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Eduardo Pinheiro, Octavian Postolache and Pedro Girão

Fixed-Point Implementation of Infinite Impulse Response Notch Filters

Many studies have been developed aiming to improve digital filters realizations, recurring to intricate structures and analyzing probabilistically the error's behavior. The work presented in this paper analyzes the feasibility of fixed-point implementation of classical infinite impulse response notch filters: Butterworth, Chebyshev I and II, and elliptic. To scrutinize the deformations suffered for distinct design specifications, it is assessed: the effect of the quality factor and normalized cut-off frequency, in the number of significant bits necessary to represent the filter's coefficients. The implications brought to FPGA implementation are also verified.

The work focuses especially on the implementation of power line notch filters used to improve the signal-to-noise ratio in biomedical signals. The results obtained, when quantizing the digital notch filters, show that by applying second-order sections decomposition, low-order digital filters may be designed using only part of double precision capabilities. High-order notch filters with harsh design constraints are implementable using double precision, but only in second-order sections. Thus, it is revealed that to optimize computation time in real-time applications, an optimal digital notch filter implementation platform should have variable arithmetic precision.

Considering these implementation constraints, utmost operation performance is finally estimated when implementing digital notch filters in Xilinx Virtex-5 field-programmable gate arrays. The influence of several design specifications, e.g. type, and order, in the filter's behavior was evaluated, namely in regard to order, type, input and coefficient number of bits, quality factor and cut-off frequency. Finally the implications and potential applications of such results are discussed.

Open access

Eduardo Pinheiro, Octavian Postolache and Pedro Girão


Embedding cardiac system sensing devices in wheelchairs is both necessary and attractive. Elders, diabetics, or stroke victims are a substantial group needing permanent cardiac monitoring, without restriction of their already limited mobility. A set of sensing devices was embedded in a wheelchair to monitor the user without his awareness and intervention. A dual-wavelength reflection photoplethysmogram (PPG), and a ballistocardiogram (BCG) based on MEMS accelerometers and on electromechanical film sensors are output by the hardware. Tests were conduced on twenty one subjects, for an immobility scenario. Additional recordings were made for helped propulsion over a tiled floor course, with good results in keeping track of acceleration BCG and PPG. A treadmill was also used for tests, providing a smooth floor and constant speed and inclination. The PPG and acceleration BCG could be continuously monitored in all the tests. The developed system proves to be a good solution to monitor cardiac activity of wheelchair users even during motion.

Open access

E. Pinheiro, O. Postolache and P. Girão

Impedance cardiography is a technique developed with the intent of monitoring cardiac output. By inspecting a few properties of the obtained signal (impedance cardiogram (ICG), the left ventricular ejection time can be derived with certainty, and an estimate of cardiac output is available. This signal is nowadays used in non-invasive monitoring, requiring the placement of electrodes over the subject’s skin, either ECG-type or in the form of encircling bands. The work here reported describes the implementation steps and the results obtained when embedding the ICG circuitry in a wheelchair’s backrest. The subject is seated normally, is normally dressed, and is completely unaware that monitoring is taking place. That means that the variation of tenths of ohm produced due to the cardiac cycle has to be detected with electrodes having substantial coupling impedance. Contactless ICG with embedded sensors was developed and tested on fourteen healthy subjects. The signal was always acquired, although respiratory activity is also important, constituting a noteworthy innovation in the area.