Electronics > Projects, Designs, and Technical Stuff

OSHW - 24bit ADC measurement system for voltage references

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timb:
I actually would be in the market for something like this, but since you can't be bothered to keep your documents current, I won't bother either. :)

TiN:
It takes 5 seconds to export PDF and attach to files, and saves all that time wasted with valid and invalid complains. If every engineer will post their hobby stuff only in software they used, discussion would be less open and wide. Viewing schematics is all that most of interested folks need, as those who will bother building something likely to use their own chassis, own PCB layout, own components set they want anyway, and PDF-schematics is more than enough to test your idea.


--- Quote ---Almost every CAD for printed circuit boards is able to import Eagle Files.
--- End quote ---
3000 EUR OrCAD can do nothing with Eagle files. Yes, it's not your problem.

I was remotely interested as just got few LTC6655's and ADCs not long time ago and going to bodge little PCB for few tests into my order, so hence came to this thread to check out on similar design ideas.. Baa , can't see files. :)

AlfBaz:
Hi Branadic, first let me thank you for posting this and Andreas for allowing it.
I'm in the process of duplicating the circuit using altium, roughly copying your PCB layout and routing.

I'm sure this circuit has been tweaked for minimum noise and as I work through the design I see some of the choices you've made but I still have a few questions

The relatively high series resistance to the optos seems to be to round off the edges on the pulses and the even higher resistances (22k and 47k) to allow the pic to be programmed
The one part I'm stuck on is why the parallel 2.2nF cap across the 3k3 resistor from the PIC to the opto. This seems to consume a large spike of current from VDD on transitions given the typical 10ns rise and fall times, am I missing something here?

B1 and B2, are they ferrite beads? If so i'm curious which frequency you chose 70, 100MHz or something else

The choice of micro, was it based on its low current consumption and adc for the thermistor? Did you create a software UART  and SPI for the interface to the ftdi usb and LTC2400 or is it just some rudimentary bit banging? I ask this because the PIC you have chosen is quite low current and don't know if I'll find anything lower

When you run this in conjunction with the 2-1 divider where do you power the whole lot from, DB9 plug or from the JP1 header on the LTC2400 board?

And finally after you tested did you end up cutting out the LTC2400's mounting square and go dead bug like Andreas did. In other words, did you end up noticing the effects of humidity
 

Andreas:

--- Quote from: AlfBaz on June 22, 2019, 07:35:35 am ---The one part I'm stuck on is why the parallel 2.2nF cap across the 3k3 resistor from the PIC to the opto.

And finally after you tested did you end up cutting out the LTC2400's mounting square and go dead bug like Andreas did. In other words, did you end up noticing the effects of humidity

--- End quote ---

Hello,

intention of the 2.2nF is to speed up the rise time at relatively low power (~1mA).
Otherwise 9600 Baud are marginal with PC817 and 1 mA.

The LTC2400 does not need decoupling from the PCB. (Self adjustment during each measurement).
Only the voltage reference shows a dependancy from PCB stress and should be decoupled.

with best regards

Andreas

branadic:
Hi,

B1 and B2 were choosen to be 600ohms at 100MHz, if I remember correct. Micro was choosen by Andreas because of battery operation and thus low power consumption required.

Power is always supplied via JP1, DB9 powers additional modules connected to it.

I didn't test the ADC to excess as Andreas did, however I have the reference still mounted directly onto pcb but NTC was changed to 0402 type and this way is much closer to the reference compared to through hole NTCs.
What I wanted to try is to place reference on a flexboard, which is then soldered to the board giving some strain-relief. Since real semi rigid pcb's are barely expensive and dead bug style is not a good production process I thought this tradeoff should give best results to keep board stress due to humidity away from the reference. But haven't tried that yet as some other projects came into focus.

-branadic-

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