Monday, 31 December 2012

DSP Tech Brief : D.C. Offset with Sigma-Delta ADCs/DACs

A question that I frequently had, back in the day when I was an FAE working for a general purpose DSP hardware company, often used to go like this :
Me : Good afternoon, technical support, how may I help
Customer : Hi, I have your ADC input card. I have shorted the inputs of my ADC and measured the samples on the input. They have magnitudes like 31, 32 or 33 instead of zero. I have purchased a 16 bit converter but if I take off these 5 odd bits I am only left with 10, which is not what I paid for.

How do you answer a question like this ?
Well the answer always revolved around the type of ADC/DAC that was being used. In almost all cases it was a Sigma-Delta converter and many of these devices introduce a D.C. offset in the conversion process. These devices are designed for low cost, high quality consumer audio applications that are typically A.C. coupled, so a couple of mV D.C. offset is not an issue.

Is this an issue ?
D.C. offset does not necessarily lead to a loss of resolution provided that the converter is being used in the correct way. Of course, if D.C. measurement accuracy is critical to an application then Sigma-Delta converters are probably not the correct device to use unless they include D.C. offset correction capabilities.

Another issue commonly seen with Sigma-Delta converters is the group-delay associated with the digital filtering element of the converter. This delay can sometimes be in the order of 10s of milliseconds for both ADCs and DACs. So these devices are not suitable for embedded control applications where you may require a servo loop time in the order of 1 ms. For these types of applications something like a Successive Approximation device might be more appropriate.

Despite all of these issues, they are very cheap and very high quality.

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