There are not many worse feelings than getting through a 2500 piece puzzle only to find out that somewhere along the way your dog must have eaten a puzzle piece or two. Building an electronics system from scratch is like putting together a giant puzzle. Each piece of the puzzle must fit together perfectly and the entire picture does not make sense if you remove a single piece.
Most complicated systems that contain a variety of functionality are multi-board systems. These are found in industrial systems, computers, many consumer electronics, and in other areas where a footprint is not a major consideration. Component placement in multi-board systems can be as simple or as complex as you want it to be, and it can boil down to carefully following the functional block diagram for your system.
Customizing Layout for Multi-board System Assembly
Determining the best layout for a multi-board system is more about choosing the right board arrangement. If you go back to your block diagram for your overall system, you can get an idea of how you should separate functionality on different boards. Separating critical functions in an electronic overall system onto different boards provides a number of benefits.
First, this makes your system easier to test and service. In the event that the system has a problem and requires maintenance, each board can be tested for basic functionality individually, allowing a technician to identify the responsible board and replace it. Separating your power components, core logic units, analog components, and remaining functions onto their own boards actually simplifies the layout for each portion of the system.
For example, suppose you had a display board which contains LCD & Display LEDs, a motherboard having a mixed signal circuit, and a power supply board. This would make the overall board more complex as you will now need to use mixed signal design techniques. Separating these components into their own boards makes each board somewhat easier to design as each board will require separate design rules.
This idea of separating functionality into different boards based on a block diagram makes component placement in multi-board systems much easier from the aspect of design rules. You only need to worry about design rules within an individual board. Connecting multiple boards together into a full system requires obeying a different set of design rules.
Putting together a multi-board system doesn’t have to be significantly harder.
Linking Your Boards Together
Similar to a rigid-flex board, a multi-board system must be tied together using a number of cables and connectors. This can create a number of issues involving EMI/EMC, grounding, and synchronization across your boards. When your entire system is a mixed signal system, you’ll need to take special grounding precautions to ensure signal integrity.
Many designers will tell you that the grounding layout is the first element that should be designed in a multi-board system. This includes the ground to earth for the entire system (usually on a metal chassis) and the ground planes in each board. This is very important and should be done before you start routing signals.
All boards should ground to the chassis at a single point, which should then be placed as close as possible or directly connected to the earth ground for the system. Note that chassis ground and earth ground may be used interchangeably. Wiring ground connections to physically separate points on the chassis creates the potential for ground loops. Instead, run ground connections to the chassis in a star configuration as this helps prevent ground loops due to ground potential differences.
ADCs and DACs: Optimizing for Multi-Board Systems
When laying out components on your boards, you will find that you will need to convert analog signals to digital and send them to another board and reverse. This begs the question: where should you place your ADCs/DACs? This really depends on how you’ve designed your grounding strategy. These mixed signal devices should straddle analog and digital ground sections in order to reduce EMI susceptibility. Making this work requires taking advantage of cables and connectors to transfer a ground connection between boards.
When routing signals between a digital and analog board, you are essentially routing over a split between ground planes, which increases the chances of crosstalk arising elsewhere in the system. It is a good idea to use shielded cables to connect boards, with one wire providing a ground connection between boards. This allows you to ground an ADC or DAC on one of your boards.
The best choice is to route signals between boards is to use differential signaling in your cables and connectors. If this is not possible, you can take a cue from telecommunications and route a signal cable with its reference ground as a twisted pair. If preventing crosstalk is a major concern, you should use shielded twisted pair cables where possible.
Don’t forget to double-check your signal integrity.
One thing to note about multi-board system assembly relates to electromagnetic compatibility (EMC). Individual boards might function properly and may not radiate strongly when tested individually. However, once multiple boards are linked together into a larger system, EMI coupling between boards can degrade the EMC of the overall system.
Modeling potential EMI and EMC problems between boards in a multi-board system is difficult with SPICE models. Although SPICE models are great for analyzing the behavior of a single board, they are not so great for analyzing EMI between boards. Analyzing signal integrity issues and validating multi-board designs is much easier when you use a signal integrity package that includes a 3D field solver that is specifically tailored to electronics.
Thankfully, Cadence has both the PCB layout and analysis systems to handle your design challenges and the simulation and modeling tools to verify and optimize any design need. Work above your potential with Cadence’s tremendous software opportunities.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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