[sdiy] Another SSM2164 question

[harrybissell at wowway.com]

On Tue, 12 Aug 2008 18:21:19 +0100, Tom Wiltshire wrote
> Thanks guys, that's helpful.

<snip>

> I shall also experiment with these values to see if it has any 
> effect/ influence on the resonance's frequency dependence.

IMHO your "digging in the wrong place"  (was I the ONLY one it the theatre
who ROTFLMFAO when that was said in "Raiders of the Lost Ark")

Ian (Fritz) mentioned the "Q enhancement" that is documented
in many places, including (and especially) Electronotes. The usual
technique is small capacitors across certain feedback resistors to
try and compensate the effect. This would be the best bang for the buck.

The RC networks play at a frequency much higher than the normal resonance so
I'd suspect they will not have much effect until you move them so far that
they allow instability.

These resistors are where you would expect them to cure something called
"noise gain". Bob Pease wrote a column on it... I think the use of this RC
network is related to this subject.

http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=7164

H^) harry




> 
> Thanks again,
> Tom
> 
> On 12 Aug 2008, at 17:09, Tom Farrand wrote:
> 
> > Tom,
> >
> > Am guessing this is a roll-off network to keep the chip from
> > oscillating at a high frequency.  From the datasheet, check out
> > figure 14.  The device still has gain beyond 1MHz.  However, note that
> > a complete phase change from 180 to -180 degrees happens about 7MHz.
> > That means the device tends to (will) oscillate at high frequencies,
> > which is a "bad thing".
> >
> > Since the desired frequency span of interest is out to maybe 50KHz
> > (without introducing unwanted artifacts to the sound), what happens
> > much higher is of no concern, unless something "bad" can happen.  The
> > combination 500 Ohms and 560pF works out to a 90 degree phase shift at
> > about 568KHz ... ten times the higher limit of "interest" in the
> > signals being passed (50KHz).  That keeps the device from oscillating
> > at low levels at higher frequencies while still passing all the
> > frequencies of interest.
> >
> > AD engineers have thought that through and offer a cheap and
> > predictable cure.  I suggest you incorporate their network on each
> > input as prescribed.  It only costs pennies and it does work.  By not
> > going into a detailed explanation as to "why" this happens, one feels
> > a bit left out.  I suspect the "why" has something to do with how the
> > chip works without violating existing patents.  A full disclosure
> > explanation might have to reveal proprietary information which is
> > something AD might not want to do.
> >
> > All guesses ... YMMV.
> >
> > Peace.
> > Tom Farrand
> >
> >
> > On Tue, Aug 12, 2008 at 9:54 AM, Tom Wiltshire  
> > <tom at electricdruid.net> wrote:
> >>
> >> Hi all,
> >>
> >> The subject says it all really. The VCAs in the SSM22164 require a  
> >> 500R resistor and 560pF capacitor to ground on each VCA input, as  
> >> well as a resistor to convert voltage to current (30K in the  
> >> datasheet).
> >>
> >> Now, what I'd like to know is what do the 500R/560pF do and how  
> >> are their values calculated? All the datasheet says about it is  
> >> that "a 500R resistor in series with a 560pF capacitor must be  
> >> added from each input to ground to ensure stability". Great.
> >>
> >> Practically, it seems you can vary the values a bit without  
> >> incident (I've used 560R/470pF and nothing bad has happened) but  
> >> it's a bit hard to know when you're not sure what they're supposed  
> >> to prevent.
> >>
> >> The datasheet is at:
> >> http://www.analog.com/static/imported-files/data_sheets/SSM2164.pdf
> >>
> >> Thanks,
> >> Tom
> >>
> >>
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> >
> 
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Harry Bissell & Nora Abdullah 4eva