ZHCSMK4A september 2022 – july 2023 ADS131B26-Q1
PRODUCTION DATA
The ADC1y ΔΣ modulator bitstream feeds into a digital filter. The digital filter is a linear phase, finite impulse response (FIR), low-pass sinc3 filter that attenuates the out-of-band quantization noise of the ΔΣ modulator. The low-resolution, high-speed modulator output is decimated and downsampled by the digital filter to produce high-resolution ADC data at an output data rate of fDATA. The decimation factor, defined as per Equation 5, is called the oversampling ratio (OSR).
The OSR determines the amount of averaging that is applied to the modulator output in the digital filter and, therefore, the filter bandwidth and conversion noise. Higher OSRs lead to lower filter bandwidth and better noise performance.
The OSR is programmable within a range of 64 and 8192 using the OSR13y[2:0] bits. Table 9-4 lists the OSR settings for ADC1y together with the corresponding output data rates and –3-dB frequencies of the sinc3 filter for a nominal MCLK frequency of 8.192 MHz.
OSR | OUTPUT DATA RATE (fDATA) | –3-dB FREQUENCY |
---|---|---|
64 | 64 kSPS | 16.8 kHz |
128 | 32 kSPS | 8.4 kHz |
256 | 16 kSPS | 4.2 kHz |
512 | 8 kSPS | 2.1 kHz |
1024 | 4 kSPS | 1.0 kHz |
2048 | 2 kSPS | 524 Hz |
4096 | 1 kSPS | 262 Hz |
8192 | 500 SPS | 131 Hz |
Figure 9-3 and Figure 9-4 show the filter frequency response of the sinc3 filter. The sinc3 filter has infinite attenuation at integer multiples of the output data rate except for integer multiples of fMOD. As with all digital filters, the digital filter response repeats at integer multiples of the modulator frequency, fMOD. The data rate and filter notch frequencies scale with fMOD.