You can use an ADT7461 single-channel temperature monitor; an ADG708 low-voltage, low-leakage CMOS 8-to-1 multiplexer; and three standard 2N3906 p-n-p transistors to measure the temperature of three separate remote thermal zones (Figure 1). Multiplexers have resistance, RON, associated with them; the channel matching and flatness of this resistance normally result in a varying temperature offset. This system uses the ADT7461 temperature monitor, which can automatically cancel resistances in series with the external temperature sensors, allowing its use as a multichannel temperature monitor. The resistance automatically cancels out, so RON flatness and channel-to-channel variations have no effect. Resistance associated with the pc-board tracks and connectors also cancels out, allowing you to place the remote-temperature sensors some distance from the ADT7461. The design requires no user calibration, so the ADT7461 can connect directly to the multiplexer. The ADT7461 digital temperature monitor can measure the temperature of an external sensor with ±1 °C accuracy. The remote sensor can be a monolithic or a discrete transistor and normally connects to the D+ and D– pins on the ADT7461. In addition to the remote-sensor-measurement channel, the ADT7461 has an on-chip sensor.
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| Figure 1. | This system measures the temperature of three remote thermal zones. | |
The diode-connected transistors have emitters that connect and then connect to the D+ input of the ADT7461, and each of the base-collector junctions connects to a separate multiplexer input (S1 to S3). You connect the selected remote transistor to the D– input on the ADT7461 by addressing the multiplexer, which address bits A2, A1, and A0 digitally control. The ADT7461 then measures the temperature of whichever transistor is connected through the multiplexer. The ADT7461 measures the temperature of the selected sensor without interference from the other transistors. Figure 2 shows the results of measuring the temperature of three remote temperature sensors. The sensor at address 000 is at room temperature, the sensor at address 001 is at a low temperature, and the sensor at address 010 is at a high temperature. When you select no external sensor, the “open-circuit” flag in the ADT7461 register activates, and the Alert interrupt output asserts. You can expand the system to include as many external temperature sensors as your design requires. The limiting factor on the number of external sensors is the time available to measure all temperature sensors. If your design requires two-wire serial control of the multiplexer, you can use an ADG728 in place of the ADG708.
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| Figure 2. | The system in Figure 1 measures ambient (address 000), cold (address 001), and hot (address 010) temperatures. |
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