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ADS1213UG4 Datasheet(PDF) 11 Page - Texas Instruments |
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ADS1213UG4 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 51 page  ADS1212, 1213 11 SBAS064A f DATA = f XIN • Turbo Mode 128 • Decimation Ratio +1 () f MOD = f XIN • Turbo Mode 128 fXIN—The frequency of the crystal oscillator or CMOS compatible input signal at the XIN input of the ADS1212/13. fMOD—The frequency or speed at which the modulator of the ADS1212/13 is running, given by the following equation: fSAMP—The frequency or switching speed of the input sampling capacitor. The value is given by the following equation: fDATA, tDATA—The frequency of the digital output data produced by the ADS1212/13 or the inverse of this (the period), respectively, fDATA is also referred to as the data rate. Conversion Cycle—The term “conversion cycle” usually refers to a discrete A/D conversion operation, such as that performed by a successive approximation converter. As used here, a conversion cycle refers to the tDATA time period. However, each digital output is actually based on the modu- lator results from the last three tDATA time periods. DIGITAL FILTER The digital filter of the ADS1212/13 computes the output result based on the most recent results from the delta-sigma modulator. The number of modulator results that are used depend on the decimation ratio set in the Command Regis- ter. At the most basic level, the digital filter can be thought of as simply averaging the modulator results and presenting this average as the digital output. While the decimation ratio determines the number of modu- lator results to use, the modulator runs faster at higher Turbo Modes. These two items, together with the ADS1212/13 clock frequency, determine the output data rate: Also, since the conversion result is essentially an average, the data rate determines where the resulting notches are in the digital filter. For example, if the output data rate is 1kHz, then a 1kHz input frequency will average to zero during the 1ms conversion cycle. Likewise, a 2kHz input frequency will average to zero, etc. In this manner, the data rate can be used to set specific notch frequencies in the digital filter response (see Figure 1 for the normalized response of the digital filter). For example, if the rejection of power line frequencies is desired, then the data rate can simply be set to the power line frequency. Figures 2 and 3 show the digital filter response for a data rate of 50Hz and 60Hz, respectively. f SAMP = f XIN • Turbo Mode • Gain Setting 128 FILTER RESPONSE Frequency (Hz) –40 –60 –80 –100 –120 –140 –160 45 46 47 48 49 50 51 52 53 54 55 FILTER RESPONSE Frequency (Hz) 0 –20 –40 –60 –80 –100 –120 –140 –160 0 50 100 150 200 250 300 NORMALIZED DIGITAL FILTER RESPONSE Frequency (Hz) 0 –20 –40 –60 –80 –100 –120 –140 –160 01 2 3 4 5 6 FIGURE 3. Digital Filter Response at a Data Rate of 60Hz. FIGURE 1. Normalized Digital Filter Response. FIGURE 2. Digital Filter Response at a Data Rate of 50Hz. If the effective resolution at a 50Hz or 60Hz data rate is not adequate for the particular application, then power line fre- quencies could still be rejected by operating the ADS1212/13 at 25/30Hz, 16.7/20Hz, 12.5/15Hz, etc. If a higher data rate is needed, then power line frequencies must either be rejected before conversion (with an analog notch filter) or after conversion (with a digital notch filter running on the main controller). FILTER RESPONSE Frequency (Hz) –40 –60 –80 –100 –120 –140 –160 55 56 57 58 59 60 61 62 63 64 65 FILTER RESPONSE Frequency (Hz) 0 –20 –40 –60 –80 –100 –120 –140 –160 0 50 100 150 200 250 300 f DATA = f XIN • Turbo Mode 128 • Decimation Ratio +1 () ,t DATA = 1 f DATA |
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