[volt-nuts] bits beyond 24 from the ADC

[Andreas Jahn]

Hello John,

Yes this PCB is etched by myself.
Good to know that the LTC2440 does behave somewhat different in transferring 
the data than my LTC2400.
I have looked to the data and on the LTC2400 all 4 sub LSBs are toggling.

To the 50 or 60 Hz output rate:
The output rate is not necessarily equal to integration time.
When I measure the input of a LT2400 with a oscilloscope,
I see the swiching noise from sigma delta converter
only the 2nd half 80 ms out of  160 ms output rate (configured for 50 Hz).
The first half of the measurement time the input is passive.

I guess that the LTC2400 does a kind of self-calibration every measurement 
cycle.
Otherwise he would not have the offset and gain drift specs.

Good also to know that the AD7190 has gaussian noise.
So I will have to look up the two devices for a further ADC.

With best regards

Andreas


----- Original Message ----- 
From: "beale" <beale at bealecorner.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Wednesday, January 30, 2013 11:46 PM
Subject: Re: [volt-nuts] bits beyond 24 from the ADC


>>  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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