[sdiy] Another SSM2164 question

[Dave Manley]

If you have a reasonably fast scope, you could also look for oscillation 
on the output as you play with the RC.

-Dave

Tom Wiltshire wrote:
> Thanks guys, that's helpful.
> 
> What I really needed to know was whether these are some "magic values" 
> that might cause all sorts of weirdness if altered. I'm not thinking of 
> leaving the network out, but I needed to know if I can replace it with 
> easier-to-get cap values like 470pF or 680pF. If it's just a HPF then I 
> can. Especially if I move the resistor value in the other direction to 
> keep the overall frequency similar (which, by chance, is what my 
> 560R/470pF combination does).
> 
> I shall also experiment with these values to see if it has any 
> effect/influence on the resonance's frequency dependence.
> 
> 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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