[volt-nuts] bits beyond 24 from the ADC

beale beale at bealecorner.com
Wed Jan 30 17:46:06 EST 2013

>  From: Andreas Jahn <Andreas_-_Jahn at t-online.de>
> ...the 4 sub-bits below the 24 bits (giving a readout of totally 28 bits) would help on a low noise ADC like AD7190 to get a more gaussian distribution of the values.

Thanks for the well-explained photo Andreas, I guess you etched your own PCBs?  

By the way, the data sheet for the LTC2440 shows that it sends a 32 bit word, including 4 LSBs beyond the 24 bit result. I found that while those bits are logically present, only two of them are "real" data bits, the last two are fixed. b1 is always low and b0 is always high. I contacted Linear Tech support and they confirmed it:

"I ran your schematics and questions through our applications engineer and he mentioned what you are seeing is normal. The two LSBs are far below the noise floor and have no real influence but I agree this should be clarified in the datasheet." -Uma D, Product Marketing Engineer, Linear Tech  Nov.5 2012

If enclosed PNG image makes it, the scope screenshot shows an example. Data is valid on the rising edge of the clock. Light blue traces on the DATA channel (2) show previous data words (scope display-persist mode).

Both the LTC2440 and AD7190 can run at higher output rates, with higher noise. So, for example if you run at 10 Hz output rate (or even 50 Hz or 60 Hz, depending on your local AC frequency) and then do your own averaging, I think you can get useful data beyond 24 bits if your Vin / Vref is clean enough.

I previously sent Andreas a screen capture of the AD7190 eval board software which included a lopsided histogram, but that was in fact a software problem on the PC host (LT was sloppy in making their LabView app).  For histograms with only a few bins, the labview auto-scale function sometimes crops the right-hand edge off the histogram. You see this clearly if you watch it acquiring data live. If you export the raw AD7190 data and look at it in an external graphing program, you can confirm the histogram is symmetric as expected, for any large enough dataset.

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