18.4.2.3.2 Heterodyne (Synchronous Demodulation)

The CIC filter can also be used to digitally demodulate the incoming signal. This can be activated or deactivated using the DEMODEN (CICCON1[29]) bit. Digital demodulation is useful if the required signal is modulated onto a carrier signal (amplitude modulation). The demodulation is performed by multiplying the input data with either ‘1’ or ‘-1’, followed by the low-pass filtering provided by the CIC filter to recover the carrier amplitude.

The filter can perform automatic demodulation when it is operated in external signal source mode based on the excitation signal value when POLSEL(CICCON1[30]) = 1. In this mode, the polarity of the external signal is obtained when the input data sample is acquired. When the polarity input is 1, the demodulator passes the input sample without changes (multiplies by ‘1’). When the polarity input is 0, the demodulator multiplies the input sample by -1. Figure 18-10 illustrates the timing diagram of the CIC filter's demodulation in external signal source mode, where the signal is sampled by the ADC at its peak.

When POLSEL = 0, the demodulation will be based on the value of the POLOVR (CICCON2[0]) bit. In this mode, when POLOVR is set to ‘1’, the module multiplies the input sample by ‘1’, and if POLOVR is set to ‘0’, the module multiplies the input sample by ‘-1’.