I have been wondering for quite a while whether it's good practice when connecting two boards together (powered from the same power supply, although each has its own regulator) to send ground connections along with the digital connections.
For instance, I had a board which contained a AVR atmega microcontroller (controlling an external ADC) connected directly to a FTDI USB to serial converter. This worked well. Now, I'm splitting the boards into an ADC board with the micro+ADC components and a board containing only the FTDI chip. The connectors on these boards are screw terminals, and I'm going to use SIP pins to connect the terminals together instead of wire. This ensures minimum distance the 8 logic signal has to travel between boards. The rise time of these signals are on the order of ns, and are running at 1MHz.
The question I'm having is whether it's good practice to add more pins for ground connections between boards. I understand that giving each digital pin its own ground pin will reduce coupling between pins and provide better ground return paths. However, that will add to the pin count. Also, I'm worried it will feed back noise between the boards.
Previously, when sending wires between other boards I've had capacitive coupling between them. So I'm trying to find what the standard practice for such a situation is.
Thanks in advance, M