Pressure sensor receives the cuff
pressure signal from the cuff. A High CMRR instrument
amplifier is used to amplify this signal and then the
pre-amplified signal goes through a 4-order high-pass filter
and a 4-order low-pass filter. Finally, the AC signal
(bloodstream oscillate signal) is obtained from the output of
low-pass filter and DC signal (pressure value signal) is
obtained from the output of instrument amplifier. AC signal
and DC signal are both sent to ARM platform and
transformed into digital signal by the built-in ADC of
STM32F103ZE, sampling rate is 200Hz. The gain of this
instrument amplifier is 34. The cut-off frequency of the
high-pass filter is 0.5 Hz, and the cut-off frequency of the
low-pass filter is 40 Hz. Total gain of AC signal after the
instrument amplifier is 300.