), which in turn generates a digital pulse train. The duty cycle of this pulse train is directly proportional to the ambient temperature. A 1-MHz crystal defines the full-scale frequency on f_OUT. In noisy environments, you can use a single optocoupler to feed f_OUT back to the host computer. The optocoupler provides more than 2 kV of isolation between the transducer and the host. The host counts the f_OUT pulses. The resolution of this system is a function of the number of f_OUT pulses counted for each temperature reading. A count interval of 2^N/f_OUT(maximum) corresponds to N-bit resolution. Hence, a trade-off exists between resolution and conversion time. The synchronous nature of the AD7740 produces a more temperature-stable transfer function than asynchronous VFCs, which are prone to errors introduced by an external capacitor.

Both the temperature sensor and the VFC operate from the 5V supply. Connecting the REFIN of the AD7740 to the same supply eliminates the need for an external precision reference. Because the circuit is fully ratiometric, the outputs of both the temperature sensor and the VFC scale with the supply voltage, and any errors caused by supply variations cancel each other out. The AD22100A temperature sensor operates over –40 to +85°C. The corresponding output-voltage range is nominally 0.475 to 3.288V. The AD7740 converts this voltage range to a frequency range of 176 to 626 kHz. The transfer functions of these devices are as follows:

AD22100A:

V_OUT–V_DD/5×[1.375+(22.5 mV/°C×T_A)]V.

AD7740:

f_OUT=CLKIN×[0.1+(0.8×V_IN/V_DD)]Hz.

For the circuit in Figure 1, f_OUT=(320+3.6T_A) kHz. The AD22100A is available in a TO-92 package, and the AD7740 comes in an eight-lead SOT-23 package. The ICs require minimal external components. When V_IN is buffered (BUF is at logic 1), the power consumption in the two ICs is typically 8 mW. This figure does not include the power consumed by the crystal, which is a function of the effective series resistance and the associated capacitor values.