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LTC1608ACG Datasheet(PDF) 9 Page - Linear Technology |
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LTC1608ACG Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LTC1608 Timing and Control Conversion start and data read operations are controlled by three digital inputs: CONVST, CS and RD. A falling edge applied to the CONVST pin will start a conversion after the ADC has been selected (i.e., CS is low). Once initiated, it cannot be restarted until the conversion is complete. Converter status is indicated by the BUSY output. BUSY is low during a conversion. We recommend using a narrow logic low or narrow logic high CONVST pulse to start a conversion as shown in Figures 5 and 6. A narrow low or high CONVST pulse prevents the rising edge of the CONVST pulse from upset- ting the critical bit decisions during the conversion time. Figure 4 shows the change of the differential nonlinearity error versus the low time of the CONVST pulse. As shown, if CONVST returns high early in the conversion (e.g., CONVST low time <300ns), accuracy is unaffected. Simi- larly, if CONVST returns high after the conversion is over t2 t1 CS CONVST RD 1608 F03 Figure 3. CS top CONVST Setup Timing Figure 4. Change in DNL vs CONVST Low Time. Be Sure the CONVST Pulse Returns High Early in the Conversion or After the End of Conversion 0 1750 2000 1608 F04 250 500 1000 750 1250 1500 4 3 2 1 0 CONVST LOW TIME, t5 (ns) tCONV tACQ APPLICATIO S I FOR ATIO (e.g., CONVST low time >tCONV), accuracy is unaffected. For best results, keep t5 less than 500ns or greater than tCONV. Figures 5 through 9 show several different modes of operation. In modes 1a and 1b (Figures 5 and 6), CS and RD are both tied low. The falling edge of CONVST starts the conversion. The data outputs are always enabled and data can be latched with the BUSY rising edge. Mode 1a shows operation with a narrow logic low CONVST pulse. Mode 1b shows a narrow logic high CONVST pulse. In mode 2 (Figure 7) CS is tied low. The falling edge of CONVST signal starts the conversion. Data outputs are in three-state until read by the MPU with the RD signal. Mode 2 can be used for operation with a shared data bus. In slow memory and ROM modes (Figures 8 and 9), CS is tied low and CONVST and RD are tied together. The MPU starts the conversion and reads the output with the com- bined CONVST-RD signal. Conversions are started by the MPU or DSP (no external sample clock is needed). In slow memory mode, the processor applies a logic low to RD (= CONVST), starting the conversion. BUSY goes low, forcing the processor into a wait state. The previous conversion result appears on the data outputs. When the conversion is complete, the new conversion results appear on the data outputs; BUSY goes high, releasing the processor and the processor takes RD (= CONVST) back high and reads the new conversion data. In ROM mode, the processor takes RD (= CONVST) low, starting a conversion and reading the previous conversion result. After the conversion is complete, the processor can read the new result and initiate another conversion. DIFFERENTIAL ANALOG INPUTS Driving the Analog Inputs The differential analog inputs of the LTC1608 are easy to drive. The inputs may be driven differentially or as a single- ended input (i.e., the AIN– input is grounded). The AIN+ and AIN– inputs are sampled at the same instant. Any un- wanted signal that is common mode to both inputs will be reduced by the common mode rejection of the sample- and-hold circuit. The inputs draw only one small current |
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