The finite impulse response (FIR) filter of ADC1 is a coefficient-based filter that provides simultaneous rejection of 50-Hz and 60-Hz line cycle frequencies and harmonics. The FIR filter data rates are 2.5, 5, 10 and 20 SPS. All of the FIR data rates settle within a single conversion cycle. As shown in Figure 9-13, the FIR filter section receives data from the second-stage sinc filter at 600 Hz. The FIR filter section decimates by 30 to yield the output data rate of 20 SPS. A first-order averager (sinc1) with variable decimation provides the data rates of 10 SPS, 5 SPS, and 2.5 SPS.
As shown in Figure 9-22 and Figure 9-23, the FIR filter frequency response has a series of response nulls close to 50 Hz and 60 Hz. The response nulls repeat close to the 50-Hz and 60-Hz harmonics. The FIR frequency response superimposes with the response of the 600-SPS pre-stage filter.
Figure 9-24 is the FIR filter response at 10 SPS. As a result of the sinc1 averager in the FIR filter block, new frequency-response nulls are superimposed to the response in Figure 9-22. The first of the added response nulls occur at 10 Hz. Additional nulls occur at folded frequencies around 20-Hz multiples. These additional nulls are seen at 10 Hz and 30 Hz.
Similar to the response of the sinc filter, the overall FIR filter frequency has a low-pass response that rolls off high frequencies of the signal. The response is such that the FIR filter limits the bandwidth of the input signal. The FIR filter signal bandwidth depends on the output data rate. Table 9-5 lists the –3-dB filter bandwidth of the FIR filter. The total system bandwidth is the combined individual responses of the digital filter, the ADC antialias filter, and external filter components.
|DATA RATE (SPS)||–3-dB BANDWIDTH (Hz)|