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Session: Telemedicine - and Fetal ECG
Session starts: Friday 25 January, 13:00
Presentation starts: 13:00
Room: Lamoraalzaal

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Senad Hiseni (Delft University of Technology)
Rachit Mohan (IMEC Belgium)
Wouter Serdijn (Delft University of Technology)

Abstract:
There is a growing need to provide personalized and preventive healthcare solutions to reduce the burden on the existing infrastructure. This requires monitoring devices to be small and hence there is a need for fully integrated ExG solutions. An important challenge in the design is integrating a large time-constant required for implementing the high-pass characteristic, needed to reject low frequency interference such as baseline wander. For example, for an electrocardiography (ECG) device, the high-pass cutoff frequency is usually specified to be < 1 Hz [1]. Moreover, a resolution of at least 10 bits is required to detect low amplitude signal variation [2]. Pseudo-resistors are widely used to integrate large time constants. However, they are inaccurate as their resistance is determined by leakage currents and they are also non-linear as the resistance depends on the voltage across them. Since the cutoff frequency will vary with the momentary value of the input voltage, there will be distortion near the cutoff frequency, thereby limiting the resolution for frequencies in that region. The switched capacitor technique can also been used for obtaining large time-constants. It is more accurate than pseudo resistors. However, it will require high capacitor ratios and very low frequency clocks to achieve such a low cut-off frequency. Moreover, an anti-alias filter will also be needed which increases the overall system power. Recently a mixed-signal feedback technique has been used to implement the high-pass characteristic [3], [4]. In this technique the signal is processed in the digital domain and fed back to the previous analog stage to achieve the required frequency characteristic. It utilizes the fact that in general, the objective of any front-end is to convert the physiological signal in to a digital one. Filtering can, in principle, be done in either the digital or the analog domain. The choice for an analog or a digital filter would depend on the power budget and the DAC resolution. In [3] and [4] a 7-bit signal converted from a 8-bit one and a 1-bit signal converted from a 12-bit one, respectively, are used to obtain a low cut-off frequency. In both cases, similar to pseudo-resistors, the linearity of the signal near the cut-off frequency will be limited. This paper proposes an ExG front-end with a 10-bit linear high-pass response using the mixed-signal feedback technique along with a ΣΔ ADC. Instead of feeding back the fully converted digital signal, the output of the ΣΔ modulator is fed back via a 1-bit DAC and an analog filter. Since a 1-bit DAC is inherently linear, the overall resolution and linearity depend on the oversampling ratio, the order of the ΣΔ modulator and the linearity of the filter in the feedback path, which can be designed to obtain the desired linearity.