10 years, 5 months ago.

Isolated to minimise noise: what does this mean?

On the user manual of the LPC1768, there are several pins that state that they are the same as others, but should be isolated to minimise noise and error.

e.g. Vssa (11) says it should have the same voltage as Vss (31..) , but should be isolated etc.

On the schematic, 11 is connected to ground the same as 31.

Does something different happen between the design and the manufacture of the mbed?

I have tried searching for an answer to this, but mostly just come up with people talking about isolating full circuits, usually with some kind of transformer device, DC-DC converter etc.

Any help would be appreciated.

Thanks, Adam.

1 Answer

10 years, 5 months ago.

Vssa & Vss are analog and "digital" ground respectively, these are often separated to reduce noise carrying over from one to the other, in this case, the objective is to prevent noise from the digital circuitry interfering with the signal on the ADC but it could be the other way round (noise from motors, fans, etc. can make a uC behave like a kid on a E102 overdose).

The ground planes should be separated but connected! the issue is complicated and boring, google "star ground" & "ground bounce"

I haven't used the ADC (LPC1768 mbed) but i know there have been complaints about noise and accuracy although i suspect it has more to do with a noisy supply, a separate linear regulator for the analog part usually helps but given the limited space and cost (both power & monetary) the mbed solution (L1 & C14) seems adequate.

if you are rolling your own LPC1768 board and need precise ADC, i suggest using an external ADC, and in this case, joining Vssa & Vss at the pins is usually recommended! it all comes down to the routing and layout of the board.

Accepted Answer