I had this question about integrated circuits that have multiple power supply pins: the ADF4351 for example, has the SDVDD, AVDD, DVDD, VP, VVCO and VVCO2 pins listed on the datasheet. To minimize interference, i have used 2 separate voltage regulators (named VR1 and VR2): one for SDVDD, AVDD, DVDD, VP; and one for VVCO and VVCO2. I have one common ground plane that couples everything together.
This brings me to the problem: the pins of the SPI-interface are connected directly to an FPGA, which uses an own power supply. I was wondering if it is necessary to use this same power line to power the DVDD pin? My opinion: the ADF4351 has CMOS-inputs, and the FPGA has CMOS-outputs.
When one of the pins of the SPI-interface of the ADF4351 is driven low, current from DVDD is sinked to the FPGA.
When one of the pins of the SPI-interface of the ADF4351 is driven high, current from the FPGA-supply is sourced into the ADF4351.
As the return paths are on the common ground plane, I presume it is NOT necessary to use the same power line to power the DVDD pin. I just wanted to make sure this is true in the GENERAL case, and not only in the case of CMOS-logic.