onsdag 10. februar 2016

XM8 DAC board works after all

It's been almost a year since I designed the XM8 DAC and sample and hold boards. When testing the original DAC board I couldn't make it work.I suspected I had broken the DAC chip, and at $20-30 per chip I did not want to waste another untill I had verified my design.

The AD5547 DAC is a 38 pin smd component, with 0.5mm pitch. A real pain in the ass to solder. To breadboard it I had to order an adapter PCB. First time off I got an 0.8mm pitch adapter which of course was useless. Then I designed and ordered another adapter. Due to lack of time and other projects having priority, I never got around to soldering and trying it.

About a week ago I finally worked up the courage and found the time to try breadboarding the circuit. It worked flawlessly and let me write a code example that could be adapted to the PCB version.

Today, after about one hour of trial and error, and with six toddlers screaming and crying around me, I got the original board working! Without any modification! (I did however mess up and somehow connected analog or digital ground to +3v3 (I think) when trying to connect the two grounds, giving me some very weird results. After reconnecting however, everything works like it should.

Here is a picture of the result. The code runs at almost maximum speed on an 80MHz PIC32MX. The output is increased by 10 every time the DAC is updated, giving us about 6500 steps per rise. The scope is set to 2v/square vertically which shows a perfect +/-5V swing.

Horisontally the scope is set to 0.5ms/square which means that each rise takes about 2,5ms which gives a frequency of about 400Hz and a sample rate of 2.5MHz

The assembly code of the loop is 23 commands long. On the 80MHz microcontroller we should be able to run the loop at about 3.5MHz if each command takes one clock cycle, so 2.5MHz seems entirely reasonable. I may have misunderstood something but I think this is the correct speed.

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