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AD7818AR bảng dữ liệu(PDF) 11 Page - Analog Devices |
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11 / 20 page REV. C AD7816/AD7817/AD7818 –11– MSB LSB 0000 000 1 MINIMUM TEMPERATURE = –95 C 11111111 MAXIMUM TEMPERATURE = +152 C OVERTEMPERATURE REGISTER (DEC) = TALARM + 103 C TALARM RESOLUTION = 1 / LSB OVERTEMPERATURE REGISTER DB0 DB1 DB2 DB3 DB4 DB5 DB6 DB7 Figure 5. The Overtemperature Register (OTR) CIRCUIT INFORMATION The AD7817 and AD7818 are single- and four-channel, 9 µs conversion time, 10-bit A/D converters with on-chip tempera- ture sensor, reference, and serial interface logic functions on a single chip. The AD7816 has no analog input channel and is intended for temperature measurement only. The A/D converter section consists of a conventional successive-approximation converter based around a capacitor DAC. The AD7816, AD7817, and AD7818 are capable of running on a 2.7 V to 5.5 V power supply and the AD7817 and AD7818 accept an analog input range of 0 V to +VREF. The on-chip temperature sensor allows an accurate measurement of the ambient device temperature to be made. The working measurement range of the temperature sensor is –55 °C to +125°C. The part requires a 2.5 V reference, which can be provided from the part’s own internal reference or from an external reference source. The on-chip reference is selected by connecting the REFIN pin to analog ground. CONVERTER DETAILS Conversion is initiated by pulsing the CONVST input. The conversion clock for the part is internally generated so no exter- nal clock is required except when reading from and writing to the serial port. The on-chip track/hold goes from track-to-hold mode and the conversion sequence is started on the falling edge of the CONVST signal. At this point the BUSY signal goes high and low again 9 µs or 27 µs later (depending on whether an analog input or the temperature sensor is selected) to indicate the end of the conversion process. This signal can be used by a microcontroller to determine when the result of the conversion should be read. The track/hold acquisition time of the AD7817 and AD7818 is 400 ns. A temperature measurement is made by selecting the Channel 0 of the on-chip MUX and carrying out a conversion on this channel. A conversion on Channel 0 takes 27 µs to complete. Temperature measurement is explained in the Temperature Measurement section of this data sheet. The on-chip reference is not available to the user, but REFIN can be overdriven by an external reference source (2.5 V only). The effect of reference tolerances on temperature measurements is discussed in the section titled Temperature Measurement Error Due to Reference Error. All unused analog inputs should be tied to a voltage within the nominal analog input range to avoid noise pickup. For mini- mum power consumption, the unused analog inputs should be tied to AGND. TYPICAL CONNECTION DIAGRAM Figure 6 shows a typical connection diagram for the AD7817. The AGND and DGND are connected together at the device for good noise suppression. The BUSY line is used to interrupt the microcontroller at the end of the conversion process and the serial interface is implemented using three wires—see Serial Interface section for more details. An external 2.5 V reference can be connected at the REFIN pin. If an external reference is used, a 10 µF capacitor should be connected between REFIN and AGND. For applications where power consumption is of concern, the automatic power-down at the end of a conversion should be used to improve power performance. See Power vs. Throughput section of this data sheet. VDD AIN1 CONVST AGND DGND REFIN SUPPLY 2.7V TO 5.5V 0.1 F 10 F 10 F EXTERNAL REFERENCE OPTIONAL EXTERNAL REFERENCE AD780/ REF-192 0V TO 2.5V INPUT D OUT RD/ WR AIN2 AIN3 AIN4 D IN BUSY OTI C/ P SCLK 3-WIRE SERIAL INTERFACE CS AD7817 Figure 6. Typical Connection Diagram ANALOG INPUTS Analog Input Figure 7 shows an equivalent circuit of the analog input struc- ture of the AD7817 and AD7818. The two diodes D1 and D2 provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 200 mV. This will cause these diodes to become forward biased and start conducting current into the substrate. The maximum current these diodes can conduct without causing irreversible damage to the part is 20 mA. The capacitor C2 in Figure 7 is typically about 4 pF and can mostly be attributed to pin capacitance. The resistor R1 is a lumped component made up of the on resistance of a multiplexer and a switch. This resistor is typically about 1 k Ω. The capacitor C1 is the ADC sampling capacitor and has a capacitance of 3 pF. |
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