USB Measurement Interface Development

Last couple of months I had a deep dive into a wonderful world of mixed signal electronics. And now I know how deep the rabbit hole goes! I went through all this trouble to finally upgrade my main audio measurement device – ESI Juli@ sound card. I bought it somewhere around 2008 and it served me well since. Sure, I managed to fry output muting transistors couple of times but that’s my bad.

Apart of my own misuse I have not much to complain about. For the price it’s a wonderful sound card capable of -115dB SFDR loop-back and max 192Khz sample rate.  I’m not a nano-volt-distortion hunter/believer, so when used as audio measurement interface for THD and FRQ response it’s more then adequate for my hobby needs. But as with all things in life – it became obsolete. It’s almost impossible to find an adequate PC’s with PCI slot and whole PCI/PCI-E riser thing is kinda messy. So it’s time for upgrades.

I would love to sink my teeth into something like APx555, but depending on config it’s a >10k€ device. I have no means of justifying such an investment. There are cheaper alternatives, but nothing that cought my attention for a good price/features ratio. So rolling my own solution doesn’t seem like a bad idea. I must admit that I was contemplating on this idea for a couple of years now. And I finally feel like I have enough time/experience and resources to tackle a project of this magnitude. Here are the main design goals:

  • Full galvanic isolation from PC side
  • Only USB bus powered
  • <-120dB SFDR loop-back and <192Khz SR
  • 1Vrms (+0dBV, +2.2dBu) Balanced in/out
  • 2Vrms (+6dBV, +8.2dBu) SE in/out
  • -20dB, -40dB Input attenuation
  • 1M input impedance (for tube measurements)
  • Over-voltage in/out protection
  • Smallest workbench footprint possible

This might not seem like much, but this particulate set of goals has required quite an elaborate dance of engineering compromises from my side. And it had me cornered couple of times pretty hard, especially the power budget part. Whole system design and integration was also non trivial. I had to make an accurate 3D models and check all tolerances in CAD.

 

Just When I Thought I Was Out, They Pull Me Back In!

-Michael Corleone, 1990

This quote goes to the last DAC/ADC manufacturer that makes any kind of innovation in this field for the past 20 years.  I’m talking about AKM factory and fire that put IC’s at the heart of this project out of production for indefinite time… I managed to snatch a few ADC’s until stock lasted, but DAC’s went dry the same day news broke out. I have ideas on how to get my hands on couple IC’s but it’s already obvious that this particulate design will not be reproducible for some time.  Right now I’m planing on making 2 prototype units and if all goes well 1 unit will be for sale.

Current project status:

  • PCB design – finished 100%
  • PCB production – order submitted (50€)
  • BOM – ordered from digikey (240€)
  • Mechanical parts – ordered (box, knobs, etc.) (30€)

Preliminary project price (without PCB and mechanical assembly’s) – 320€. Stay tune for the updates, this roller-coaster is only starting!

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