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 14 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 42 shows a frequency response plot for the HPF filter.
|P0_R107_D[1:0] : HPF_SEL[1:0]||-3-dB CUTOFF FREQUENCY SETTING||-3-dB CUTOFF FREQUENCY AT 16-kHz SAMPLE RATE||-3-dB CUTOFF FREQUENCY AT
48-kHz SAMPLE RATE
|00||Programmable 1st-order IIR filter||Programmable 1st-order IIR filter||Programmable 1st-order IIR filter|
|01 (default)||0.00025 × fS||4 Hz||12 Hz|
|10||0.002 × fS||32 Hz||96 Hz|
|11||0.008 × fS||128 Hz||384 Hz|
Equation 1 gives the transfer function for the first-order programable IIR filter:
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 15 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. These programmable coefficients are 32-bit, two’s complement numbers. Table 15 shows the filter coefficients for the first-order IIR filter.
|FILTER||FILTER COEFFICIENT||DEFAULT COEFFICIENT VALUE||COEFFICIENT REGISTER MAPPING|
|Programmable 1st-order IIR filter (can be allocated to HPF or any other desired filter)||N0||0x7FFFFFFF||P4_R72-R75|