For digital connections, you need to do timing analysis.
Your receiving device (whatever receives digital data from ADC) will sample the data at the clock edge. You need to ensure that when the edge comes, the correct data is on the line. For that, the receiving device will have two characteristics (typically in the datasheet):
- setup time - how much before the clock edge the data must be on the line. For example if setup is 3 ns, you must make sure that the data is on the line at least 3 ns before the clock edge.
- hold time - how long the data must be kept on the line after the clock edge. For example if hold is 2 ns, you must make sure that the data is still on the line for at least 2 ns after the clock edge.
The circuit/PCB design must make sure that the setup/hold requirements are met. So, you calculate two entities:
First, you calculate the longest delay, which would consist of
- clock jitter (the edge may be a little late)
- the maximum delay which is necessary for the clock to get out of your device
- the clock PCB trace delay
- the maximum ADC delay (see the datasheet)
- the longest data PCB trace delay on the way back
- clock jitter (the receiving edge may be a little earlier)
Then you calculate the difference between the receiver and ADC clock edges. Say, if ADC posts data on the rising edge and you want to receive on the falling edge, it is half of the clock cycle - about 7ns at 60 MHz. Then you subtract the setup requirement from that number. The result tells you how much of a delay can be tolerated. You compare it to your longest delay. If your longest delay is bigger, your interface won't work.
Similarly, you calculate what happens if things happen fast. The shortest delay will consist of:
- clock jitter (the edge may be a little early)
- the minimum delay which is necessary for the clock to get out of your device
- the clock PCB trace delay
- the minimum ADC delay (see the datasheet)
- the shortest data PCB trace delay on the way back
- clock jitter (the receiving edge may be a little late)
Also, you calculate what is the shortest delay you can tolerate. The short delay might be bad because ADC may post the next data too soon - before your receiver is done with the previous data point. So, you calculate the difference between the clock edge where your receiver samples the data and the clock edge where the ADC produces the next data point. This may be negative (which is a good thing). You add your device's hold requirement to this, which may turn it into positive. You compare the result to the shortest delay you have just calculated. If the delay is too short, your interface won't work.
Once you do such timing analysis, you will see if your data connection works or not. If it is on the edge of working, you will have to change something. One of the things you can do is to make PCB traces shorter (to decrease delays), longer (to increase delays), or more even (to decrease the difference between minimum and maximum delays). If your timing analysis shows that there's a need to make data lines more even, you need to worry about the length matching.