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AD7827BRZ Datasheet(PDF) 8 Page - Analog Devices |
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AD7827BRZ Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD7827 –8– REV. 0 POWER-UP TIMES The AD7827 has a 1 µs power-up time when using an external reference and a 25 µs power-up time when using the on-chip reference. When VDD is first connected, the AD7827 is in a low current mode of operation. In order to carry out a conversion the AD7827 must first be powered up. The AD7827 is pow- ered up by a rising edge on the CONVST pin and a conversion is initiated on the falling edge of CONVST. Figure 9 shows how to power up the AD7827 when VDD is first connected or after the ADC has been powered down using the CONVST pin when using either the on-chip, or an external, reference. When using an external reference the falling edge of CONVST may occur before the required power-up time has elapsed; however, the conversion will not be initiated on the falling edge of CONVST but rather at the moment when the part has com- pletely powered up, i.e., after 1 µs. If the falling edge of CONVST occurs after the required power-up time has elapsed, it is upon this falling edge that a conversion is initiated. When using the on-chip reference, it is necessary to wait the required power-up time of approximately 25 µs before initiating a con- version, i.e., a falling edge on CONVST may not occur before the required power-up time has elapsed, when VDD is first con- nected or after the AD7827 has been powered down using the CONVST pin as shown in Figure 9. EXTERNAL REFERENCE ON-CHIP REFERENCE CONVERSION INITIATED HERE CONVERSION INITIATED HERE VDD CONVST VDD CONVST tPOWER-UP 1 s tPOWER-UP 25 s Figure 9. Power-Up Time POWER VS. THROUGHPUT Superior power performance can be achieved by using the automatic power-down (Mode 2) at the end of a conversion (see Operating Modes section of this data sheet). Figure 10 shows how the automatic power-down is implemented using the CONVST signal to achieve the optimum power per- formance for the AD7827. The duration of the CONVST pulse is set to be equal to or less than the power-up time of the de- vices (see Operating Modes section). As the throughput rate is reduced, the device remains in its power-down state for longer and the average power consumption over time drops accordingly. For example, if the AD7827 is operated in a continuous sam- pling mode, with a throughput rate of 100 kSPS and using an external reference, the power consumption is calculated as follows. The power dissipation during normal operation is 30 mW, VDD = 3 V. CONVST tPOWER-UP 1 s tCONVERT 330ns POWER-DOWN tCYCLE 10 s @ 100kSPS Figure 10. Automatic Power-Down If the power-up time is 1 µs and the conversion time is 330 ns (@ 25 °C), the AD7827 can be said to dissipate 30 mW for 1.33 µs (worst case) during each conversion cycle. If the throughput rate is 100 kSPS, the cycle time is 10 µs and the average power dissipated during each cycle is (1.33/10) × (30 mW) = 3.99 mW. Figure 11 shows the Power vs. Throughput rate for automatic full power-down. THROUGHPUT – kSPS 100 10 0.1 0 500 50 100 150 200 250 300 350 400 450 1 Figure 11. Power vs. Throughput FREQUENCY – Hz 0 –40 –80 0 500 50 100 150 200 250 300 350 400 450 –10 –20 –60 –70 –30 –50 2048 POINT FFT SAMPLING 1MSPS FIN = 30kHz Figure 12. AD7827 SNR |
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