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ADS8344EB Datasheet(PDF) 11 Page - Texas Instruments |
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ADS8344EB Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 25 page  ADS8344 11 SBAS139E THEORY OF OPERATION The ADS8344 is a classic Successive Approximation Register (SAR) A/D converter. The architecture is based on capacitive redistribution that inherently includes a sample- and-hold function. The converter is fabricated on a 0.6 µs CMOS process. The basic operation of the ADS8344 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. 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 ADS8344. The analog input to the converter is differential and is provided via an 8-channel multiplexer. The input can be provided in reference to a voltage on the COM pin (which is generally ground) or differentially by using four of the eight input channels (CH0 - CH7). The particular configura- tion is selectable via the digital interface. A2 A1 A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 00 0 +IN –IN 00 1 +IN –IN 01 0 +IN –IN 01 1 +IN –IN 10 0 –IN +IN 10 1 –IN +IN 11 0 –IN +IN 11 1 –IN +IN TABLE II. Differential Channel Control (SGL/DIF LOW). TABLE I. Single-Ended Channel Selection (SGL/DIF HIGH). FIGURE 1. Basic Operation of the ADS8344. A2 A1 A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM 00 0 +IN –IN 10 0 +IN –IN 00 1 +IN –IN 10 1 +IN –IN 01 0 +IN –IN 11 0 +IN –IN 01 1 +IN –IN 11 1 +IN –IN ANALOG INPUT See Figure 2 for a block diagram of the input multiplexer on the ADS8344. The differential input of the converter is derived from one of the eight inputs in reference to the COM pin, or four of the eight inputs. Table I and Table II show the relationship between the A2, A1, A0, and SGL/DIF control bits and the configuration of the analog multiplexer. The control bits are provided serially via the DIN pin (see the Digital Interface section of this data sheet for more details). When the converter enters the hold mode, the voltage difference between the +IN and –IN inputs is captured on the internal capacitor array (see Figure 2). The voltage on the –IN input is limited between –0.2V and 1.25V, allowing the input to reject small signals that are common to both the +IN and –IN input. The +IN input has a range of –0.2V to +VCC + 0.2V. The input current on the analog inputs depends on the conver- sion rate of the device. During the sample period, the source must charge the internal sampling capacitor (typically 25pF). After the capacitor has been fully charged, there is no further input current. The rate of charge transfer from the analog source to the converter is a function of conversion rate. 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 ADS8344 Single-ended or differential analog inputs 1 µF to 10µF 1 µF 0.1 µF + External V REF |
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