Re: Texas TMS1232 Delta Sigma converters

John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, 01 Nov 2005 17:11:13 +1000, Adrian Jansen <adrian@xxxxxxxxx>
> wrote:
>
> >Sorry, I meant ADS1232, too used to typing T for Texas parts.
> >
> >They did some neat tricks in the internals of this, using a 10 Hz
> >sampling frequency automatically rejects a lot of 50/60 Hz signal.
> >
> >One of the objects in using this widget is that we can cut the external
> >part count down from around 60 precision components on an existing
> >design to less than 10 non precision ones, so I prefer not to add analog
> >filtering unless we *really* have to.
> >
> >My bench power supply is quiet, 5.00 volts with less than 0.1mv residual
> >noise, and using fully isolated double transformer with separate
> >inter-winding shields. Still get some hum pickup on the bench of
> >course, but its about as low as I can get till I do a proper board layout.
> >
> >The app isnt for weighing, and I can jigger the test transducer so its
> >inputs are essentially true zero for test purposes, its just noise
> >picked up between the transducer and the ADC I'm trying to eliminate.
> >As well as the ref noise and stuff of course.
> >
> >Thanks for the tips.
>
>
> Past about 18 bits, the reference problem is serious. Most references
> have a heap of 1/f noise, which will be a problem around 10 Hz...
> bandgaps are bad. We're using an Analog Devices Xfet ref, ADR421,
> which is pretty good. The Intersil things are interesting too, basicly
> a voltage trapped on a capacitor forever.
>
> One could build a 0.xx Hz active lowpass filter (a non-trivial
> problem) or maybe average a bunch of references, four maybe, but the
> math is discouraging here, improvement as to the square root of the
> number of refs.

One can just make the measurement ratiometric, i.e. use the
"reference" to supply the transducer excitation as well as the ADC ref
input.

--

John Devereux
.

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