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AD7829BRW-1RL7 Scheda tecnica(PDF) 10 Page - Analog Devices |
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AD7829BRW-1RL7 Scheda tecnica(HTML) 10 Page - Analog Devices |
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10 / 20 page  AD7829-1 Rev. 0 | Page 10 of 20 CIRCUIT INFORMATION TIMING AND CONTROL LOGIC R1 HOLD SAMPLING CAPACITOR A B SW2 R16 R15 R14 R13 T/H 1 VIN D7 D6 D5 D4 D3 D2 D1 D0 14 15 13 1 REFERENCE CIRCUIT DESCRIPTION The AD7829-1 consists of a track-and-hold amplifier followed by a half-flash analog-to-digital converter. These devices use a half-flash conversion technique where one 4-bit flash ADC is used to achieve an 8-bit result. The 4-bit flash ADC contains a sampling capacitor followed by 15 comparators that compare the unknown input to a reference ladder to achieve a 4-bit result. This first flash, that is, coarse conversion, provides the four MSBs. For a full 8-bit reading to be realized, a second flash, that is, a fine conversion, must be performed to provide the four LSBs. The 8-bit word is then placed on the data output bus. Figure 4 and Figure 5 show simplified schematics of the ADC. When the ADC starts a conversion, the track-and-hold goes into hold mode and holds the analog input for 120 ns. This is the acquisition phase as shown in Figure 4, when Switch 2 is in Position A. At the point when the track-and-hold returns to its track mode, this signal is sampled by the sampling capacitor as Switch 2 moves into Position B. The first flash occurs at this instant and is then followed by the second flash. Typically, the first flash is complete after 100 ns, that is, at 220 ns, while the end of the second flash and, hence, the 8-bit conversion result, is available at 330 ns (minimum). The maximum conversion time is 420 ns. As shown in Figure 5. ADC Conversion Phase HOLD HOLD 120ns CONVST EOC CS RD DB0 TO DB7 t2 TRACK TRACK VALID DATA t1 t3 Figure 6, the track-and-hold returns to track mode after 120 ns and starts the next acquisition before the end of the current conversion. Figure 8 shows the ADC transfer function. TIMING AND CONTROL LOGIC R1 HOLD SAMPLING CAPACITOR A B SW2 R16 R15 R14 R13 T/H 1 VIN D7 D6 D5 D4 D3 D2 D1 D0 14 15 13 1 REFERENCE Figure 6. Track-and-Hold Timing TYPICAL CONNECTION DIAGRAM Figure 7 shows a typical connection diagram for the AD7829-1. The AGND and DGND are connected together at the device for good noise suppression. The parallel interface is implemented using an 8-bit data bus. The end of conversion signal (EOC) idles high, the falling edge of CONVST initiates a conversion, and at the end of conversion the falling edge of EOC is used to initiate an interrupt service routine (ISR) on a microprocessor (see the Parallel Interface section). VREF IN/OUT and VMID are connected to a voltage source, such as the AD780, while VDD is connected to a voltage source that can vary from 4.5 V to 5.5 V (see Table 5 in the Analog Input section). When VDD is first connected, the AD7829-1 powers up in a low current mode, that is, power-down. Ensure that the CONVST line is not floating when VDD is applied, because this can put the AD7829-1 into an unknown state. Figure 4. ADC Acquisition Phase |
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