Monte-Carlo simulations can be used to generate a statistical estimate of typical variations from component tolerances. The Monte-Carlo Simulations shown in Figure 2-2 demonstrate the expected spectral response of the AAF on ADS127L21EVM-PDK. The mean and sigma displayed in the blue box supports the calculated expectations of the cutoff and the large swing of cutoffs spanning about 13kHz.

Monte-Carlo 100 runs of AAF as installed on the ADS127L21

In contrast, the same simulation with the new anti-aliasing filter gives a result shown in Figure 2-3 The same mean bandwidth is more than 100kHz larger with a sigma three times tighter than that seen in Figure 2-2.

Monte-Carlo 100 runs of AAF as installed on the ADS127L21

Phase is another important metric since a consistent time delay between the input and output of the AAF can be accounted for by the computer. If the variations are inconsistent, voltages can be collected on unexpected clock cycles which can cause issues when processing data. For example, when taking power measurements with a power analyzer, if the phase is not accounted for when measuring the current and voltage, the calculated power can be inaccurate. In the same filter simulation, the phase can be seen in Figure 2-4 with a somewhat constant slope between 0Hz and 100kHz.

Group delay can be calculated from phase or measured directly. The linear phase leads to a very flat group delay The simulation in Figure 2-5 shows that the group delay is very flat from DC to 100kHz.

The ADS127L21 has four speed modes and a programmable digital filter with data rates up to 512kSPS using the wideband filter and up to 1365kSPS using the low-latency filter, offering trade-offs between latency, bandwidth, resolution, and power consumption. In max-speed mode, ADS127L21 can have a modulator clock frequency of 16.384MHz. The filter must have good attenuation at the modulator frequency. The stopband attenuation at the modulator clock frequency minus the filter bandwidth (16.2MHz) is -112dB in simulation, as seen in Figure 2-7. The digital filter can attenuate all signals between the digital filter bandwidth, up to the modulator frequency.