Hi Atsushi,
The ADS1241 is a delta-sigma dataconverter that continuously oversamples the input signal. The conversion period is defined as from one falling edge of DRDY to the next falling edge. The actual conversion ends at the t18 (page 6 of the datasheet) period just prior to the falling edge of DRDY. So the conversion runs continuously. You do not start a conversion as you would do with a SAR converter.
However, you can restart the conversion which basically resets the modulator and the digital filter. You can do this by either pulsing the DSYNC pin or by issuing the DSYNC command. The information regarding the DSYNC operation is discussed on page 14 of the datasheet.
If you do not read the results data, DRDY will pulse at the chosen data rate. The pulse will be the t18 period, going from low to high back to low. DRDY will also go high after a read of the data has been completed. Unfortunately we do not have specific timing as to when this happens. It will depend on the clock polarity chosen. I have attached a picture of data I collected with SCLK dwelling low on idle and data being read by the RDATA command. You can see that the DRDY goes high on the last clock.
You need to finish reading the data before the next update or the results will be corrupt when reading through the t18 update period. It is best to read the results as soon as possible after the falling edge of DRDY. Interrupt driven systems work the best, but because of the slow data rates of the ADS1241, you can also poll the DRDY pin to look for data to be read from the device.
Best regards,
Bob B