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AD7417ARUZ-REEL7 датащи(PDF) 11 Page - Analog Devices |
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AD7417ARUZ-REEL7 датащи(HTML) 11 Page - Analog Devices |
11 / 24 page AD7416/AD7417/AD7418 Rev. I | Page 11 of 24 THEORY OF OPERATION CIRCUIT INFORMATION The AD7417 and AD7418 are single-channel and four-channel, 15 μs conversion time, 10-bit ADCs with on-chip temperature sensor, reference, and serial interface logic functions on a single chip. The AD7416 has no analog input channel and is intended for temperature measurement only. The ADC section consists of a conventional successive approximation converter based around a capacitor DAC. The AD7416, AD7417, and AD7418 are capable of running on a 2.7 V to 5.5 V power supply, and the AD7417 and AD7418 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 −40°C to +125°C. The parts require a 2.5 V reference that can be provided from the part’s own internal reference or from an external reference source. CONVERTER DETAILS Conversion is initiated on the AD7417/AD7418 by pulsing the CONVST input. The conversion clock for the part is internally generated so that no external clock is required except when reading from and writing to the serial port. The on-chip track- and-hold goes from track mode to hold mode, and the conversion sequence is started on the falling edge of the CONVST signal. A conversion is also initiated in the automatic conversion mode every time a read or write operation to the AD7416/AD7417/ AD7418 takes place. In this case, the internal clock oscillator (which runs the automatic conversion sequence) is restarted at the end of the read or write operation. The track-and-hold goes into hold mode approximately 3 μs after the read or write operation is complete, and a conversion is then initiated. The result of the conversion is available either 15 μs or 30 μs later, depending on whether an analog input channel or the tempera- ture sensor is selected. The track-and-hold acquisition time of the AD7417/AD7418 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 30 μs to complete. Temperature measurement is explained in the Temperature Measurement section. The on-chip reference is not available to the user, but REFIN can be overdriven by an external reference source (2.5 V only). All unused analog inputs should be tied to a voltage within the nominal analog input range to avoid noise pickup. For minimum power consumption, the unused analog inputs should be tied to GND. TYPICAL CONNECTION DIAGRAM Figure 9 shows a typical connection diagram for the AD7417. Using the A0, A1, and A2 pins allows the user to select from up to eight AD7417 devices on the same serial bus, if desired. 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 GND. SDA and SCL form the 2-wire I2C compatible interface. For applications where power consump- tion is of concern, the automatic power-down at the end of a conversion should be used to improve power performance (see the Operating Modes section.) + + SUPPLY 2.7V TO 5.5V 2-WIRE SERIAL INTERFACE 0.1µF 10µF 10µF FOR EXTERNAL REFERENCE OPTIONAL EXTERNAL REFERENCE AD780/ REF192 0V TO 2.5V INPUT SDA SCL GND OTI CONVST VDD REFIN AD7417 A0 A1 A2 AIN1 AIN2 AIN3 AIN4 MICROCONTROLLER/ MICROPROCESSOR Figure 9. Typical AD7417 Connection Diagram ANALOG INPUTS Figure 10 shows an equivalent circuit of the analog input structure of the AD7417 and AD7418. 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 to prevent these diodes from becoming 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. Capacitor C2 in Figure 10 is typically about 4 pF and can primarily be attributed to pin capacitance. Resistor R1 is a lumped component made up of the on resistance of a multiplexer and a switch. This resistor is typically about 1 kΩ. Capacitor C1 is the ADC sampling capacitor and has a capacitance of 3 pF. VDD VBALANCE AIN R1 1kΩ CONVERT PHASE: SWITCH OPEN TRACK PHASE: SWITCH CLOSED D1 D2 C2 4pF C1 3pF Figure 10. Equivalent Analog Input Circuit ON-CHIP REFERENCE The AD7417/AD7418 have an on-chip 1.2 V band gap reference that is amplified by a switched capacitor amplifier to give an output of 2.5 V. The amplifier is only powered up at the start of the conversion phase and is powered down at the end of the conversion. The on-chip reference is selected by connecting the REFIN pin to analog ground, which causes SW1 (see Figure 11) to open and the reference amplifier to power up during a conver- sion. Therefore, the on-chip reference is not available externally. |
Аналогичный номер детали - AD7417ARUZ-REEL7 |
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Аналогичное описание - AD7417ARUZ-REEL7 |
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