Insane Hydraulics
Bold and audacious blog about fluid power
Now that I have the schematic for the Super Pressure Gauge ADC board all figured out, I need to design the PCB. But before I go into that, I want to explain why I believe that making a separate board for the analog-to-digital converter is such a good idea.
If you look at how the "king" of digital pressure gauges - the trusty Parker SCJN - is designed, you'll see that both the analog and digital parts of its circuit "live" on a single-piece PCB, placed in a die-cast zinc alloy housing that shields it against electromagnetic interference. This design is simple and effective, and there's nothing wrong with it.
But the "super pressure gauge" is different - I will be using a Silicon Labs Bluetooth module for the "brains", and along with employing its 32-bit micro-controller for driving the ADC and the graphic display, I also intend to use its wireless functionality for live transmission of pressure readings to a smartphone app. This means that if I use a single PCB, I will have a micro-volt sensitive analog circuit (with four "antenna" wires connecting ADC to the transducer) sitting right next to an 8 dBm 2.4 GHz active transmitter!
So, my (big) idea is to make a small round PCB that will contain just the ADC and the driver chip for the AC excitation of the bridge (see the diagram from my last post for reference) - and then place it above the transducer in a steel enclosure, exposing just the power and digital communication lines to the outside world:
This approach, theoretically, still leaves one sensitive wire "in the open," namely, the 5V power line for the analog part of the ADC, but I suppose it can be shielded too, if necessary. In any case - since I don't even know if this thing is going to work or not - I'll not worry too much about it, at least for now. But I do like the idea of having an ADC and the respective pressure transducer inside a compact shielded container - seems like a solid way to address EMI issues.
Now I can go back to the PCB layout - and I confess I spent quite a lot of time "pushing the footprints around" before I came up with this "masterpiece":
This is a simple two-sided round PCB (22 mm in diameter), all of the components are located on one side (the side that will be facing the transducer), and the analog half of the ADC sits above a solid copper plane, which (hopefully) will provide adequate shielding.
So, what's next?
"In the old days", I would have to take care of making this PCB, then sourcing the components, and then painstakingly assembling the super tiny SMD components over a hot plate. Luckily, we live in an age of progress, and all I have to do now is send the bill of materials and the gerber files to a fabrication house, and wait for a couple of weeks for the professionally assembled PCBs to arrive! So, yes - the next step is - have the PCBs manufactured. Hopefully, I'll have something "tangible" to experiment with real soon!