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ADS8320EB2K5 Datasheet(PDF) 9 Page - Texas Instruments |
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ADS8320EB2K5 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 21 page ADS8320 9 SBAS108D www.ti.com NOISE The noise floor of the ADS8320 itself is extremely low, as can be seen from Figures 1 and 2, and is much lower than competing A/D converters. It was tested by applying a low- noise DC input and a 5.0V reference to the ADS8320 and initiating 5000 conversions. The digital output of the A/D 2 2510 3 2490 4 Code 56 00 00 1 FIGURE 1. Histogram of 5000 Conversions of a DC Input at the Code Transition. FIGURE 2. Histogram of 5000 Conversions of a DC Input at the Code Center. 2 72 3 4864 4 Code 56 64 0 00 1 converter varies in output code due to the internal noise of the ADS8320. This is true for all 16-bit SAR-type A/D converters. Using a histogram to plot the output codes, the distribution should appear bell-shaped with the peak of the bell curve representing the nominal code for the input value. The ±1σ, ±2σ, and ±3σ distributions represents the 68.3%, 95.5%, and 99.7%, respectively, of all codes. The transition noise can be calculated by dividing the number of codes measured by 6 and this yields the ±3σ distribution or 99.7% of all codes. Statistically, up to 3 codes could fall outside the distribution when executing 1000 conversions. The ADS8320, with < 3 output codes for the ±3σ distribution, yields a < ±0.5LSB transition noise. Remember, to achieve this low noise performance, the peak-to-peak noise of the input signal and reference must be < 50 µV. AVERAGING The noise of the A/D converter can be compensated by averaging the digital codes. By averaging conversion re- sults, transition noise is reduced by a factor of 1/ √n, where n is the number of averages. For example, averaging four conversion results reduces the transition noise by 1/2 to ±0.25 LSBs. Averaging should only be used for input signals with frequencies near DC. For AC signals, a digital filter can be used to low-pass filter and decimate the output codes. This works in a similar manner to averaging; for every decimation by 2, the signal- to-noise ratio improves 3dB. DIGITAL INTERFACE SIGNAL LEVELS The digital inputs of the ADS8320 can accommodate logic levels up to 5.5V regardless of the value of VCC. Thus, the ADS8320 can be powered at 3V and still accept inputs from logic powered at 5V. The CMOS digital output (DOUT) swings 0V to VCC. If VCC is 3V and this output is connected to a 5V CMOS logic input, then that IC may require more supply current than normal and may have a slightly longer propagation delay. |
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