[sdiy] Another SSM2164 question

[Tom Farrand]

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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