10mV ripple on ADCIN

10mV ripple on ADCIN

Postby phoeny » Mon Nov 04, 2019 11:05 am

Hy, my dso has a 10mV ripple on ADCIN even without the analogboard plugged in.

When i measure VAC on 3.3V and GND i have no ripple
but on ADCIN and GND i do have 1.65V and 10mV ripple.

I thought about unstable 3.3v but since i have no ripple on 3.3v the problem could come from somewhere else.

As far as i have seen on the schematic ADCIN on the digital board is connected to a 2x 100kOhm voltage divider (R21 & R22) and nothing else. Where is that ripple comming from?

Measurements taken with a Fluke 87V


I have to add my digitalboard is modified.
* replaced Header J4 with a FFC connector,
* removed R4 (lcd backlight) and connected it with a smaller resistor to 3.3v
* basically V+ only powers the 3.3v Regulator
+ analogboard has its own filtered boost converter. but its not connected for testing.
phoeny
 
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