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sinara-hw / Sayma_RTM

RTM board with 8-channel GS/s DAC, 125MS/s ADC and flexible clock circuit
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LDO Vset bypass #60

Closed hartytp closed 5 years ago

hartytp commented 5 years ago

All the LDO Iset capacitors are electrolytic. What's the reason for that when we've always used ceramic for this elsewhere? Does it actually affect the cost? It's a relatively low set current (100uA) se we are somewhat sensitive to capacitor leakage if we actually want to achieve the LDO's rated performance (if we don't need the ultra-low noise then we should just use a cheaper LDO).

gkasprow commented 5 years ago

I used them due to lower noise, especially microphonic noise caused by the piezo effect that is a nightmare in low noise circuits with big X7R capacitors... If we use big ceramic ones, one can notice vibrations from fans in output spectra :)

hartytp commented 5 years ago

ACK. Fair enough. I've never seen issues with microphonics in this kind of application, but maybe the uTCA fans vibrate more than I'm used to

gkasprow commented 5 years ago

It depends how deep you dig with spectrum analyzer :D I probably overestimated the influence of this effect on LDO performance, but it's better to be too careful than to ignore it.

hartytp commented 5 years ago

okay, sounds good to me.