Using a dual ADC raspberry pHAT for 20Msps+ acquisition

What does it look like?

Information

What is it supposed to do?

The aim of this module is to achieve 20Msps, at 9bits or more.

How does it work: block diagram

Block schema

  • Raw Signal->ITF-12_RPIn->Selector->ADC->Buffer->Pi->Signal Digitalized
  • Raw Signal->Onboard header->Selector
  • +Vref/2 jumper->ADC
  • GND->Selector
  • External->Selector
  • ITF-4_RawSig->Doj Jumper->ITF-12_RPIn
  • ITF-11_OffSig->Doj Jumper
  • ITF-3_ENV->Doj Jumper

About the module

Pros

Cons

  • SMDs is tough to assemble
  • Linux is not real time: there will be inaccuracies in the measurements!
  • Need a custom kernel module

Constraint and limits

  • See above ;)

Discussions

Let's start with the end: it works!

Testing the module with the signal emulator module, which yields a signal at 1.9MHz, it well seen at 1.9MHz, with a sampling speed equal to two times this freq, ie 24Msps ! See the details below:

Problem and solution

Problem was that the Pi0 or PiW, with the current kernel module, can't run faster than around 10 to 12 Msps. Clock can be faster, but the memcpy limits the transfer to memory. Soooo.. what can we do? As a reminder, I wanted at least a 10Msps 10bit ADC.

Solution Why not interleaving two ADCs, with clocks in opposite phase? Hum that means 2 times 10 bits, and I had only 18 pins left once the Ultrasound SDK board connected. That means.. quite obviously, 2 times 9 bits.

So now I can run at twice the speed of the transfer, so a bit over 20Msps, over this time 9 bits. Or by oversampling, that could be 10Msps at 10 bits. With more flexibility !

Win.

At least for the moment, now I need to order the PCBs and some components.

-> BOM is around 30$ components, 5$ OSHPark PCB.. that's quite neat.

Raw signal acquisition and enveloppe with this ADC pHAT

With the enveloppe

With the raw signal

First acquisitions:

See the jupyter notebook:

Issues: bugs in connected ADC pins + no offset really

What does v2 look like ?

and the corresponding schematics (PDF).

What does v1 look like ?

Top in 3D

Bottom in 3D

TODO

  • Lots

DONE

People

  • @ivan for his help on altium

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