8.3.13.2 FIR Stage

The second stage of the ADS1282-SP digital filter is an FIR low-pass filter. Data are supplied to this stage from the sinc filter. The FIR stage is segmented into four sub-stages, as shown in Figure 39. The first two sub-stages are half-band filters with decimation ratios of 2. The third sub-stage decimates by 4 and the fourth sub-stage decimates by 2. The overall decimation of the FIR stage is 32. Two coefficient sets are used for the third and fourth sections, depending on the phase selection. Table 20 (in 器件支持) lists the FIR stage coefficients. Table 6 lists the data rates and overall decimation ratio of the FIR stage.

Figure 39. Fir Filter Sub-Stages

As shown in Figure 40, the FIR frequency response provides a flat passband to 0.375 of the data rate (±0.003-dB passband ripple). Figure 41 shows the transition from passband to stop band.

Figure 40. FIR Passband Magnitude Response (F_DATA = 500 Hz)

Figure 41. FIR Transition Band Magnitude Response

Although not shown in Figure 41, the passband response repeats at multiples of the modulator frequency (Nƒ_MOD – ƒ₀ and Nƒ_MOD + ƒ₀, where N = 1, 2, and so forth, and ƒ₀ = passband). These image frequencies, if present in the signal and not externally filtered, fold back (or alias) into the passband and cause errors. A low-pass signal filter reduces the effect of aliasing. Often, the RC low-pass filter provided by the PGA output resistors and the external capacitor connected to CAPP and CAPN provides sufficient signal attenuation.