The Saga with AD9850

Just when you think you have covered all the bases and have finally got it right, along comes the inevitable. If you’re lucky the circuit works as planned, works but not quite right, and in some cases you let the smoke out. 

After a few rounds of testing and trying hard to not pay attention to that little voice in the back of your head, you unplug the power and take a closer look at the board, datasheets, and the schematic.  In the case of my lpf/mar-6 board, a missing cap, a resistor that should not be there, and another missing capacitor.  So the cure was cutting a trace, adding stuff, removing stuff, and voila, it works.

It’s amazing to me the feeling you get when something you build from scratch works and to go from the frustrations when it doesn’t work to when you fix it, even more.  Mind you it would be nice once and a while to have a circuit work right out of the gate.

It’s an interesting project where on one hand it first appears to be a simple project.  Combine an lpf circuit with an amplifier and away you go.  So where did this get complicated?

1, Board layout. Need to go through 3 or 4 revisions at least before getting it close enough to actually etch.

2, Components you thought you had on hand, you don’t, or are of the wrong footprint.  Now you have to beg or borrow, wait a month for that slow boat, or pay some hefty shipping charges.

3, Errors you make due to stupidity or a lack of knowledge. The latter being ok as it is all about learning. The former, well make up an excuse to explain to your buddies.

4, We now have the option of simulating circuits. Confirming that the circuit at least has the chance of behaving the way we expect or at least get you in the ballpark.  LT spice to the rescue. However, there is a learning curve that is well worth the time spent in my opinion.

5, By now you are in so deep with the prototype, it can be hard to let go of it.  Recognizing when you can continue with the present board or do start over.

6, Working with a team on a project. Unquestionably a great way to go about a project. I would never have learned so much on my own as I do being part of a team.  Individuals bring different strengths and knowledge to the group, and besides, it’s way more fun. So why do I have it listed under complications? Well, plainly stated, information overload. It’s all inversely proportional of course to the amount of time one has. The better organized you are the better you can track, remember, and implement those new ideas and suggestions.  Who would have thought that I can be busier at my hobby than I was at work before I retired? 

For my latest board, going to give it another go. Couple more modifications and some more testing that hopefully turns into a proof of concept.   Pictured below, shows the board in its present state.  The fixed resistors for biasing the MAR6 were removed and a pot substituted. This will give me better control over setting the current. Missing cap added in and another one still to be added. The thru hole cap visible has a trace broken below it to allow for circuit changes required due to oversights on my behalf…. To put in nicely.