I am building a small ESP32 control panel and powering using a PoE splitter, I need both 5V and 3.3V to power various devices. I am using this to convert to 5V and this to convert to 3.3V
Can I share grounds between 5V and 3.3V or do I need to have separate ground blocks for each? For instance, can I simply share the PoE splitter ground for both 5V and 3.3V?
Most people provide 5V and then just use a linear regulator to get 3V3 from 5V. It all depends on exactly what you are powering. But, it is generally fine and usually necessary to have the power supplies share a ground reference, i.e. be hooked together.
Most esp boards have a 3V3 regulator and can supply enough current for many sensors.
They are isolated DC-DC converters. So you can choose to join the output 0V rails or not. Both modes are supported.
If you are using 5V sensors that connect to your ESP board powered by 3.3V you will need to tie the 0V outputs together so that you have a common reference.