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PlainDAQ - open source DAQ module for Raspberry Pi Pico

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Marco:
A 220k series resistor can hold off mains voltage to the extent the opamp input protection diodes can conduct the current with the resistor burning less than 0.25W, though some external clamp diodes for the opamp still make sense (it's on the edge). With a 10nf parallel capacitor the resistor contributes noise up to around a 100 Hz and won't really be an issue give the ADC precision. R4 needs to go and the RC filter needs to be put behind the opamp.

About the opamp, why not something nice like say MCP6V69?

palpurul:

--- Quote from: Marco on August 16, 2022, 01:46:22 am ---A 220k series resistor can hold off mains voltage to the extent the opamp input protection diodes can conduct the current with the resistor burning less than 0.25W, though some external clamp diodes for the opamp still make sense (it's on the edge).
--- End quote ---

I get it now. with 10nF cap my bandwidth goes down to 450Hz my bandwidth is going to be around 100kHz.


--- Quote from: Marco on August 16, 2022, 01:46:22 am ---With a 10nf parallel capacitor the resistor contributes noise up to around a 100 Hz and won't really be an issue give the ADC precision. R4 needs to go and the RC filter needs to be put behind the opamp.

--- End quote ---

Wouldn't resistor contribute to the noise of the overall system until the equivalent noise bandwdith of the input filter (220k, 10nF) which should be more than 450Hz.


--- Quote from: Marco on August 16, 2022, 01:46:22 am ---About the opamp, why not something nice like say MCP6V69?

--- End quote ---
That's too expensive, I am trying make something affordable with jellybean parts.


Marco:

--- Quote from: palpurul on August 16, 2022, 06:52:25 am ---I get it now. with 10nF cap my bandwidth goes down to 450Hz my bandwidth is going to be around 100kHz.

--- End quote ---
As I said, the RC filter has to be moved behind the opamp. The 10 nF capacitor would be across the 220k resistor, not across the opamp input. At high frequencies it forms a capacitive divider with the input capacitance of the opamp (almost unity).

palpurul:

--- Quote from: Marco on August 16, 2022, 10:52:37 am ---As I said, the RC filter has to be moved behind the opamp. The 10 nF capacitor would be across the 220k resistor, not across the opamp input. At high frequencies it forms a capacitive divider with the input capacitance of the opamp (almost unity).

--- End quote ---

Sorry for my late response,

Could you just refer to a schematic? I am just curious to learn about this technique of protection because it's probably something I am not familiar with :)

palpurul:
As raspberry pi pico W is out, there is really no reasons for me have a wi-fi module in PlainDAQ.

I am considering removing the Wi-Fi Module and adding new features.

Any thoughts?‚Äč

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