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AD7151BRMZ bảng dữ liệu(PDF) 10 Page - Analog Devices |
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10 / 28 page AD7151 Rev. 0 | Page 10 of 28 ARCHITECTURE AND MAIN FEATURES DIGITAL FILTER POWER-DOWN TIMER CLOCK GENERATOR SERIAL INTERFACE POWER SUPPLY MONITOR CIN CX EXC Σ-Δ CDC EXCITATION CAPDAC THRESHOLD SCL VDD GND AD7151 SDA PROGRAMMING INTERFACE DIGITAL OUTPUT OUT 3.3V Figure 20. AD7151 Block Diagram The AD7151 core is a high performance capacitance-to-digital converter (CDC) that allows the part to be interfaced directly to a capacitive sensor. The comparator compares the CDC result with thresholds, either fixed or dynamically adjusted by the on-chip adaptive threshold algorithm engine. Thus, the output indicates a defined change in the input sensor capacitance. The AD7151 also integrates an excitation source and CAPDAC for the capacitive inputs, an input multiplexer, a complete clock generator, a power-down timer, a power supply monitor, control logic, and an I2C®-compatible serial interface for configuring the part and accessing the internal CDC data and status, if required in the system (see Figure 20). CAPACITANCE-TO-DIGITAL CONVERTER Figure 21 shows the CDC simplified functional diagram. The converter consists of a second-order sigma delta (Σ-Δ), charge balancing modulator and a third-order digital filter. The measured capacitance CX is connected between an excitation source and the Σ-Δ modulator input. The excitation signal is applied on the CX during the conversion, and the modulator continuously samples the charge going through the CX. The digital filter processes the modulator output, which is a stream of 0s and 1s containing the information in 0 and 1 density. The data is processed by the adaptive threshold engine and output comparators; the data can be also read through the serial interface. The AD7151 is designed for floating capacitive sensors. Therefore, both CX plates have to be isolated from ground or any other fixed potential node in the system. The AD7151 features slew rate limiting on the excitation voltage output, which decreases the energy of higher harmonics on the excitation signal and dramatically improves the system electromagnetic compatibility (EMC). DIGITAL FILTER 0x000 TO 0xFFF DATA CLOCK GENERATOR CAPACITANCE TO DIGITAL CONVERTER (CDC) CIN CX 0pF TO 4pF EXC EXCITATION Σ-Δ MODULATOR Figure 21. CDC Simplified Block Diagram CAPDAC The AD7151 CDC core maximum full-scale input range is 4 pF. However, the part can accept a higher capacitance on the input, and the offset (nonchanging component) capacitance of up to 10 pF can be balanced by a programmable on-chip CAPDAC. 0x000 TO 0xFFF DATA CIN EXC CAPDAC 10pF 0pF TO 4pF CDC CSENS 10pF TO 14pF Figure 22. Using CAPDAC The CAPDAC can be understood as a negative capacitance connected internally to the CIN pin. The CAPDAC has a 6-bit resolution and a monotonic transfer function. Figure 22 shows how to use the CAPDAC to shift the CDC 4 pF input range to measure capacitance between 10 pF and 14 pF. |
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