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ADS8324E bảng dữ liệu(PDF) 8 Page - Texas Instruments |
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ADS8324E bảng dữ liệu(HTML) 8 Page - Texas Instruments |
8 / 19 page ADS8324 8 SBAS172A www.ti.com different settling times. This may result in offset error, gain error, and linearity error that changes with both temperature and input voltage. If the impedance cannot be matched, the errors can be lessened by giving the ADS8324 additional acquisition time. The input current on the analog inputs depends on a number of factors: sample rate, input voltage, and source impedance. Essentially, the current into the ADS8324 charges the inter- nal capacitor array during the sample period. After this capacitance has been fully charged, there is no further input current. The source of the analog input voltage must be able to charge the input capacitance (25pF) to the 14-bit settling level within 4.5 clock cycles. When the converter goes into the hold mode, or while it is in the power-down mode, the input impedance is greater than 1G Ω. Care must be taken regarding the absolute analog input voltage. The +In input should always remain within the range of GND – 100mV to VCC + 100mV. The –In input should always remain within the range of GND – 100mV to 2V. Outside of these ranges, the converter’s linearity may not meet specifications. REFERENCE INPUT The external reference sets the analog input range. The ADS8324 will operate with a reference in the range of 500mV to VCC /2. There are several important implications of this. As the reference voltage is reduced, the analog voltage weight of each digital output code is reduced. This is often referred to as the Least Significant Bit (LSB) size and is equal to 2 • VREF divided by 16,384. This means that any offset or gain error inherent in the A/D converter will appear to increase, in terms of LSB size, as the reference voltage is reduced. The noise inherent in the converter will also appear to increase with lower LSB size. With a 0.9V reference, the internal noise of the converter typically contributes only 5LSB peak-to-peak of potential error to the output code. When the external reference is 500mV, the potential error contribution from the internal noise will be 7LSBs. The errors due to the internal noise are gaussian in nature and can be reduced by averaging consecutive conversion results. For more information regarding noise, consult the typical performance curve “Noise vs Reference Voltage.” Note that the Effective Number of Bits (ENOB) figure is calculated based on the converter’s signal-to-(noise + distortion) ratio with a 1kHz, 0dB input signal. SINAD is related to ENOB as follows: SINAD = 6.02 • ENOB + 1.76 With lower reference voltages, extra care should be taken to provide a clean layout including adequate bypassing, a clean power supply, a low-noise reference, and a low-noise input signal. Because the LSB size is lower, the converter will also be more sensitive to external sources of error such as nearby digital signals and electromagnetic interference. FIGURE 4. Histogram of 5,000 Conversions of a DC Input at the Code Transition. NOISE The noise floor of the ADS8324 itself is extremely low, as can be seen from Figure 4, and is much lower than compet- ing A/D converters. It was tested by applying a low noise DC input and a 0.9V reference to the ADS8324 and initiat- ing 5,000 conversions. The digital output of the A/D con- verter will vary in output code due to the internal noise of the ADS8324. This is true for all 14-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 will represent 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 will yield the ±3σ distribution or 99.7% of all codes. Statistically, up to 3 codes could fall outside the distribution when executing 1000 conversions. The ADS8324, with five output codes for the ±3σ distribu- tion, will yield a ±0.8LSB transition noise. Remember, to achieve this low-noise performance, the peak-to-peak noise of the input signal and reference must be < 50 μV. 3FFEH 583 560 3857 3FFFH 0000H Code 0001H 0002H 0 0 AVERAGING The noise of the A/D converter can be compensated by averaging the digital codes. By averaging conversion re- sults, transition noise will be reduced by a factor of 1/ √n, where n is the number of averages. For example, averaging 4 conversion results will reduce the transition noise by 1/2 to ±0.25LSBs. 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 will improve 3dB. |
Số phần tương tự - ADS8324E |
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Mô tả tương tự - ADS8324E |
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