So far weíve looked offset and gain mismatch between interleaved ADCs. Once again, letís up the complexity just a bit more and take a look at timing mismatch. There have been lots of great questions and comments related to the topic of interleaved ADCs, so letís keep this thing going and continue with some great discussion.
Judging from the comments it seems there is a good bit of interest in calibrating for the various mismatches that exist between ADCs. I do hope to make it at least to a high-level discussion of that topic (high-level because the details can get pretty nasty pretty quickly), but first letís take things one step at a time and continue to peel the onion one layer at a time. Besides, this is a blog which should be reasonably short and not too detail-ridden with the risk of making you terribly bored.
We must first understand what we are up against and define the problem before we set out to tackle a solution. Just as an architect draws up his plans before the builder pours the concrete for the foundation, we need to make sure we understand the interleaving spurs before we try to do any kind of calibration for them.
As with the case of the gain mismatch before, an input signal must be applied to see the resultant spur in the output spectrum. The timing mismatch has two components, group delay in the analog section of the ADC and clock skew. The analog circuitry within the ADC has an associated group delay, and the value can be different between the two ADCs.
In addition, there is clock skew that has an aperture uncertainty component in each of the ADCs and has a component related to the accuracy of the clock phases that are input to each converter. Figure 1 gives a visual representation of the mechanism and effects of the timing mismatches in the ADCs. What we find is quite interesting: The resultant spur is located at fS/2 ± fin. Does that sound familiar? It should, it is at exactly the same spot in the output spectrum as the spur resulting from the gain mismatch!
Similar to the gain mismatch spur, the timing mismatch spur is also a function of the input frequency and the sample rate. On top of that, isnít it nice that it appears at fS/2 ± fin, which is at the exact same location as the spur resulting from gain mismatch?
This is getting more fun by the minute, huh? The good news is that we can do some tests in order to tell how much each of the two mismatches contributes to the spur. Letís hold onto that thought for now though. Weíll come back to that soon as we continue down on this path looking at interleaving. Letís stay focused on the timing mismatch for now.
In order to minimize the spur caused by the timing mismatch, the group delay through the analog section of each converter needs to be properly matched with good circuit design techniques. In addition, the clock path designs need to be closely matched to minimize aperture uncertainty differences. Letís not forget that the clock phase relationships need to be precisely controlled such that the two input clocks are as close to 180° apart as possible.
As with the other mismatches, the goal is to attempt to minimize the mechanisms that cause the timing mismatch. The better job done up front to control these mismatches, the easier the job will be to try to calibrate for them later.
As I mentioned earlier, letís make sure we get a good set of plans to start with before we pour any of the foundation. We must also consider that we want to start with a good foundation when the time comes so that we have a nice firm place for our building to begin. This foundation begins with understanding all these mismatches so we know what we are getting ourselves into.
Stay tuned as we continue on in our discussion of interleaved ADCs and talk about bandwidth mismatch and some ways to distinguish the interleaving spur products. Keep those questions and comments coming as well.