To remove the DC offset component and attenuate the undesired low-frequency noise content in the record data, the device supports a programmable high-pass filter (HPF). The HPF is not a channel-independent filter setting but is globally applicable for all ADC channels. This HPF is constructed using the first-order infinite impulse response (IIR) filter, and is efficient enough to filter out possible DC components of the signal. Table 8-17 shows the predefined –3-dB cutoff frequencies available that can be set by using the HPF_SEL[1:0] register bits of P0_R107. Additionally, to achieve a custom –3-dB cutoff frequency for a specific application, the device also allows the first-order IIR filter coefficients to be programmed when the HPF_SEL[1:0] register bits are set to 2'b00. Figure 8-17 shows a frequency response plot for the HPF filter.

Figure 8-17 HPF Filter Frequency Response Plot

Equation 1 gives the transfer function for the first-order programmable IIR filter:

Equation 1.

The frequency response for this first-order programmable IIR filter with default coefficients is flat at a gain of 0 dB (all-pass filter). The host device can override the frequency response by programming the IIR coefficients in Table 8-18 to achieve the desired frequency response for high-pass filtering or any other desired filtering. If HPF_SEL[1:0] are set to 2'b00, the host device must write these coefficients values for the desired frequency response before powering-up any ADC channel for recording. Table 8-18 shows the filter coefficients for the first-order IIR filter.