В datasheet ADS1253 сказано, что требуется пять внутренних циклов преобразования.
When multiplexing inputs it is possible to achieve sample rates
close to 4kHz. This is due to the fact that it requires five internal
conversion cycles for the data to fully settle, the data also must
be read before the channel is changed. The DRDY signal
indicates a valid result after the five cycles have occurred