[Diy_efi] PCB layout/circuit design

Mon Feb 26 02:38:53 UTC 2007

We have a 1 farad - ie 10x or more your need in front of this
already :-)

On Sun, 2007-02-25 at 17:50 -0800, Heitor Lima wrote:
> 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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> 
> 
> 
>  
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