ADS7038-Q1: daisy chain SPI

I am reading following application note.

Daisy Chain Implementation for Serial Peripheral Interface (Rev. A)

Thanks Tom. Will you introduce other SPI ADC daisy chainable?

Nice!  That is a great app note but it is not an overall commentary on standard SPI interfaces.  Data in and Data out are most often two separate entities, as is the case with the ADS7038.

This is very difficult to identify either from reading of Datasheet.

Maybe you can start here:

https://www.circuitbasics.com/basics-of-the-spi-communication-protocol/

'Standard SPI' does not have a serial shift register from DIN to DOUT.  There is an input buffer and/or output buffer.  Some ADC's do not have an SDI, some DAC's do not have an SDO.  Some devices can have a serial shift from DIN to SDO (see ADS8887 as an example), but it is not a native SPI operating mode.

ADS7038 has SDI and SDO, and I understand these were designed by different shift registers by your explain. and I think datasheet need to be explaining the internal block diagram of SPI in ADS7038.

If you look at the ADS7038-Q1 datasheet, page 32 thru page 34 lists various registers found inside the device.  The only way to program those registers is through the SDI.  Register read back or conversion results come out of the SDO pin.