In today's world, ribbon cables are widely used as a way to interconnect PCBs with each other.
They are low-cost and very easy to install.
In spite of this, they come at a price. It is very important that you know how to choose how the current flows on each pin, otherwise, you may get into a lot of trouble during the EMC tests.
It is important to recognize that the performance of a ribbon cable is dependent on how well the designer and engineer understand the importance of a good ground return within a ribbon cable as a whole.
I believe it is safe to say that the wider the loop created by the closest return current, the greater the parasitic inductance you are creating and the bigger the problem you are going to encounter when testing for EMC is going to be.
During the course of the last year, for example, I had the opportunity to work on a project that was going through some serious difficulties. In that design, one of the major problems that were encountered was the poor selection of the pins on two ribbon cables that were part of this design. This is why I would like to share some examples of bad and good pinouts with you in order to help you make an informed decision.
Now let's take a look at the next figure.
FIG 1. several configuration of ribbon cables
Using the first example, we can see that there is a separation between the VDD and the GND at the two edges of the ribbon cable. As a result of this configuration, we have a large inductance between the VDD and the GND, and we have current loops of different sizes between the VDD and the GND. If, for example, those signals were to be sent to an ADC, I would expect the signal that is closest to the GND pin would be the least affected, whereas the signal that is closest to the VDD pin would be the most affected.
FIG 2. Try to minimize W (for each signal) and you will minimize both emission and suscettibility
I would like to point out that in the second drawing down, we have improved the effectiveness of the Vdd/gnd pair, but overall, it is still a poor design for the same reasons mentioned above.
In the next drawing down, it is possible to see an intermediate solution where we have at least one GND return pin for every two or three signals that we have. There is no doubt that this solution is, in most cases, acceptable, but it is not my favourite.
Next, we will see a solution that I have used many times before, where I place a gnd return on each data signal that I receive.
In most cases, this is a good enough solution. In spite of this, the cost is still relatively low. In spite of this, as you can see, the number of wires in your design has increased, and as a result, the cost of your unit has increased as well. Therefore, you need to balance the benefits of the project against the risks of the project.
As can be seen in the last drawing, we have a shielded ribbon cable. Several manufacturers sell shielded ribbon cable and this type of cable is available from quite a few manufacturers. In spite of the fact that I have never had to use one of these solutions in my designs, I have shielded ribbon cables during pre-compliance in order to prove to my customer that their ribbon cable design was very poor.
Comments