XIX IMEKO World Congress Fundamental and Applied Metrology September 611, 2009, Lisbon, Portugal ON THE DESIGN OF LOW-POWER SIGNAL CONDITIONERS FOR RESISTIVE SENSORS Ramon Casanella 1 , Ramon Pallàs-Areny 1 1 Instrumentation, Sensors and Interfaces Group (Technical School of Castelldefels, Universitat Politècnica de Catalunya), Castelldefels (Barcelona), Spain, email: Ramon.Casanella@upc.edu , ramon.pallas@upc.edu Abstract  This work analyzes power consumption in signal conditioning circuits for resistive sensors. We show that, for a given dynamic range for the measurand, simple conditioners based on voltage dividers or Wheatstone bridges directly connected to an analog-to-digital converter (ADC) do not usually have minimal power consumption. We develop analytical guidelines to achieve the optimal power design for signal conditioners and apply them to the actual design of the conditioner for an RTD sensor. We show that by adding a low-power amplifier plus a passive low-pass filter to a voltage divider sensor interface, the power consumption can be significantly reduced as compared to that of standard voltage dividers designed for maximal sensitivity. Keywords: optimal power design, signal conditioning circuits, resistive sensors 1. INTRODUCTION Power consumption has lately become a main issue in digital [1] and analog [2] electronic circuit design. Low- power design, traditionally associated to autonomous portable equipment [3], has gained momentum with the development of sensor networks and distributed data acquisition systems [4]. These two last areas in particular need low-power analog signal conditioning circuits but research efforts to reduce power consumption in these applications have mainly addressed communication protocols [5] without considering that many autonomous systems spend far more time (and energy) in measuring than in communicating. On the other hand, low-power analog design has traditionally focused more on reducing power consumption in each individual IC than on the optimal power design of the whole signal chain. Furthermore, to the best of our knowledge, power analysis or low-power design criteria are not usually considered for the most common sensor signal conditioners such as voltage dividers or Wheatstone bridges [6, 7, 8]. Rather, the trade-off between high sensitivity and linearity is addressed. In this work we analyze power consumption in the measurement chain for resistive sensors. First, we establish the design guidelines that yield minimal power dissipation for a given dynamic range of the measurand. Then, we compare power dissipation in two designs for an RTD conditioner: one according to common practice and the other one according to the guidelines developed, and experimentally assess the benefits of the novel design approach proposed. 2. THEORETICAL ANALYSIS Fig. 1 shows the basic measurement chain to convert a measurand into a digital value. We divide the analog signal conditioning block into the interface circuit, defined as the minimal circuitry to convert the sensor quantity into a voltage, and the analog processor, defined as the additional circuitry to match levels (e.g. gain/offset stages), reduce noise (e.g. filtering stages), demodulate, linearize, or any other analog signal processing function previous to the ADC [9]. Any measurement of a quantity x has a desired range x FS (= x max – x min ) and resolution x, whose quotient determines the so-called dynamic range [9] that should be ensured by each block of the measurement chain: x x   FS DR (1) On the other hand, the total power consumption P T of the measuring system is the sum of the power dissipated in each block, ADC a i s T P P P P P     (2) being P s , P i , P a , and P ADC the power consumption of sensor, interface circuit, analog processor and ADC respectively. The best design from the point of view of power dissipation is achieved when P T is minimal for the dynamic range required for the application in hand. Because the sensor interface circuit depends on the particular sensor type Fig. 1. Measurement chain to obtain a digital value from a measurand. Measurand Sensor Interface circuit Analog Processor ADC x y V i V a 787 ISBN 978-963-88410-0-1 © 2009 IMEKO