ADS1158: sampling questions

Hi Nicholas,

I agree that the autoscan is a little confusing.  Basically for autoscan you need a sinc1 response so that the digital filter is single-cycle settling (each conversion result is fully valid).  The fastest rate for this mode of operation is 23.7ksps.  So the scan rate becomes 23.7ksps/channel for actual throughput.  The fixed channel mode allows up to 125ksps which uses a different digital filter response.

As the ADC is oversampling there is some latency, but with the sinc1 filter you can think of this as basically an average over the conversion period.  As the mux changes internally for autoscan, there is no additional latency due to digital filter as it is flushed for each conversion when the conversion starts.

Where you might see an issue is with analog settling between channels.

Best regards,

Bob B

Hi Nicholas,

This is kind of a complicated part as there is a sinc5 followed by an sinc1 averager.  What actually happens depends on fixed channel or auto-scan mode selection, CHOP and DLY settings.

Let's go back to your original questions.

Nicholas Carley said:

The datasheet says that the fixed-channel data rate is 125 kSPS, and the auto-scan data rate for 16 channels is 23.7 kSPS/Channel.   This doesn’t seem possible to me, because 125ksps/16 = 7.8ksps.  How can this part sample each channel at 23.7kSPS, yet only be capable of 125ksps on a single channel?

The answer is 125ksps/5 is about 25ksps.  Note that the maximum for a single channel in auto-scan is 23.7ksps.  Does that help clarify a little?  The full 16 channel conversions takes about 700us (1/(23.7ksps/16)) which is stated in the datasheet on page 11.

Nicholas Carley said:

Since this is a delta-sigma converter, I believe there is latency between when a sample is captured, and when it appears at the digital interface.  How do I figure out what this latency is?

I'm not quite sure here what you are asking for here.  Delta-Sigma ADCs are oversampling and there is always some latency involved.  For a sinc5 digital filter, the data requires 5 complete conversions for valid data and thus the lower data rate when using auto-scan.

Nicholas Carley said:

When switching between ADC channels (or auto-scanning), is there extra delay required to “flush” the digital filter data from the previous ADC channel? 

There is some overhead to complete the conversion, switch channels and flush the digital filter.  The overhead is the difference in the theoretical and the actual times which is about 2.2us.  Additional delay can be added to compensate for any analog settling, but will reduce the overall throughput.

Nicholas Carley said:

So, I think a sinc1 filter is probably not present.  Thoughts?

The digital filter output response will follow the sinc5 response in the graphs.  The averager that follows is a sinc1 response.  For the 23.7ksps, the averager is set to 'no averaging'.  The auto-scan mode does all of the work for you in the end and removes the overhead of manually communicating for the mux switching, restarting the conversion, and monitoring the DRDY for a single channel result after 5 conversions periods.

I hope that makes it more clear.

Best regards,

Bob B

Hi Nick,

Bob was supporting this question for me while I was out.

Yes, your statement is correct assuming the parameters in the customer's system match those in footnote #1 (f_CLK = 16MHz, Chop = 0, and Delay = 0)

-Bryan

Hi Ranjita,

I saw you asked this same question in a different e2e post. I will close out this thread then and answer your question there.

-Bryan