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ADC7802 bảng dữ liệu(PDF) 11 Page - Burr-Brown (TI) |
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ADC7802 bảng dữ liệu(HTML) 11 Page - Burr-Brown (TI) |
11 / 13 page 11 ® ADC7802 INPUT PROTECTION The input signal range must not exceed ±V REF or VA by more than 0.3V. The analog inputs are internally clamped to V A. To prevent damage to the ADC7802, the current that can flow into the inputs must be limited to 20mA. One approach is to use an external resistor in series with the input filter resistor. For example, a 1k Ω input resistor allows an overvoltage to 20V without damage. REFERENCE INPUTS A 10 µF tantalum capacitor is recommended between V REF+ and V REF– to insure low source impedance. These capacitors should be located as close as possible to the ADC7802 to reduce dynamic errors, since the reference provides packets of current as the successive approximation steps are carried out. V REF+ must not exceed VA. Although the accuracy is speci- fied with V REF+ = 5V and VREF– = 0V, the converter can function with V REF+ as low as 2.5V and VREF– as high as 1V. As long as there is at least a 2.5V difference between V REF+ and V REF–, the absolute value of errors does not change significantly, so that accuracy will typically be within ±1LSB. (1/2LSB for a 5V span is 610µV, which is 1LSB for a 2.5V span.) The power supply to the reference source needs to be consid- ered during system design to prevent V REF+ from exceeding (or overshooting) V A, particularly at power-on. Also, after power-on, if the reference is not stable within 42,425 clock cycles, an additional calibration cycle may be needed. POWER SUPPLIES The digital and analog power supply lines to the ADC7802 should be bypassed with 10 µF tantalum capacitors as close to the part as possible. Although ADC7802 has excellent power supply rejection, even for higher frequencies, linear regulated power supplies are recommended. Care should be taken to insure that V D does not come up before V A, or permanent damage to the part may occur. Figure 10 shows a good supply approach, powering both V A and V D from a clean linear supply, with the 10Ω resistor between V A and VD insuring that VD comes up after VA. This is also a good method to further isolate the ADC7802 from digital supplies in a system with significant switching cur- rents that could degrade the accuracy of conversions. GROUNDING To maximize accuracy of the ADC7802, the analog and digital grounds are not connected internally. These points should have very low impedance to avoid digital noise feeding back into the analog ground. The V REF– pin is used as the reference point for input signals, so it should be con- nected directly to AGND to reduce potential noise problems. EXTERNAL CLOCK OPERATION The circuitry required to drive the ADC7802 clock from an external source is shown in Figure 11a. The external clock must provide a 0.8V max for LOW and a 3.5V min for HIGH, with rise and fall times that do not exceed 200ns. The minimum pulse width of the external clock must be 200ns. Synchronizing the conversion clock to an external system clock is recommended in microprocessor applications to prevent beat-frequency problems. Note that the electrical specification tables are based on using an external 2MHz clock. Typically, the specified accuracy is maintained for clock frequencies between 0.5 and 2.2MHz. INTERNAL CLOCK OPERATION Figure 11b shows how to use the internal clock generating circuitry. The clock frequency depends only on the value of the resistor, as shown in “Internal Clock Frequency vs R CLOCK” in the Typical Performance Curves section. SFR AIN0 AIN1 AIN2 AIN3 V + V – DGND V D7 D6 D5 D4 D3 V AGND CAL A1 A0 CLK BUSY HBE WR CS RD D0 D1 D2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 28 27 26 25 24 23 22 21 20 19 18 17 16 15 REF REF D A + 10nF 10µF + 10µF 10nF +5V 10 Ω 5V REF + 10nF 10µF FIGURE 10. Power Supply and Reference Decoupling. FIGURE 11. Internal Clock Operation. To ADC7802 Pin 23 CLK 74HC-Compatible Clock Source To ADC7802 Pin 23 +5V R f CLOCK (in Hz) = 10 /R 11 (a) External Clock Operation (b) Internal Clock Operation |
Số phần tương tự - ADC7802 |
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Mô tả tương tự - ADC7802 |
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