Column: JESD204 standard
Figure 6: Timing mismatch
Bandwidth mistmatch Te last mismatch to look at is probably the most difficult to understand and also to manage – that of bandwidth mismatch – with a gain and a phase/frequency component (Figure 7), making it more difficult. Here we see different gain values at different frequencies. In addition, the bandwidth has a timing component which causes signals at different frequencies with different delays through each converter. Te best way to minimise the
bandwidth mismatch is to have very good circuit design and layout practices.
Figure 7: Bandwidth mismatch
through the analogue section of each converter must be properly matched, with good circuit design techniques. In addition, the clock path designs need to be closely matched to minimise aperture uncertainty differences. And lastly, the
clock phase relationships need to be precisely controlled such that the two input clocks are as close to 180° apart as possible; the goal is to minimise the mechanisms that cause the timing mismatch.
Common traits Now that we’ve discussed four different mismatches that cause issues when interleaving ADCs, it is apparent that a commonality has emerged: three of the four mismatches produce a spur in the output spectrum at fS
/2 ± fIN . Te offset
mismatch spur can be easily identified since it alone resides at fS
/2 and can
be compensated fairly easily. Te gain, timing and bandwidth mismatches all produce a spur at fS
/2 ± fIN in the output
spectrum, so the question is how to identify the contribution of each. Figure 8 shows how to identify the sources of the spurs from the different mismatches of interleaved ADCs. If looking at gain mismatch alone, it is a
Figure 8: The inter-related nature of interleaving mismatches 18 September 2021
www.electronicsworld.co.uk
low frequency, or DC mismatch. Te gain component of the bandwidth mismatch can be separated from the gain mismatch by making a gain measurement at low frequency near DC, and then performing gain measurements at higher frequencies. Te gain mismatch is not a function of frequency like the gain component of the bandwidth mismatch. A similar approach is used for the timing mismatch. A measurement is performed at near- DC frequency and then subsequent measurements are performed at higher frequencies to separate the timing component of bandwidth mismatch from the timing mismatch.
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