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ADS8345N Arkusz danych(PDF) 10 Page - Texas Instruments |
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ADS8345N Arkusz danych(HTML) 10 Page - Texas Instruments |
10 / 18 page ADS8345 SBAS177C 10 www.ti.com THEORY OF OPERATION The ADS8345 is a classic Successive Approximation Register (SAR) A/D converter. The architecture is based on capacitive redistribution which inherently includes a sample- and-hold function. The converter is fabricated on a 0.6 µm CMOS process. The basic operation of the ADS8345 is shown in Figure 1. The device requires an external reference and an external clock. It operates from a single supply of 2.7V to 5.25V. The external reference can be any voltage between 500mV and +VCC/2. The value of the reference voltage directly sets the input range of the converter. The average reference input current depends on the conversion rate of the ADS8345. The analog input to the converter is differential and is provided via an eight-channel multiplexer. The input can be provided in reference to a voltage on the COM pin (which is generally +VCC/2) or differentially by using four of the eight input channels (CH-CH7). The particular configuration is selectable via the digital interface. ANALOG INPUT The analog input is bipolar and fully differential. There are two general methods of driving the analog input of the ADS8345: single-ended or differential (see Figure 2). When the input is single-ended, the COM input is held at a fixed voltage. The CHX input swings around the same voltage and the peak-to-peak amplitude is 2 • VREF. The value of VREF FIGURE 1. Basic Operation of the ADS8345. determines the range over which the common voltage may vary (see Figure 3). When the input is differential, the amplitude of the input is the difference between the CHX and COM input (see Figure 4). A voltage or signal is common to both of these inputs. The peak-to-peak amplitude of each input is VREF about this common voltage. However, since the input are 180 °C out-of- phase, the peak-to-peak amplitude of the difference voltage is 2 • VREF. The value of VREF also determines the range of the voltage that may be common to both inputs (see Figure 5). In each case, care should be taken to ensure that the output impedance of the sources driving the CHX and COM inputs are matched. If this is not observed, the two inputs could have different settling times. This may result in offset error, gain error, and linearity error which changes with both temperature and input voltage. If the impedance cannot be matched, the errors can be lessened by giving the ADS8345 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 ADS8345 charges the inter- nal capacitor array during the sample period. After this capacitance has been fully charged, there is no further input current. Care must be taken regarding the absolute analog input voltage. Outside of these ranges, the converter’s linearity may not meet specifications. Please refer to the Electrical Characteristics table for min/max ratings. CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM SHDN 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 +V CC DCLK CS D IN BUSY D OUT GND GND +V CC V REF Serial/Conversion Clock Chip Select Serial Data In Serial Data Out +2.7V to +5V 1 µF to 10µF ADS8345 Single-ended or differential analog inputs. V REF 1 µF to 10µF +1.25V to +2.5V 0.1 µF |
Podobny numer części - ADS8345N |
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Podobny opis - ADS8345N |
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