Georgy.Moshkin
Full Member level 5
I am observing interesting effect: LC filter on power supply output worsens noise rejection performance between modules. Main power module (A) provides filtered output at 8v through 100uH + 1000uF LC filter. Two secondary modules (B) and (C) have 5V/3.3V LDOs and 100uH+68uF / 100uH+100uF input LC filters. Here is interesting observation:
case 1 - unmodified
(A) power module (LC on output)
(B) analog module (opamps, etc., LC on input)
(C) digital module (MCU, I2S ADC, etc., LC on input)
digital module (C) contaminates analog module (B) with noise (0-10kHz).
Noise starts right after MCU boot and initialization sequences are complete.
case 2 - removed 100uH inductor from module (A):
noise levels in analog part opamp preamplifier output are lowered by 20-30dB, very good performance
My hypothesis is that large inductor between 8v linear regulator and digital module is "slowing down" regulation, so digital module current spikes are larger at LC input of digital module. When 8V linear regulator output inductor is removed, 8V regulation becomes "fast" enough to deal with currents at LC input of digital module.
Maybe another explanation?
case 1 - unmodified
(A) power module (LC on output)
(B) analog module (opamps, etc., LC on input)
(C) digital module (MCU, I2S ADC, etc., LC on input)
digital module (C) contaminates analog module (B) with noise (0-10kHz).
Noise starts right after MCU boot and initialization sequences are complete.
case 2 - removed 100uH inductor from module (A):
noise levels in analog part opamp preamplifier output are lowered by 20-30dB, very good performance
My hypothesis is that large inductor between 8v linear regulator and digital module is "slowing down" regulation, so digital module current spikes are larger at LC input of digital module. When 8V linear regulator output inductor is removed, 8V regulation becomes "fast" enough to deal with currents at LC input of digital module.
Maybe another explanation?