Monday, December 18, 2017

Custom Build-A-Box

One thing that many Analog Engineers who work on very low noise and low frequency circuits figure out very quickly is that even simple room air currents destroy the achievable accuracy of our circuits.

Most of us first figured this out when we hooked a Strip Chart recorder to our designs output for a day and, much to our surprise (and dismay) the output would invariably track the day and night room temperature variations very well. We managed to build a thermometer when all we really wanted was a very low drift amplifier.

Air is not only good to breathe but, it has a thermal mass, it has a temperature and it has a thermal transfer coefficient. When air circulates around a low noise / low frequency design it either heats or cools the circuit and normally in a not so evenly manner. This gives rise to all sorts of thermocouple effects that are exasperated by the air induced temperature gradients.

The first thing that most of us grabbed when we observed this effect was observed was some cardboard box that was dutifully placed on the circuit in question to keep the room air currents off of it.

But the ‘Fun’ has just begun...

Modern semiconductor packages are so small and thin now that many designs are now sensitive to IR light. Yes the IR light that your fluorescent lights give off is enough to bombard the IC’s transistors with photons because the packages are now transparent to IR light. This can also cause unexplained circuit drifts. Again a ‘Cardboard Box’ is the solution (Keep the circuit in the dark!).

Even light and air currents take a back seat to Long Wave and AM radio stations messing with your circuits. If you have never worked a few blocks from an AM radio transmitter, well let’s just say you have sure missed some fun hours of debugging there [1]. Those Florescent lights are major EMI ‘aggressors’ here too. Not the ones on the ceiling, as they are usually far enough away. The real trouble makers are the ones on your lab bench or even the microscope light. Yes, my microscope light is a real circuit ‘destabilizer’ - a ‘double whammy’ with IR and EMI bombardment!

Even a “Dark Cardboard Box” won’t filter out RF, so naturally, we just wrapped the box with aluminum foil or copper tape! There! A job sorrta well done...

Then Vs. Now

That was then, when our breadboards were spread all over the lab bench. Today our breadboards are finished products and may well ship to customers. This makes finding a suitable “Cardboard Box” to ‘tighten up’ the finished design very difficult indeed.

3D Printing to the Rescue

A 3D printer can quickly make any size or shape “Plastic Box” that you want, when you want it.

In a current design, I needed to get some air current isolation from the main circuit to the very sensitive and high gain Analog Front End. Years ago I would have hacked something together with scissors, an X-Acto knife and the cardboard off the back of a paper tablet. This would take a while and it would have looked pretty amateurish by the time I wrapped it with Electrical or Kapton tape.

For this design I just imported the Step model of the Analog PCB [2] into Design Spark Mechanical [3] as shown in the figure below. In less than an hour I had drawn the top and bottom of the box I wanted fabricated right around the PCB model and exported it as a STL file to my 3D printer.

The Custom Box was designed around a 3D accurate STEP model produced by my Altium PCB software. In less than 2 hours the box was printed as shown below.



The finished design as viewed from Design Spark Mechanical. The design is simply 'drawn' around the 3D accurate exported Altium PCB Model.


The actual finished two halves of the 3D Printed Box. Note the cutouts where the IO connectors have to go. For a real design I would have used Black Material because it looks more ‘professional’ but yellow shows up better in the photos.  


Bottom of the custom 3D box installed on the main board. The Analog Front End IO Connectors fits in the blue connectors that poke through the cutouts that I made in the 3D printed box bottom.
  



The base and top of the 3D box as it fit on the main electronics PCB. Way better fit than cardboard ever was.
 

The custom 3D box covers the analog front end board well. The Analog Input connectors in this particular design are BNC connectors.



Since I wanted EMI shielding also (you never know where your design may end up [1]), I sealed the box together with my some of my favorite copper tape. Be sure to always use the copper tape with conductive adhesive. The Copper tape enclosure is ‘grounded’ by electrically connecting the copper tape to the BNC’s with an overwrap.


Conclusion

3D printers are certainly handy, not only in Robotics Projects but in making custom, 21st Century “Cardboard Box” replacements in less than 3 hours start to finish – and they look better too.

Certainly a more professionally looking finished job and better working instrumentation too boot!


References / /Notes:

[1] I once worked for a company making instrumentation products that had a manufacturing plant in Puerto Rico. About a mile from the plant was a 1000 foot tall Navy Communications antenna. It did not broadcast continually, but when it did you couldn’t test anything at the plant site! Thank goodness the broadcasts happened very infrequently, otherwise we would have had to move!

[2] Altium has native 3D PCB design. Exporting a 3D Step model of any design is a “one click” operation. Then that model can be imported into any modern #d Drafting Package.

[3] Design Spark Mechanical – A very easy too use, free program from RS / Allied.
https://www.rs-online.com/designspark/mechanical-software


Article By: Steve Hageman www.AnalogHome.com

We design custom: Analog, RF and Embedded systems for a wide variety of industrial and commercial clients. Please feel free to contact us if we can help on your next project.


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