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ADS7815UG4 Datasheet(PDF) 6 Page - Texas Instruments |
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ADS7815UG4 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 16 page ADS7815 6 SBAS065B www.ti.com TIMING The timing shown in Figure 2 and Table II is the recom- mended method of operating the ADS7815. The falling edge of CS initiates the conversion. During the conversion, the digital outputs are tri-stated and BUSY is LOW. Near the end of the conversion, the digital outputs become active with the most recent conversion result. After a brief delay (see time t11 in Figure 2 and Table II), BUSY rises. The rising edge of BUSY is used to latch the digital result in Figure 1. R/C AND CS The R/C (read/convert) and CS signals control the start of conversion and, when a conversion is not in progress, the status of the digital outputs D15 through D0. It is possible to start a conversion by taking CS LOW and then taking R/C LOW. However, this is not recommended and will result in a significant decrease in signal-to-noise ratio. This is due to FIGURE 3. Bus Timing. SYMBOL DESCRIPTION MIN TYP MAX UNITS t1 CS to R/C Delay 200 ns t2 CS to BUSY Delay 40 ns t3 Aperture Delay 40 ns t4 BUSY LOW 3.3 µs t5 R/C LOW to CS LOW 100 ns t6 BUSY HIGH to CS HIGH 250 ns t7 Bus Access Time 10 83 ns t8 Bus Relinquish Time 83 ns t9 Throughput Time 4 µs t10 Conversion Time 3.3 µs t11 Data Valid to BUSY HIGH 25 35 ns t12 CS to R/C Setup Time 40 ns TABLE II. Conversion Timing. FIGURE 2. ADS7815 Timing. t 12 CS R/C t 7 t 8 Hi-Z State D15 - D0 Hi-Z State MODE Acquire DataValid CS t 9 t 6 t 5 BUSY t 4 R/C t 1 t 2 Hi-Z State D15 - D0 t 11 t 8 Hi-Z State Acquire MODE Convert t 10 t 3 Acquire DataValid the digital outputs tri-stating while the sample and hold transitions to the hold mode. The change in digital outputs results in noise being coupled onto the hold capacitor. If a conversion is not in progress or is just about to finish, the digital outputs will be active when R/C is HIGH and CS is LOW. This is shown in Figure 2 and Figure 3. It is possible to return CS HIGH during the initial part of the conversion (as is done with R/C) and prevent the digital outputs from becoming active. At a later time, the digital results could be read by taking CS LOW. It is also possible to leave R/C LOW, take CS HIGH during the conversion, and read the results at a later time by taking R/C HIGH and CS LOW. Following a conversion, if R/C and CS are both LOW 250ns after BUSY rises, then a new conversion will be initiated without allowing the proper acquisition period for the sample and hold. R/C must remain HIGH or CS must be taken HIGH within 250ns of BUSY rising. |
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