SLASEC8B February 2017 – January 2019 PGA460-Q1
The ultrasonic echo signal is an amplitude-modulated signal with an underlying carrier frequency equal to the drive frequency of the ultrasonic transducer. The DSP band-pass filter block allows the frequencies outside of the observed frequency band to be filtered out and therefore reducing the amount of noise influencing the ultrasonic echo signal.
The center frequency of the filter is automatically adjusted based on the driving frequency set by the FREQ bit while the bandwidth of the filter can be adjusted from 2 kHz to 8 kHz in steps of 2 kHz by setting the BPF_BW bit in the INIT_GAIN EEPROM register.
The band-pass filter is a second-order Butterworth IIR type filter. On power up, the PGA460-Q1 device calculates the coefficients and places them in the BPF_A2_xSB, BPF_A3_xSB, and BPF_B1_xSB registers. These registers can be overwritten by the user to reconfigure the filter. However, if the FREQ or BPF_BW bit is changed, the coefficient calculation sequence is rerun and the device rewrites these registers. In case the FREQ_SHIFT bit is set to 1 (80- to 480-kHz driving frequency range), the band-pass filter coefficients are not calculated automatically by the PGA460-Q1 device. In this case the MCU is required to write these values through the UART or USART interface.