[Diy_efi] PCB layout/circuit design

Heitor Lima heitorlima
Mon Feb 26 01:50:09 UTC 2007


Bill

  We need to use two capacitors , one big as 100uF ,
electrolytic type and another one , 100nF , ceramic
type.
   Regards

   Heitor


--- Bill Washington <bill.washington at nec.com.au>
wrote:

> James,
>     You said "all ICs should have at least a100uF
> cap as close as 
> possible to the supply " do you mean 100nF?
> Steven,
>     In PCB layout it is good practise to have a
> large reservoir Cap 
> (your 1 Farad) close by plus smaller (ceramic 100nF
> and sometimes 10nF 
> as well) caps right on the IC supply pins (as close
> as you can get 
> them). The reason for this is that the large
> reservoir cap has some 
> inductance and resistance in its construction which
> slows down its 
> response time - the ceramic Caps (which very low
> ESR) on the IC supply 
> pins handle the sharp rising and falling edge surges
> and minimise ground 
> bounce which can cause all sorts of problems ....
> Regards
> Bill
> >
> > Subject:
> > [Diy_efi] RE: More ECU progress (Steven P.
> Donegan)
> > From:
> > "James Holland" <j_holland at btopenworld.com>
> > Date:
> > Sat, 24 Feb 2007 22:41:30 -0000
> > To:
> > <diy_efi at diy-efi.org>
> >
> > To:
> > <diy_efi at diy-efi.org>
> >
> >
> > I have been following this project with some
> interest. I had a look on the
> > website but you seem to have the same schematic
> for the datalogger and I/O
> > board. I presume the I/O board schematic hasn't
> been posted. I've designed
> > electronics for military automotive issues so I'm
> aware of the problems of
> > designing for this kind of environment.
> > I like the idea of this but I think that you will
> have some noise issues
> > with the design as it stands. The PCB design lacks
> a good ground plane and
> > the decoupling could do with being improved, all
> ICs should have at least a
> > 100uF cap as close as possible to the supply. The
> PIC should have two, one
> > on each supply pin. I'm a bit surprised that the
> PIC doesn't have a separate
> > Analogue supply for the A/D as it stands the
> analogue and digital returns
> > are mixed which isn't good. The A/D inputs should
> really have anti-aliasing
> > filters close to the inputs, a simple RC would do
> and only the C really
> > needs to be up by the input. The R would also
> provide some input protection
> > to the PIC. Input voltage clamping is also a good
> idea. The PIC is pretty
> > well protected but voltages outside of its supply
> rails throw the A/D
> > readings all over the place. 
> > It looks like you are using the 5V supply as the
> reference for the A/D, what
> > supply are the I/O lines referenced to?
> > The digital I/O could do with some resistance in
> line as well, a couple of
> > hundred ohms would provide output short circuit
> protection and give some
> > filtering on the inputs. If your digital inputs
> are coming off another board
> > then you could see some ringing on them, that
> could be more of a problem on
> > PortB which doesn't have Schmitt inputs. 
> > The 5V regulator has plenty of protection, that's
> good. Automotive supplies
> > are horribly noisy. The 28V military stuff I
> design has to be protected
> > against +280V/-130V spikes. The EMC test involves
> 100V for 100mS. You could
> > reduce the protection needed by using a regulator
> specifically designed for
> > automotive use, there are a few of them out there.
> I have used the LM2940S
> > in the past and that works well but I think the
> best spec only comes in an
> > SMD package.
> > I hope this doesn't sound too harsh, I don't see
> any reason why it won't
> > work but I think you will lose some A/D resolution
> by having to digitally
> > filter the inputs. I learnt the hard way with a
> few board redesigns along
> > the way. The Microchip website has a very good on
> line seminar about PCB
> > layout and design for A/Ds, its well worth
> checking out.
> > HTH
> > James
> >
> >
> >   
> >
> >
>
------------------------------------------------------------------------
> >
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