Electronic Design

Simplify Isolated Temperature Sensing With Single-Wire Sensors

Galvanic isolation of sensors, such as temperature sensors, is required since these sensors are often mounted in "mechanically inconvenient" locations. "Mechanically inconvenient" often implies an electrically noisy environment, an environment where poor grounding can cause ground loops, or perhaps an equipment or human safety issue with the relative potential between the location of the sensor and the data-acquisition system. New families of temperature sensors ease the design of such isolated systems by virtue of a new, simple, single-wire method of digitally multiplexed outputs. In addition, they bring very low levels of power consumption, simplifying the choices and cost of providing power to the isolated side.

Isolating a temperature sensor with a unidirectional output is a straightforward design, such as when using a temperature sensor with a frequency or period output, or a thermostatic switch output (Fig. 1). This circuit takes advantage of a new family of sensors whose outputs are either a period proportional to temperature (MAX6576) or frequency proportional to temperature (MAX6577). Simple temperature switches whose outputs become active at a designed-in temperature also can be used in this circuit.

Digitally addressable sensors have been available for some time, and prior articles have demonstrated how to interface such devices on the I2C or SMBus1,2. However, the I2C and SMBus buses require two isolation circuits for their separate clock and data lines.

A new type of sensor family, the MAX6575H/L, enables the multiplexing of up to eight sensors on a simple single-wire bidirectional digital bus. These sensors require a command signal from the processor, to which they respond after a predetermined delay (covered in detail in the data sheets). The bidirectional bus requirement is readily satisfied by the same isolation method used for the I2C bus, with the simplification that only a single bidirectional isolator circuit is required.

Although the design of the isolator in Figure 2 is symmetrical, for the sake of this description, the left side connects to the isolated slave devices while the right side goes to the master (typically a microcontroller, microprocessor, or a digital I/O line to a computer). If the computer takes the right line low, the photodiode in optocoupler 1 turns on the transistor, pulling the slave data line low. The transistor in optocoupler 1 pulls the data line down (via D2) in such a way that the photodiode can't be activated. This would return the signal around the loop and latch the circuit.

Data transfer in the opposite direction functions identically except via optocoupler 2. The circuit of Figure 2 is capable of driving a total of eight MAX6575 devices in any combination of isolated or non-isolated sensors. Also shown in Figure 2 is a block depicting the isolated power supply or an alternative battery power.

The typical bidirectional communication sequence used by the sensors is shown in Figure 3. Once the DO pin is pulled low and then released, the control of the DO pin is then transferred to the MAX6575L/H. The temperature conversion begins on the falling edge of the externally triggered pulse.

The DO line is pulled low by the MAX6575 at a later time, determined by the device temperature and the Time Select pins (TS1, TS0). The DO line remains low for 5T µs, where T is the temperature in degrees Kelvin. The temperature of the device is represented by the edge-to-edge delay of the externally triggered pulse and the falling edge of the subsequent pulse originating from the device. The Start pulse should be low for at least 2.5 ms (tSTART).

The temperature, in degrees Celsius, may be calculated as follows:

T(°C) = \[tDx(µs)/timeout multiplier (µs/°K)\] − 273.15°K

The multiplier value for each sensor, determined by their TS0 and TS1 line selections (as well as exact model number, refer to data sheet for details) is:

For IC3, tD1 = 5 µs/°K
For IC4, tD2 = 20 µs/°K
For IC5, tD3 = 80 µs/°K

References:

  1. Steele, J., "Isolate Data Acquisition On A Bidirectional Bus," Electronic Design, Nov. 6, 1995.
  2. Steele, J., "Temperature Sensor Uses I2C Isolator," EDN, June 6, 1996.
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