My design requires a 16-bit DAC with 100kS/s plus and SPI communication. Strangely to me, the cheapest 16-bit DACs that meet the requirement (parametric search) have a 24-bit SPI word, like the AD5662 (pdf) I'm trying to use. The least significant 16-bits are the output value to update and the upper 8-bits are configuration (only 2 of the 8 are actually used). This mode of operation is inconvenient for my 16-bit micro-controller, as the associated ADC in the design has a 16-bit SPI word, ADC161S626 (pdf). This means my micro-controller needs to write x3 8-bit packets instead of x1 16-bit.
Why do manufactures do this? What would be the best way of perform the communication?
If both DAC and ADC were 16-bit SPI word I could read and write at the same time without having to reconfigure the SPI module and data in the micro-controller.