Friday, 8 September 2017

AD9850 LPF Redesign and Amplifier Board

Almost finished the AD9850 LPF/amplifier board.  Just a couple of parts to left to install and they are being mailed to me. Hopefully Canada Post can beat the slow boat from China and I’ll get them by Tuesday next week.

When designing the board, there were 2 options for selecting a footprint for components. Example, 1210 or 1210 handsolder.  I chose handsolder but I really don’t know exactly the differences. I assumed the pads are a little larger, but the tolerance between the contact area on the device and nearby ground spacing is tight. As well the distance between the solder pads seems to be a little on the far side.  Not much room for error.  Anyway no big deal as I check for shorts every time I place a component and a good thing I do that as I do get shorts. 

My boards are on the smaller side of things lately and I’ve noticed that my printer has an easier time with the print density.  Not as many pin holes on the ground planes.

One change I did this time round was to use a pin header for test points instead of individual test pins located throughout the board.  Since this board is not overly complex, I was able to group 5 pins together and this gives a solid support so badly needed.  My previous methods didn’t last too long as the constant attachment of probes eventually ripped up the pad that the tp was soldered to.  Even enlarging the pad didn’t help much.

I did manage to damage one trace and needed to put a jumper in to repair. It was the 2nd component I installed so I almost was going to scrap the board and re etch another but went ahead and carried on.  It’s a prototype board and not the finished product.

I had to change the 2 15pf caps in the LPF to 18pf as I was out of the 15’s. Fortunately LT Spice basically shows not much of a change in the characteristics of the curve so all’s good with that.
I have an Altoids tin available, almost out of them, so I’ll install the board in that to minimize outside influences. 

That’s it for this board until next week unless the other part of the design team points out an oversight. Well actually that has already happened but I was well into this board so I ignored it…for now.  Next version of the board will have a couple of attenuator pads etched in. One ahead of the filter and the other right after.  Thank you Dave.

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