If ground ratio does refer to the ratio of signal to ground pins in a connector. The assumption is that it refers to single ended signals like TTL or CMOS. This can be a complicated calculation. The general rule of thumb that I use is this. For interconnect between PCBs in the same chassis and common power supply. Must have a low impedance ground connection between the boards preferable with a ground plane. Use one ground per signal. Never use single ended signals to connect between circuits that do not have a common power supply and low impedance ground connection. For example you would never Run CMOS between two chassis with different power supplies.
I will attempt to explain why.
A single ended signal is one that used the ground for the return path like TTL or CMOS. RS422 is a good example of a differential signal. It does not use the ground for a return. The high input impedance of the receiver approx 4-12 K ohms breaks the ground connection between the two chassis. There are many other differential drivers and receivers. See reference below.
The problem with single ended signals is that power supply currents flow in the signal return and the signal current flow in the power supply return. This is the definition of a ground loop. Two methods of eliminating ground loops are a Star ground topology and using isolated power supplies. If you run a signal ground across the star topology it causes a ground loop.
Now to calculate the ground loop current required to cause a logic error.
The noise margins for the various logic famlies are shown below:.
Noise margin = Voh-Vih or Vil-Vol which ever is less. = 0.4volts for all but 1.8VCMOS.
If your ground connection in the signal cable has a resistance of 0.1ohms it will take 4 amps of power supply current flowing in the signal return to cause an error. This could happen if you had a Motor sharing the power with a start up current of 8 amps and a .1ohm ground resistance. Half of the moter current would flow in your signal cable causing the error. If your cable has a resistance of 1 ohm then it would only take 400milliamps of current. This could happen if enough signals of the same polarity switched at the same time. Now you see why a low impedance ground is beneficial.
For more detailed info see these two documents.
Logic Guide 2009
**broken link removed**
Advanced Bus Interface Logic Selection Guide
https://www.ti.com/litv/pdf/scyt126
HCT ACT HC AC LV-A LVT AUP LV-A AUC LVC AVC AUC AUP LVC GTLP
5V 5V 3.3-V 2.5V 1.8V 1.8V
Logic Family TTL CMOS LVTTL CMOS CMOS CMOS
Voh 2.4 4.4 2.4 2.3 1.2 1.5
Vih 2 3.5 2 1.7 1.17 1.05
Vil 0.8 1.5 0.8 0.7 0.63 0.95
Vol 0.4 0.5 0.4 0.2 0.45 0.55
noise margin high 0.40 0.90 0.40 0.60 0.03 0.45
noise margin low 0.40 1.00 0.40 0.50 0.18 0.40