8.3.2.1 High-Speed, Low-Power, And Low-Speed Modes

The digital filter configuration is the same in High-Speed, Low-Power, and Low-Speed modes with the oversampling ratio set to 64. Figure 54 shows the frequency response in High-Speed, Low-Power, and Low-Speed modes normalized to f_DATA. Figure 55 shows the passband ripple. The transition from passband to stop band is shown in Figure 56. The overall frequency response repeats at 64x multiples of the modulator frequency f_MOD, as shown in Figure 57.

Figure 54. Frequency Response For High-Speed, Low-Power, And Low-Speed Modes

Figure 55. Passband Response For High-Speed, Low-Power, And Low-Speed Modes

Figure 56. Transition Band Response For High-Speed, Low-Power, and Low-Speed Modes

Figure 57. Frequency Response Out To F_MOD For High-Speed, Low-Power, And Low-Speed Modes

These image frequencies, if present in the signal and not externally filtered, will fold back (or alias) into the passband, causing errors. The stop band of the ADS1278-SP provides 100-dB attenuation of frequencies that begin just beyond the passband and continue out to f_MOD. Placing an anti-aliasing, low-pass filter in front of the ADS1278-SP inputs is recommended to limit possible high-amplitude, out-of-band signals and noise. Often, a simple RC filter is sufficient. Table 2 lists the image rejection versus external filter order.