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AD7813YR 데이터시트(PDF) 7 Page - Analog Devices |
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AD7813YR 데이터시트(HTML) 7 Page - Analog Devices |
7 / 11 page AD7813 –7– REV. B During the acquisition phase the sampling capacitor must be charged to within a 1/2 LSB of its final value. The time it takes to charge the sampling capacitor (TCHARGE) is given by the following formula: TCHARGE = 7.6 × (R2 + 125 Ω) × 3.5 pF For small values of source impedance, the settling time associ- ated with the sampling circuit (100 ns) is, in effect, the acquisi- tion time of the ADC. For example, with a source impedance (R2) of 10 Ω the charge time for the sampling capacitor is approximately 4 ns. The charge time becomes significant for source impedances of 2 k Ω and greater. AC Acquisition Time In ac applications it is recommended to always buffer analog input signals. The source impedance of the drive circuitry must be kept as low as possible to minimize the acquisition time of the ADC. Large values of source impedance will cause the THD to degrade at high throughput rates. ADC TRANSFER FUNCTION The output coding of the AD7813 is straight binary. The de- signed code transitions occur at successive integer LSB values (i.e., 1 LSB, 2 LSBs, etc.). The LSB size is = VREF/1024. The ideal transfer characteristic for the AD7813 is shown in Figure 7. 000...000 0V ANALOG INPUT 111...111 000...001 000...010 111...110 111...000 011...111 1LSB +VREF–1LSB 1LSB = VREF/1024 Figure 7. Transfer Characteristic POWER-UP TIMES The AD7813 has a 1 µs power-up time. When VDD is first con- nected, the AD7813 is in a low current mode of operation. In order to carry out a conversion the AD7813 must first be pow- ered up. The ADC is powered up by a rising edge on an inter- nally generated CONVST signal, which occurs as a result of a rising edge on the external CONVST pin. The rising edge of the external CONVST signal initiates a 1 µs pulse on the internal CONVST signal. This pulse is present to ensure the part has enough time to power up before a conversion is initiated, as a conversion is initiated on the falling edge of gated CONVST. See Timing and Control section. Care must be taken to ensure that the CONVST pin of the AD7813 is logic low when V DD is first applied. When operating in Mode 2, the ADC is powered down at the end of each conversion and powered up again before the next conversion is initiated. (See Figure 8.) t POWER-UP 1 s t POWER-UP 1 s t POWER-UP 1 s MODE 1 MODE 2 VDD EXT CONVST INT CONVST VDD EXT CONVST INT CONVST Figure 8. Power-Up Times POWER VS. THROUGHPUT RATE By operating the AD7813 in Mode 2, the average power con- sumption of the AD7813 decreases at lower throughput rates. Figure 9 shows how the Automatic Power-Down is implemented using the external CONVST signal to achieve the optimum power performance for the AD7813. The AD7813 is operated in Mode 2, and the duration of the external CONVST pulse is set to be equal to or less than the power-up time of the device. As the throughput rate is reduced, the device remains in its power- down state longer and the average power consumption over time drops accordingly. EXT CONVST INT CONVST POWER-DOWN t POWER-UP 1 s t CONVERT 2.0 s t CYCLE 100 s @ 10kSPS Figure 9. Automatic Power-Down For example, if the AD7813 is operated in a continuous sam- pling mode, with a throughput rate of 10 kSPS, the power con- sumption is calculated as follows. The power dissipation during normal operation is 10.5 mW, VDD = 3 V. If the power-up time is 1 µs and the conversion time is 2.3 µs, the AD7813 can then be said to dissipate 10.5 mW for 3.3 µs (worst case) during each conversion cycle. If the throughput rate is 10 kSPS, the cycle time is 100 µs and the average power dissipated during each cycle is (3.3/100) × (10.5 mW) = 346.5 µW. |
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