[time-nuts] 31bit ADC, 1000 samples per second

Bob Camp lists at rtty.us
Tue Dec 11 01:01:45 UTC 2012


It's not at all uncommon for the seismic guys to go a bit uber-nuts on clean supplies and references.


On Dec 10, 2012, at 7:54 PM, David <davidwhess at gmail.com> wrote:

> On Mon, 10 Dec 2012 23:01:49 +0000, "Poul-Henning Kamp"
> <phk at phk.freebsd.dk> wrote:
>> In message <tilcc892a6ntfk64t2nljm92idr92df078 at 4ax.com>, David writes:
>>> The gain drift is specified at 2ppm per degree C.  There are
>>> provisions for a calibration cycle but that requires external
>>> multiplexing.
>> That's the really smart thing about this particular chip:  You can
>> drive the geophone calibration signal in through the second input
>> and out onto the first, which eliminates the external calibration
>> mux and the errors that usually cause.
> I see now from the data sheet that the 3000 ppm typical gain mismatch
> applies between the PGA gain settings and not between the multiplexor
> input channels so you can use the internal multiplexor for external
> calibration.  The other instrumentation delta-sigma ADCs I am familiar
> with do a gain and offset calibration for every conversion which
> knocks the drift down by an order of magnitude unless you disable that
> feature.
>> And yes, in seismology you do care about frequencies from 0.1Hz and up
>> (0.01Hz if you're involved with the CTBT) and most voltage references
>> will be stable enough for that.
> Voltage reference 1/f noise will be a problem but only because the
> ADS1282 input stage is already chopped allowing that kind of
> sensitivity at low frequencies.  Competing instrumentation
> delta-signal converters have the same problem if you consider that a
> problem. 
>> For longer term you calibrate your entire system, starting with the
>> geophone, so the voltage reference is caught that way.
> What they need is a ratiometric geophone . . . :)
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

More information about the time-nuts mailing list