Now fix those drooping DAC and ADC responses

June 11, 2012 // By Kendall Castor-Perry
The Filter Wizard aka Kendall Castor-Perry continues to examine solutions to the problems posed by unexpected droop when dealing your DAC and ADC responses.

In Why Are My DAC and ADC Responses Drooping? , we looked at the two common places where the sinc() function crops up in a data acquisition system.  A DAC with zero-order hold has a frequency response that follows a sinc() response, falling as signal frequency rises.  Commonly, instrumentation-grade delta-sigma ADCs have responses that are higher powers of the sinc() function.  This is a Filter Wizard column, so you just know that filters are going to be involved in the remedy for this response droop.  In the ADC case, a filter was the cause of it in the first place!     

In a system that suffers from both types of droop when measured from input to output, we could perhaps just try to apply one fix for both.  But I recommend that you treat the input side and the output side separately.  That's because your data acquisition system probably shares data with other systems and analysis computers, so you want both the 'record' and 'replay' processes to be correct.  I'll assume from now on that you do have a droop problem on one or both ends.  If you don't, it's either because you already recognized the issue and fixed it, or you used components (for instance, audio-grade converters) that didn't suffer from the issue in the first place.  I hope the rest of the column will still be interesting and useful for you, if that's the case!     

Let’s look at the DAC side of things first, just because that’s the order implied by the title.  Matters are broadly similar for the ADC case, as we’ll see.  In a typical data 'replay’ path, your data might pass through a digital filter on the way to the DAC, and might also go through an analog 'reconstruction’ filter after conversion, before it reaches the final output of the system.  If you have neither an analog nor a digital filter in your system

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