Electronics > Projects, Designs, and Technical Stuff
OSHW - 24bit ADC measurement system for voltage references
AlfBaz:
--- Quote from: Andreas on June 22, 2019, 09:56:27 am ---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.
--- End quote ---
My mistake, LTC2400 on the brain, meant LT1236
Thank you gentlemen, about to order parts and PCB, will post pics when complete. Lots of measurement fun ahead :-+
Just one more thing, is there any reason to go without solder mask? I seem to recall seeing you guys post pictures of your various pcb's without it and always assumed it was an artifact of an in house manufacturing process. I see Dr Frank's LTZ1000 boards I got from you Branadic have it though
Regards, Alf
branadic:
Andreas made is first boards at home and I once had a cheap and fast german pcb manufaturer with the only drawback of having no solder stop mask nor silk screen. Unfortunately he's no longer in business.
-branadic-
AlfBaz:
Ok thanks
Andreas:
Hello,
Thanks for the summary.
But noise is not all.
For a absolute measurement the offset and gain drift and INL is also important.
That is why I love the LTC2400:
Offset + Gain drift is self adjusted.
INL can be easily modeled and canceled out to below 1 ppm.
with best regards
Andreas
chickenHeadKnob:
--- Quote from: 3roomlab on January 31, 2020, 12:00:21 pm ---
2 of the interesting ADC in the list is ADS125H and ADS8699. it has built in DMM gain scale.
i have also included some relative levels of 6.5D MM noise.
there is a chance i made some fundamental errors ... i have not seen any ADC being able to oversample to hell at nV level of noise ... pls feel free to highlight any corrections.
to note : it would be good to view the chart with lesser importance on rank but to analyze which sampling speed to use to maximise brute force lowering of noise
--- End quote ---
Thank you for the work you put into this, My first impression is that you have been overly optimistic in two dimensions. Both in using the data-sheet specmanship Enob of the ADC's as well as relying on getting more than 2 extra bits from averaging. Two extra bits with simple averaging in the presence of gaussian noise, easy. The next bits progressively more difficult, then you will hit some system noise floor that becomes an insurmountable limit.
The ADS8699/8689 is indeed interesting, I have been planning a design for a year now and I need to get off my ass. One complicating factor is the bandwidth of the analogue front end. From the data sheet (page 25):
In order to mitigate the noise of the front-end amplifier and gain resistors of the PGA,the AFE circuit of the device features a second-order,anti-aliasing LPF at the output of the PGA.The magnitude and phase response of the analog anti-aliasing filter are shown in Figure51 and Figure52, respectively.For maximum performance,the –3-dBcutoff frequency for the anti-aliasing filter is typically set to 15 kHz.The performance of the filter is consistent across all input ranges supported by the ADC.
I read this as you won't get near nyquist performance suggested by the sample rate if you are simply brute force adding these puppies in parallel.
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