In reading the datasheet for the HMC625, there is a bit of vague information. If I plan on using the SPI interface only to control the gain, is it possible to leave D0-D6 unconnected? Are there internal pullups or pulldowns on any of these signals?
Also can I leave PUP1 and PUP2 unconnected?
Moved to RF and Microwave.
On the first question, it's best to terminate all unused logic pins to gnd with a 10K ohm resister.
For the SPI bus, as shown on pg 5 of the datasheet, in the "Control Voltage Table", you can use any logic above 2 volts, whether Vdd = 3 or 5 volts.