Re: Re: SD24 to sample a resistive bridge with pulsed bridge supply

"Jon Kirwan [email protected] [msp430]" <[email protected]>:

> On Sun, 13 Sep 2015 23:08:19 +0000 (UTC), you wrote:

>>Isn't it possible to just "pause" the sampling of the delta sigma
>>converter (let it sleeping, all registers keep their values) and awake
>>it after 100ms with nearly the same input level as before the sleep?
> 
> I haven't seen one work like that. They are basically very
> fast "1 bit" converters with a lot of state behind it. It
> takes a long time for those "bits" to fill the state. Once
> you get the pipeline filled up, you can get very precise
> values at a fairly reasonable rate.... if it is still
> attached to the same input which is "drifting" around the
> nominal value at less than some specified rate. But the
> sample rate is based upon an ALREADY FILLED pipeline. You
> have to fill it before the first value is really any good.
> You can find out how long it takes to fill the state and, in
> my limited experience, that will take a LOT LONGER than the
> specified sample rate for the unit.

The question is: how does the decimation filter work. In case it is working 
like a FIR, then it should be filled after a fixed time and I should ne able 
to find out, how long it take, in case it is a IIR, well then the fill-time 
is not really fixed....
 
>>The sd24_a input is
>>just a 18pF capacitor. The 350ohms will charge it quickly... 
> 
> Hehe. That's true. Have you computed your expected S/N for
> the analog portion of the input system, yet? Just curious.

No, not yet. :-) Maybe the noise from the resistors will be quite large and I 
have to add additinal capacitance to get it lower? I am not experienced with 
calculating noise figures, but surpricingly according to

https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise

it seems the noise in a RC system does not depend from R, just
from C, and with 18pF it is 15uV/sqrt(Hz) and at 100nF it is at 
0.2uV/sqrt(Hz), and it takes (7*tau) 250us to stabilize.
I am at 10Hz sample rate, so I think I do not have to add a sample and hold 
circuit to further reduce this time and increase accuracy..

> I'm interested in what you find out. I think you already have
> figured out enough to take a fair shot at it. Sounds like you
> may be able to do just fine. [Personally? I'd have preferred
> to use an ADC12 or ADC10 section and use over sampling to
> achieve the desired resolution... after making sure that my
> noise is where it needs to be (for an ADC12 the noise should
> be about 1000 ppm or so, I think, and for an ADC10 perhaps
> 5000 ppm; then use averaging to squeeze back down.)]

I will give it a try; on last week I was still in the "find out how" phase of 
the project, now it is getting more and more out of the dust, it is nice to 
share ideas with you. :-)

Matthias



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Posted by: Matthias Weingart <[email protected]>
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