second array. The squared samples are then numerically low-pass filtered to estimate their mean. The numerical square root of the mean squared value is taken and stored for the kth epoch. The process is repeated until M epochs have been processed; then the average of M estimates of the RMS signal is finally calculated. The only components required are the anti-aliasing LPF, an ADC, some interface chips, and, of course, a PC.

12.7 IC Thermometers

12.7.1Introduction

Temperature measurement is very important in medicine and biology. Many means have been devised to measure temperature, based on the fact that many physical phenomena vary with temperature, including, but not limited to: physical volume expansion (mercury and alcohol thermometers); resistance; EMF generated by the Seebeck (thermoelectric) effect; change in pemittivity of materials; change in reverse current through a pn junction; etc.

Many electronic means have been devised to circumvent use of the slow (and toxic) mercury thermometer. The fact that the resistance of metals increases with temperature has been the basis of one important class of electronic thermometer. The platinum resistance temperature detector (RTD) is widely used in scientific applications (Northrop, 1997). Its resistance is modeled by the truncated power series:

where Ro is the Pt RTD’s resistance at 0∞C and T is the RTD’s temperature in degrees Celsius.

RTD resistance changes are sensed by using a Wheatstone bridge and suitable electronic amplification. Often a look-up table is used to correct for slight nonlinearity in the platinum RTD’s resistance vs. T characteristic. The look-up table can be in the form of an ROM in which correction values are stored. Other metals (Ni, W, Cu, Si) can be used for RTD design, but Pt is the one most widely encountered because its R(T) is fairly linear compared to other metals and it can be used at elevated (industrial) temperatures.

Thermistors are also used with Wheatstone bridges to sense temperature. Thermistors are amorphous semiconductor resistors; they are very nonlinear, but have much greater thermal sensitivity compared to metal RTDs. The resistance of a negative temperature coefficient (NTC) thermistor is modeled by the relation (Northrop, 1997):

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If the temperature coefficient of a resistor is defined by:

then the NTC thermistor’s tempco is:

Beta is typically 4000 K and, for T= 300 K, α = −0.044. By contrast, the α tempco of Pt is +0.00392. It should be remarked that there are also PTC thermistors. The author has used NTC thermistors to measure Ts on the order of 0.0001∞C in in-vitro chemical assays of blood glucose using the enzyme glucose oxidase.

Other electrical/electronic means of temperature measurement use the minute DC voltages generated by thermocouples and thermocouple arrays called thermopiles used for photonic radiation power measurements. The interested reader should consult texts by Northrop (1997), Lion (1959), and Pallàs–Areny and Webster (2001) for further details on thermocouples and thermopiles.

Still other temperature measurement devices have been invented that measure the long-wave infrared (LIR) blackbody radiation from the eardrum. This class of fever thermometer is characterized by a fast response time (seconds), a minimally invasive implementation (inserted in the ear canal), and reasonable expense. The primary sensor is a thermopile or a pyroelectric material (PYM) such as triglicine sulfate or barium titanate. Further details on the Thermoscan‘ thermometers can be found in Northrop (2002).

12.7.2IC Temperature Transducers

Figure 12.35 illustrates a simplified schematic of the Analog Devices’ AD590 temperature-controlled current source (TCCS). AD also makes the AD592 precision TCCS, which uses basically the same circuit. These ICs behave as two-terminal, 1-μA K current sources for supply voltages between +4 ≤ Vcc ≤ +30 V. Analog Devices (1994) gives the following circuit description for the AD590:

The AD590 uses a fundamental property of the silicon transistors from which it is made to realize its temperature proportional characteristic: if two identical transistors are operated at a constant ratio of collector current densities, r, then the difference in their base-emitter voltages will be (kT/q)ln(r). Since both k, Boltzmann’s constant, and q, the charge of an electron, are constant, the resulting voltage is directly proportional to absolute temperature (PTAT). In the AD590, this PTAT voltage is converted to a PTAT current by low temperature coefficient thin film resistors.

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+VCC

Io

(0 V)

FIGURE 12.35

Simplified schematic of Analog Devices’ AD590 analog temperature-controlled current source (TCCS) temperature sensor.

The total current of the device is then forced to be a multiple of this PTAT current. Referring to Figure [12.35], the schematic diagram of the AD590, Q8 and Q11 are the transistors that produce the PTAT voltage. R5 and R6 convert the voltage to current. Q10, whose collector current tracks the collector currents in Q9 and Q11, supplies all the bias and substrate leakage current for the rest of the circuit, forcing the total current to be PTAT. R5 and R6 are laser trimmed on the wafer to calibrate the device at +25∞C.

The AD590 temperature-to-current transducer can operate over a −55 to +150∞C range; the AD592 operates over a −25 to +105∞C range. There are many applications for these electronic TCCSs. Figure 12.36 illustrates a simple op amp circuit that converts 1 μA K to 100 mV/∞C. A chopper-stabilized op amp (CHSOA) is used for low dc drift tempco. The trim pots are used to

© 2004 by CRC Press LLC