Hi Glen, Thank you very much for your input. I will carefully consider each of your suggested changes.
At this stage I am feeling conservative - for the Betas I want to minimise risk as any bugs we introduce from changes will results in 100 repairs. So I'm inclined to separate mods to: (i) those that are low risk to add on the Betas (ii) possible improvements we can try out in a later prototype stage. So the 100uH inductor kick is a good example of something we need to be very careful with. Might be best to replace that with a bead, as currently tested in the prototype. We could always increase R21 to form a RC filter blocking switcher hash from re-radiating. Re (4) I could hear reduced switcher noise on the speaker output when I tacked an electrolytic across C25, which was initially specified as a ceramic (not sure what material however). Had assumed a tant would be just as good? Cheers, David On 29/09/14 09:00, glen english wrote: > I have some schematic comments > > Critical : > > 1) > Try to avoid connecting input pins of the CPU to Vdd. This is bad ! > > IE PTT, SELECT, BACK (SW1,2,4) > > A latch up condition would mean destruction of the chip. > > If there needs to be input pins tied to Vdd, tie them through a low > value resistor, like 470 ohms . NOT direct to supply. > > Improvements : > > 2) A stubber should be present on the switched mode supplu switching > node. This will substantially reduce the ringing rubbish > Jn C27/L2 > > 3) A 2 ohm resistor in series with C26 will also reduce radiated rubbish > > > 4) A ceramic cap (X7R, X5R, X7S) would be a MUCH better choice for C25, > C29, C28 in the power supply. > Tantalums will under perform here . Eliminate C1. > > A bulk electrolytic may be a good choice on the input to reduce ringing > and reduce Q of the equiv circuit. Put there where C1 was. > > > 5) The 100uH inductor is a loaded gun....it may kick 2x or more voltage > into the switched mode chip at power up- suggest the electrolytic be > across C1, and/or a zener to clamp to spike > > > comments > > 4) VREF+ may have too high a value resistor (R10) - successive ADC > operations will deplete the reference stored in the C10 cap and generate > errors. IE the first conversion is good, but subsequent conversions > will show a higher conversion because The VREF+ will sag. > This may not be a problem in this application- but it is if you are > trying accurately to measure say a battery voltage. > > 5) Rather than the half rail supply splitting resistors everywhere for > the op amp Vhalf-rail (VHR) , why not drop a opamp down and provide this > voltage. this would eliminate 2 large capacitors and 2 resistors. > > > > On 29/09/2014 6:28 AM, David Rowe wrote: >> Yes I linked to the prototype schematic, as that was what I was >> debugging. The beta schematic is being worked on now. >> >> On 29/09/14 00:53, Steve wrote: >>> The articles schematic link is old, I think this is the newest one: >>> >>> http://sourceforge.net/p/freetel/code/HEAD/tree/smartmic/SM1000-C/SCH-SM1000-C.pdf >>> >>> >>> >>> ------------------------------------------------------------------------------ >>> Meet PCI DSS 3.0 Compliance Requirements with EventLog Analyzer >>> Achieve PCI DSS 3.0 Compliant Status with Out-of-the-box PCI DSS Reports >>> Are you Audit-Ready for PCI DSS 3.0 Compliance? Download White paper >>> Comply to PCI DSS 3.0 Requirement 10 and 11.5 with EventLog Analyzer >>> http://pubads.g.doubleclick.net/gampad/clk?id=154622311&iu=/4140/ostg.clktrk >>> >>> >>> >>> _______________________________________________ >>> Freetel-codec2 mailing list >>> [email protected] >>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >> ------------------------------------------------------------------------------ >> Meet PCI DSS 3.0 Compliance Requirements with EventLog Analyzer >> Achieve PCI DSS 3.0 Compliant Status with Out-of-the-box PCI DSS Reports >> Are you Audit-Ready for PCI DSS 3.0 Compliance? Download White paper >> Comply to PCI DSS 3.0 Requirement 10 and 11.5 with EventLog Analyzer >> http://pubads.g.doubleclick.net/gampad/clk?id=154622311&iu=/4140/ostg.clktrk >> _______________________________________________ >> Freetel-codec2 mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > ------------------------------------------------------------------------------ Slashdot TV. Videos for Nerds. Stuff that Matters. http://pubads.g.doubleclick.net/gampad/clk?id=160591471&iu=/4140/ostg.clktrk _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
