Electronic Design

RTD Auto-Nulling "Delta-T" Thermometer

Some precision temperature-measurement applications, such as the acquisition of the temperature rise of a heat sink in response to a thermal load, are inherently concerned with the temperature change relative to an initial value, rather than simply the absolute temperature.

In these types of applications, an important improvement in measurement resolution can be gained from the scale expansion. This is made possible by acquiring the initial baseline temperature and subtracting it from subsequent measurements prior to analog-to-digital conversion. An autonulling thermometric circuit can perform this function automatically using digitally controlled potentiometers as the baseline temperature memory (see the figure).

The basis for temperature sensing in this circuit is RT, a standard 100 Ω at 25°C platinum RTD. Such RTD devices have a highly stable and accurate temperature coefficient of +0.385 Ω/°C. Therefore the ~1 mA excitation current provided by R1 leads to a temperature-dependent signal of −385 µV/ °C.

R2, P1, and R3 complete a ratiometric bridge with A1 as the bridge amplifier. The feature of the circuit that uses digitally controlled pots to act as feedback elements is the process which automatically captures and holds the initial reference temperature, which is subtracted from subsequent readings.

Acquiring a reference temperature is initiated by bringing the 5-V logic signal ACQUIRE low. This causes S3 to open the feedback loop around A1, and simultaneously enables the S2-S3 multivibrator. This causes P1 to sample A1’s output at a 100-Hz rate and drive A1 toward null.

If ACQUIRE is held low long enough (one second will always suffice), P1 will be driven to the setting that causes A1 to dither around zero output. This indicates that P1’s setting is alternating between the two values that bracket optimum to null.

When ACQUIRE is returned to logic “1,” P1 will retain the bridge ratio needed to cancel the temperature present at RT during the nulling process. As a result, the thermometric signal presented to the scaling op amp A2 will be referenced to this initial TO. Because the P1 setpoint memory is digital, it will hold this ratio forever. That’s unless ACQUIRE is deliberately put through another 0/1 cycle to acquire a new TO, or power is removed from the thermometer.

A2 applies the necessary gain, digitally fine-adjusted by P2, of 0.01/0.000385 = 25.97 TO achieve an output scale factor of 10 mV/°C.

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