I am now working on integrating the AD5263 digital potentiometer.
Each submodule section will provide 4 quad potentiometers, for a total of 16 potentiometers (one per analog port). These will be connected to the Raspberry Pi 2 through the SPI bus.
The AD5263 is available in three options for the nominal resistance: 20 kΩ, 50 kΩ, and 200 kΩ. This resistance level is a static characteristic of the device, not something that can be dynamically set (contrary to what I had assumed originally). Therefore, we will need to define that at design time. I am not yet sure which nominal resistance would be most appropriate, or whether we need to mix and match levels.
The AD5263 supports daisy-chain operation. We might want to take advantage of this feature, and daisy-chain all four devices within every submodule. Therefore, we would have 8 clusters of 4 devices on the SPI bus. I’m still not clear whether this is better than having 32 devices on the SPI bus, and we should also keep in mind that we will need the SPI bus for driving 8 analog-to-digital converters.
Because of the ESD protection diodes of the AD5263, we will need to pay special attention to the power-up sequence. VDD and VSS will need to be powered first.
Power Supply Bypassing
The power supply should be bypassed with capacitors.