The interface response is digitally tuned, compensating non-linearities in the sensor response and undesired effects due to circuit components mismatching. The power consumption can be reduced by powering off the analogue components when the system is not sampling the sensor outputs. In this way, the battery life in portable systems is extended. The proposed circuits were integrated in a 0.35 ��m standard digital CMOS process. The following sections show the use of current-mode analogue adaptive systems in sensor conditioning: design and characteristics of the proposed circuits, experimental measurements and loading effects. The feasibility of a complex conditioning architecture based on these cells is also demonstrated.2.
?Adaptive SystemsAdaptive circuits in sensor conditioning permit tuning the circuit operation to match changes in sensor response due to ageing, environmental effects or sensor replacement, providing optimum performance under any condition by means of a tuning/calibration process. Perceptron [8-9] features make it a worthy candidate to be used in adaptive analogue-digital signal processing, where system operation is programmed by adjusting the values stored in a set of registers. Due to their robustness to circuit non-idealities, mismatches and offsets, tuning operation can be achieved by means of perturbative algorithms [10].To embed sensor network units in a portable system, they must work with compact low-voltage batteries, making it difficult to process the data in voltage mode. Current-mode electronics give better results at low bias voltages [11].
The proposed processing elements were designed to provide good transfer features and impedance matching between them. The main current-mode circuits presented are a four-quadrant analogue-digital current AV-951 multiplier (ADM) and a current amplifier that performs a logistic function. By properly combining both processing cells, it is possible to design a non-linear adaptive unit (Figure 1) which will be the basic cell in a multi-layer perceptron designed to extend the linear range of a sinusoidal sensor. [12-13].Figure 1.Proposed adaptive processing unit.3.?Arithmetic CircuitsThe conditioning circuit basically consists of two main blocks: an analog-digital current-mode four-quadrant multiplier (ADM) and a logistic circuit (LC) that performs a non-linear operation.3.1.
Four-Quadrant MultiplierThe four-quadrant current-mode multiplier (Figure 2) is based on a MOS R-2R current ladder (M-2M ladder) [14], and a current follower as the sign circuit (SC). This circuit is a modified version of a cell that has been previously reported in the literature [15-16].Figure 2.Four-quadrant analog-digital current multiplier.As shown in Figure 2, the most significant bit (b7) determines the direction of the current flow, that is, it selects the sign of the operation.