Hello Yamada-san,
Welcome to the TI E2E community!
The ADS8924B maximum SCLK frequency is specified as 70MHz for DVDD>2.35V, but can typically be much lower. Operating at the maximum sample rate of 250ksps, a minimum SCLK frequency of 15MHz will support all communication modes and frame sizes.
The minimum SCLK frequency depends on the communication mode used (including the number of SDO outputs), the data transfer zone, the number of SCLKs per frame, and the maximum sample rate.
Regards,
Keith Nicholas
Precision ADC Applications
Example:
Maximum Sample Rate: 250ksps (t-cycle=4000nsec)
Communications Mode: SPI-00-S (single SDO)
Number of SCLK per frame: 22 for an optimal command frame (can be reduced to 16 if only reading conversion result data)
Data Transfer Zone: Zone 1, data is transferred immediately after the end of conversion, during the next acquisition period, see Figure 47 in datasheet for details.
ADS8924B Conversion time: t-conv=2500nsec maximum
ADS8924B Quiet Acquisition time: t-qt_acq=30nsec
t-read_Z1 = 4000-2500-30 = 1470nsec
f-SCLK >= 22/t-read_Z1 = 15MHz
