I have an ADC attached to my uC on a custom PCB. The traces are just directly connected between the uC and the ADC. Here is a photo of an oscilloscope trace of SCK and MISO. SCK is Blue, MISO is Red. I see my clock and my data, but they are only about 940mV of the 3.3V range. I have a bypass cap on the ADC (and the uC). Where did my voltage go? Its a hobby oscilloscope. I can try a better one tomorrow.