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ADS7818EB250 bảng dữ liệu(PDF) 7 Page - Texas Instruments |
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ADS7818EB250 bảng dữ liệu(HTML) 7 Page - Texas Instruments |
7 / 21 page 7 ® ADS7818 FIGURE 1. Basic Operation of the ADS7818. THEORY OF OPERATION The ADS7818 is a high speed successive approximation register (SAR) analog-to-digital converter (A/D) with an internal 2.5V bandgap reference. The architecture is based on capacitive redistribution which inherently includes a sample/hold function. The converter is fabricated on a 0.6 µ CMOS process. See Figure 1 for the basic operating circuit for the ADS7818. The ADS7818 requires an external clock to run the conver- sion process. This clock can vary between 200kHz (12.5Hz throughput) and 8MHz (500kHz throughput). The duty cycle of the clock is unimportant as long as the minimum HIGH and LOW times are at least 50ns and the clock period is at least 125ns. The minimum clock frequency is set by the leakage on the capacitors internal to the ADS7818. The analog input is provided to two input pins: +IN and –IN. When a conversion is initiated, the differential input on these pins is sampled on the internal capacitor array. While a conversion is in progress, both inputs are disconnected from any internal function. The range of the analog input is set by the voltage on the VREF pin. With the internal 2.5V reference, the input range is 0 to 5V. An external reference voltage can be placed on VREF, overdriving the internal voltage. The range for the external voltage is 2.0V to 2.55V, giving an input voltage range of 4.0V to 5.1V. The digital result of the conversion is provided in a serial manner, synchronous to the CLK input. The result is pro- vided most significant bit first and represents the result of the conversion currently in progress—there is no pipeline delay. By properly controlling the CONV and CLK inputs, it is possible to obtain the digital result least significant bit first. ANALOG INPUT The +IN and –IN input pins allow for a differential input signal to be captured on the internal hold capacitor when the converter enters the hold mode. The voltage range on the –IN input is limited to –0.2V to 0.2V. Because of this, the differential input can be used to reject only small signals that are common to both inputs. Thus, the –IN input is best used to sense a remote ground point near the source of the +IN signal. If the source driving the +IN signal is nearby, the –IN should be connected directly to ground. The input current into the analog input depends on input voltage and sample rate. Essentially, the current into the device must charge the internal hold capacitor 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 to a 12-bit settling level within the sample period—which can be as little as 350ns in some operating modes. While the converter is in the hold mode or after the sampling capacitor has been fully charged, the input impedance of the analog input is greater than 1G Ω. Care must be taken regarding the input voltage on the +In and –IN pins. To maintain the linearity of the converter, the +In input should remain within the range of GND – 200mV to +VCC + 200mV. The –IN input should not drop below GND – 200mV or exceed GND + 200mV. Outside of these ranges, the converter’s linearity may not meet specifications. REFERENCE The reference voltage on the VREF pin directly sets the full- scale range of the analog input. The ADS7818 can operate with a reference in the range of 2.0V to 2.55V, for a full- scale range of 4.0V to 5.1V. The voltage at the VREF pin is internally buffered and this buffer drives the capacitor DAC portion of the converter. This is important because the buffer greatly reduces the dynamic load placed on the reference source. However, the voltage at VREF will still contain some noise and glitches from the SAR conversion process. These can be reduced by carefully bypassing the VREF pin to ground as outlined in the sections that follow. INTERNAL REFERENCE The ADS7818 contains an on-board 2.5V reference, result- ing in a 0V to 5V input range on the analog input. The specification table gives the various specifications for the 1 2 3 4 8 7 6 5 +V CC CLK DATA CONV V REF +In –In GND ADS7818 0.1µF +5V 0.1µF 10µF Serial Clock from Microcontroller or DSP Serial Data Convert Start + 2.2µF + 0 to 5V Analog Input |
Số phần tương tự - ADS7818EB250 |
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Mô tả tương tự - ADS7818EB250 |
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