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AD7680 データシート(PDF) 22 Page - Analog Devices |
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AD7680 データシート(HTML) 22 Page - Analog Devices |
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22 / 28 page  AD7688 Rev. A | Page 22 of 28 CHAIN MODE WITH BUSY INDICATOR This mode can also be used to daisy-chain multiple AD7688s on a 3-wire serial interface while providing a BUSY indicator. This feature is useful for reducing component count and wiring connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. Data readback is analogous to clocking a shift register. A connection diagram example using three AD7688s is shown in Figure 44 and the corresponding timing is given in Figure 45. When SDI and CNV are low, SDO is driven low. With SCK high, a rising edge on CNV initiates a conversion, selects the chain mode, and enables the BUSY indicator feature. In this mode, CNV is held high during the conversion phase and the subsequent data readback. When all ADCs in the chain have completed their conversions, the nearend ADC (ADC C in Figure 44) SDO is driven high. This transition on SDO can be used as a BUSY indicator to trigger the data readback controlled by the digital host. The AD7688 then enters the acquisition phase and powers down. The data bits stored in the internal shift register are then clocked out, MSB first, by subsequent SCK falling edges. For each ADC, SDI feeds the input of the internal shift register and is clocked by the SCK falling edge. Each ADC in the chain outputs its data MSB first, and 16 × N + 1 clocks are required to readback the N ADCs. Although the rising edge can be used to capture the data, a digital host using the SCK falling edge allows a faster reading rate and consequently more AD7688s in the chain, provided the digital host has an acceptable hold time. For instance, with a 3 ns digital host setup time and 3 V interface, up to four AD7688s running at a conversion rate of 360 kSPS can be daisy-chained to a single 3-wire port. CLK CONVERT DATA IN IRQ DIGITAL HOST CNV SCK SDO SDI AD7688 C CNV SCK SDO SDI AD7688 A CNV SCK SDO SDI AD7688 B Figure 44. Chain Mode with BUSY Indicator Connection Diagram SDOA = SDIB DA15 DA14 DA13 SCK 123 35 47 48 tEN CONVERSION ACQUISITION tCONV tCYC tACQ ACQUISITION CNV = SDIA DA1 415 tSCK tSCKH tSCKL DA0 17 34 16 SDOB = SDIC DB15 DB14 DB13 DA1 DB1DB0DA15 DA14 49 tSSDISCK tHSDISC tHSDO tDSDO SDOC DC15 DC14 DC13 DA1DA0 DC1DC0DA14 19 31 32 18 33 DB1DB0DA15 DB15 DB14 tDSDOSDI tSSCKCNV tHSCKCNV DA0 tDSDOSDI tDSDOSDI tDSDOSDI tDSDOSDI Figure 45. Chain Mode with BUSY Indicator Serial Interface Timing |
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