Asynchronous Adaptive Threshold Level Crossing ADC for Wearable ECG Sensors

The level crossing ADC generates digitized samples consisting of the magnitude of input signal and time interval between two consecutive level crossings when the input signal crosses the threshold level. This paper presents a new architecture of low power asynchronous adaptive threshold level crossing (LC) ADC suitable for wearable ECG sensors based on a novel algorithm for determining adaptive threshold. The adaptive threshold was determined by calculating the mean of maximum and minimum values of signal in a predetermined window. Polynomial interpolation was used to reconstruct the signal. A signal to noise distortion ratio of 57.50 dB and a mean square error (MSE) measure of 1.368*10^-8 V² was achieved by the proposed algorithm for a 1 mV, 10 Hz input sinusoidal signal in MATLAB. The asynchronous adaptive threshold LC ADC operating from a supply voltage of 0.8 V occupied a layout area of 266.33*331.385 μm² when implemented in CADENCE virtuoso using 180 nm technology. The designed circuit consumes an average power of 367.6 nW for a 1mVpp, 10 Hz input sinusoidal signal when simulated in Virtuoso.