What is Mixed-Signal Design?
So if you're here and reading this, chances are you have some sort of concept, or idea of what mixed signal design is. At the very least, you want to know what mixed-signal design is and why it's so challenging to design boards that span various types of circuit design.
So let's start out by using everyone's favorite source of knowledge...Google! Simply typing in "What is Mixed Signal Design?" yields a mixed bag of results depending on what you're looking for. Most of the results are about ASIC design. (An ASIC is an application specific integrated circuit - a prime example of this would be an analog-to-digital converter)
However, we can still apply the general concept and definition of a mixed-signal ASIC to mixed-signal PCB design:
So for the purposes of our discussions, we will just replace some phrases:
'Semiconductor die' with 'printed circuit board'
'Integrated circuit' with 'design'
Viola! Mixed-Signal Design is defined.
Breaking it Down Further
OK, so great, we have a definition to work with. But what does it mean, really?
Analog and digital circuits?
Isn't everything analog anyways? The world is analog, right? Digital signals are just analog signals interpreted as a '0' or a '1' right?
Yes, absolutely. All of that is correct at it's most fundamental conceptual level. However from a design perspective what you do with the signals you are generating or measuring does have a pretty significant impact on what you need to consider during your design work.
What does a mixed-signal designer mean, or think of, when they say they have to do "analog signal conditioning" or their design has to generate "analog voltages"?
The answer is (and this will become a painfully annoying theme, I promise) that it depends! At the most basic level an analog signal is a continuous signal that has a time-varying quantities. (e.g. a sine wave)
Some common examples to consider:
Measuring the analog voltage output from a transducer measuring force, temperature or pressure
Turning a small voltage into a bigger voltage (or vice-versa) using an op-amp circuit so it can be digitally sampled and processed
Using one or multiple stages of op-amp circuits to turn digital communication stream into an analog signal that controls something, like a motor for example
Generally speaking (because after all, it depends) analog signals are expected to be, and almost always need to be "quiet". Free of noise, pure and stable.
If you want to control a motor with a 0V-10V control signal, and you want that motor to run at 50% speed, you want that voltage to stay steady at 5.00V - you don't want 5.10V or 4.90V, and you certainly don't want that voltage jumping around in value.
If you are measuring a thermocouple you have very little room for error in your conditioning circuit if you want reliable measurements. A difference of 4mV will throw your reading off by 100C!
Low Speed Digital Signals
What does a mixed-signal designer mean, or think of, when they say they have low speed digital signals on their design?
Once again, it depends! At the most basic level a digital signal refers to an electrical signal that is converted into a pattern of bits, '0'and '1', our old binary friends. Unlike an analog signal, a digital signal has a discrete value at sampled points.
Low speed is also relative, in other words, it depends! For the sake of discussion we will draw a line at 10 MHz. Why 10 MHz? Because it's a round number, and we had to draw a line somewhere.
Some common examples to consider:
Using general purpose input/output pins (GPIO) to control some sort of actuator, or set a certain mode on a device. You might turn this device on or off once every minute, once every hour, or once every day
I2C communications in between integrated circuits. For example you might use an Arduino, BeagleBone, or Raspberry Pi to communicate to a digitally controlled temperature sensor, or external memory chip. I2C runs at a modest 100-400kHz
RS232 communications from your device to another device. The old faithful standby of engineers everywhere, the good old UART. (For our friends who may be too young to know what a UART is, see below)
Remember our analog voltage examples? There were some cases where 10mV mattered, some cases where 100mV mattered. There are other extreme cases where microvolts and picoamps matter!
Digital signals? They are much more forgiving in this regard - let's take a look at some common voltage levels:
Look at those ranges! For LVC logic you could deliver a voltage anywhere between 2.0V and 3.6V and still "get away with it".
You might be asking why the speed even matters - great question! The higher frequency your signals are, the more that you must consider transmission line effects, return current and loop area, and EMI/EMC (electro-magnetics - yes, we hated that class too, don't worry)
These things still matter at 10 MHz, but they matter infinitely more at 1 GHz. The processor on a BeagleBone runs near 1 GHz, so it's not crazy to consider designing at higher frequency.
High Speed Digital Design
What does a mixed-signal designer mean, or think of, when they say they have high speed digital signals on their design?
Once again, it depends! They might be talking about a high speed serial communications interface, such as RGMII (commonly used to communicate to Ethernet transceiver PHY devices), USB 3.0 or a DDR memory interface. They also might have a wireless internet chip (Bluetooth) for their IoT project. They might have both! They might have many of each!
The topic of high speed digital design could be a blog series all on its' own - in fact people have written hundreds of papers, and books about the nuances of high speed digital design.
The important thing to realize is that these types of interfaces are difficult to route on a PCB, and they are very sensitive to even minor errors! Mess up the trace impedance, routing structure or signal termination and you might have a completely non-functional interface to your network IC or memory!
(Sometimes even worse than a non-functional interface is a marginally functional interface, which is where simulation and design verification become critical...but I digress.)
The same can be said for RF design, which has all the pitfalls of high speed digital and then some!
What does a mixed-signal designer mean, or think of, when they say they have to do power design?
(By now I am sure you've caught on...it depends!)
Let's just make one assumption and carry on from there. You have been asked to design a system that runs from one "bus" voltage. This might be 24V-DC in a bench-top instrument, it might be a battery pack for a cell phone or other portable IoT device or it might be 24V-AC in an industrial application.
So if we take the above as the situation we as designers find ourselves in, think of all the different voltage levels we may need to provide
Microcontroller core and I/O voltages
5.0V, 3.3V, 2.5V, 1.8V
Auxiliary voltages for things like valves, fans and relays
48V, 24V, 12V
That is a lot of voltages that we need to create, and there might be a lot of current behind them as well. All of that power switching generates various kinds of noise (circuit to circuit interference) and heat (which might affect our sensitive analog circuits).
You also might have to design circuits to then "switch" that power you've generated on and off to turn your valves and relays on and off when your sensitive analog sensors tell you to do something!
Wrapping It Up
OK, so now you know what we mean when we say mixed-signal design. And I'm sure you can begin to think of some of the challenges involved.
But don't worry! All of this design work can be done successfully. We ourselves have designed boards with all of the above (and more even!) that are shipping in products around the world.
If we learned to do it, you can learn to do it too! Just don't be afraid to ask questions, and make mistakes. I've learned the most from mistakes, not from easy successes.
Next time on the E3 Blog we'll be taking a look in a little more detail at what makes mixed-signal design hard(er) than other PCB designs (which are by no means easy).
Hope you enjoyed this, and we hope to see you next time