Application specific DAC and ADC for a bio-impedance measurement circuit
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A transistor level implementation of a 3 bit digital harmonic canceling (DHC) digital to analog converter (DAC), and a 10 bit successive approximation register (SAR) analog to digital converter (ADC) is designed. The circuits are intended to be a part of a bio-impedance measurement system in which a current is injected into the myocardium tissue and blood pool for the purpose of determining the heart’s stroke volume. The DHC DAC injects a 100 μA[subscript rms] current at a 20 kHz fundamental frequency using appropriately weighted current mirrors from a 160 kHz square-wave reference clock. A [function subscript s] = 160 kS/s ADC is designed with synchronous SAR logic, bottom-plate sampling, and a charge-redistribution capacitive DAC.