V5, V6 - 1.8V low. May result in spikes or noise
It seems that the 1.8V supply to the ADC is low which may show up as spikes or noise in the measurements.
- The 10uF capacitor may not have that value (capacitance is lower when used below the nominal voltage), maybe less than 4uF.
- The inductor in the path for the analogue supply removes 88mV.
The report [2] from HT-859 [3] describes that the 1.8V power supply is on the low side, notably the analogue supply. The suggestions are to implement any one of these two:
- Remove L8 and short it.
- Gains 88mV on ADC_ANALOG (1.8V)
- Add another 10uF capacitor in parallel with C54 (10uF, 0603)
- To verify with the assembly workshop if this is feasible
Removing L8 may influence the noise level to the ADC which is an important parameter. I suggest that even before applying this change, the SNR/ENOB (effective number of bits) should be measured, similar to those done before [4,5].
The report suggests that only adding an additional capacitor is enough to solve the problem: “The voltage output of the LDO became insensible to temperature changes and increased the voltage to its nominal value.” and puts this change on the same level as removing L8: “Removing one inductor also gave the same result.”
My preference: • Check if just adding another C in parallel with C54 is “good enough” (after all, hundreds of cards have been installed and are functional). • If so: do this modification on the cards that stock and replace “à fur et à mésure”.
[1] https://issues.cern.ch/browse/INST-3767?focusedCommentId=382662&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-382662 [2] https://issues.cern.ch/secure/attachment/72136/failure%20analysis%20report-1.pdf [3] https://issues.cern.ch/browse/HT-859 [4] https://ohwr.org/project/fmc-adc-100m14b4cha-hw/wikis/l1_alternative_measurements [5] https://ohwr.org/project/fmc-adc-100m14b4cha/wikis/home#specifications