What Makes Mixed Signal Design Hard(er)?
First An Apology...
It has been nearly a month since the first blog post we created, so we want to apologize for the long wait between entries! We have been feverishly working with clients to meet their goals and timelines while delivering the highest quality work possible. We always prioritize our clients' goals and work ahead of our own, but we will try to do better publishing content!
On To the Blog!
Last time we went over what a PCB designer most likely meant when they spoke about doing a mixed-signal design, the different kinds of circuits that might be involved and touched on the high level differences and challenges involved with each kind of circuitry.
So you might be wondering why any of that matters - and why it makes life any harder than doing say a pure analog PCB design for example....
Don't Get Us Wrong...
All PCB design is challenging, mixed-signal or not - let's clear that up right away. Doing a design from start-to-finish means a slew of steps and decisions, mixed in with trade-offs and compromises finally resulting in a finished product that meets (hopefully!) the requirements you inherited. Let's take a quick look at some of these design steps from a PCB design perspective.
Define Your Requirements
In our opinion this is far and away the most underrated and under-utilized step in design work today! Without clear requirements, achieving success becomes a much more difficult task. (Perhaps another E3 Blog will delve into this topic)
Plan the Design
Select critical and core components early! If you are doing an analog PCB with 16 identical analog input channels then do your homework and research to find the perfect op-amp (or op-amps) to use. If you're doing a WiFi module read up on available PA/LNA devices and RF transceivers. Maybe a completed module makes more sense! The point is to do this work well before you start your schematic capture tool up. Plan, plan, plan - and THEN you can worry about execution.
Do block diagrams! Power trees, signal flow, floor planning and system interconnect are some examples of things that can benefit from early diagramming.
Think about PCB stack-up, materials and constraints!
Capture the Schematic
Once key components are picked and you've done the planning you should feel confident moving into capturing the circuit design intent.
Don't forget to document your design decisions! Historical reference is important, 2 years from now when you need to revisit the PCB for any number of reasons having captured critical decisions will be immeasurably valuable.
Layout the PCB
Put copper down on FR-4 (or whatever PCB material you may need to use) and get ready to order your PCB.
That is a good overview to have in mind for any PCB design - and every type of circuit design has its own nuances that are challenging.
So imagine doing the steps we just outlined for what we at E3 might consider a typical PCB for most industries:
ARM Processor and/or FPGA
High Speed Serial Communications
Analog Signal Conditioning and Processing
This is a fairly common mix of circuits for an embedded controller across a wide variety of industries. Here you've got so many things that interact in intended ways, and you need to keep them from interacting in unintended ways. Consider a couple of examples:
You don't want your high-speed communications to couple into the analog signals!
You don't want the high-current loops from the power conversion to couple into anything!
Now start adding more types of circuits to the mix - RF design for example - and the issues start to compound more and more, and constrain your design more and more. Pretty soon you have a hefty challenge on your hands.
Wrapping it Up
Don't worry! Sure it's challenging to do work like this, and it can absolutely seem like an impossible task at times - even for the most seasoned designer. But it can be done - we promise. And we are living proof!
When we graduated we didn't know much about any of this - and 10 years into our careers we are considered to be pretty good design engineers (excuse our lack of humility!)
The things that make mixed-signal design hard also make it fun. It pushes you to learn new things, and to think critically about fundamental engineering principles. We didn't even cover constraints that come from outside of your own design needs - like harmonized standards for safety, EMC/EMI and environmental regulations.
Then there are product driven constraints....but I digress.
Next time on the E3 Blog we'll be taking a look in a little more detail at some things to consider at the schematic level for mixed-signal designs .
Hope you enjoyed this, and we hope to see you next time