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AD7467BRT 데이터시트(PDF) 10 Page - Analog Devices |
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AD7467BRT 데이터시트(HTML) 10 Page - Analog Devices |
10 / 15 page AD7466/AD7467/AD7468 –10– REV. PrC Figure 11. REF193 as Power Supply to AD7466/AD7467/ AD7468 Analog Input Figure 12 shows an equivalent circuit of the analog input sturcture of the AD7466/AD7467/7468. 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 200mV. This will cause these diodes to become forward biased and start conducting current into the substrate. The capacitor C1 in figure 12 is typically about 4pF and can primarily be at- tributed to pin capacitance. The resistor R1 is a lumped component made up of the on resistance of a switch. This resistor is typically about 100 Ω. The capacitor C2 is the ADC sampling capacitor and has a capacitance of 16pF typically. For ac applications, removing high frequency components from the analog input signal is recommended by use of a band-pass filter on the relevant analog input pin. In applications where harmonic distortion and signal to noise ratio are critical the analog input should be driven VIN D1 VDD D2 R1 C2 16PF C1 4pF CONVERSION PHASE - SWITCH OPEN TRACK PHASE - SWITCH CLOSED from a low impedance source. Large source impedances will significantly affect the ac performance of the ADC. This may necessitate the use of an input buffer amplifier. The choice of the op amp will be a function of the par- ticular application. When no amplifier is used to drive the analog input the source impedance should be limited to low values. The maximum source impedance will depend on the amount of total harmonic distortion (THD) that can be tolerated. The THD will increase as the source impedance increases and performance will degrade. Figure 13 shows a graph of the Total Harmonic Distortion vs. analog input signal frequency for different source impedances when using a supply voltage of 2.7V and sampling at a rate of 100 kSPS. Figure 12. Equivalent Analog Input Circuit VDD VIN GND +5V SUPPLY 0.1µF 10µF REF193 0.1µF 1µF TANT +3V AD7466/67/68 0V toVDD INPUT SDATA µC/µP SCLK SERIAL INTERFACE CS 1mA 680nF The AD7468 conversion result consists of four leading zeros followed by the MSB of the 8-bit result. Alternatively, because the supply current required by the AD7466/AD7467/AD7468 is so low, a presision reference can be used as the supply source to the AD7466/AD7467/ AD7468. A REF19x voltage reference (REF193 for 3V, REF192 for 2.5 V ) can be used to supply the required voltage to the ADC - see figure 11. This configuration is especially useful if your power supply is quite noisy or if the system supply voltages are at some value other than 3V (e.g. 2.5V). The REF19x will output a steady voltage to the AD7466/AD7467/AD7468. In applications where power consumption is important, the automatic power down mode of the ADC and the sleep mode of the REF19x reference should be used to improve power per- formance. See Modes of Operation section of the datasheet. TYPICAL CONNECTION DIAGRAM Figure 11 shows a typical connection diagram for the AD7466/AD7467/AD7468. VREF is taken internally from VDD and as such VDD should be well decoupled. This pro- vides an analog input range of 0V to VDD. For the AD7466 the conversion result is output in a 16-bit word with four leading zeroes followed by the MSB of the 12- bit result. The AD7467 conversion result consists of four leading zeros followed by the MSB of the 10-bit result. pecifications |
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