[sdiy] OTA performance (was SSM chip reissue)

[Tom Wiltshire]

Different types might have different leakage currents too though, no?

I  don't suppose that's a specification they put in the datasheet, although perhaps it correlates with something that is there if you know about zener construction.

(other) Tom

On 28 Apr 2017, at 08:54, Mattias Rickardsson <mr at analogue.org> wrote:

> Hej Tom,  :-)
> 
> the consequences, if any, of capacitance in zeners would probably show
> up in high frequencies and not only low ones, no?
> 
> /mr
> 
> On 28 April 2017 at 09:47, Tom Bugs <admin at bugbrand.co.uk> wrote:
>> One thing I wondered about Zeners a couple of years ago was whether
>> different types gave different results - I wasn't entirely sure whether I
>> was barking or looking at the wrong parameters though!
>> 
>> I *think* that BZX84 type were worse than BZV55 for example. One parameter
>> in the datasheet I'd wondered on was the capacitance measured at 1MHz.
>> But I now can't remember if this changed the oscillating amplitude at all
>> freqs or just at the low freqs which is what I was trying to figure (as per
>> current discussion)
>> 
>> Barking up the wrong tree?
>> 
>> 
>> 
>> 
>> On 27/04/2017 18:25, Tom Wiltshire wrote:
>>> 
>>> On 27 Apr 2017, at 16:03, Mattias Rickardsson <mr at analogue.org> wrote:
>>> 
>>>> On 27 April 2017 at 16:57, Richie Burnett <rburnett at richieburnett.co.uk>
>>>> wrote:
>>>>>> 
>>>>>> But if the gains match well at 0dB, pretty well at -20dB, and not so
>>>>>> well
>>>>>> by the time you're down at -80dB, then you'd expect resonance to drop
>>>>>> off at
>>>>>> lower frequencies.
>>>>> 
>>>>> Or it could do the opposite, depending which of the two integrator gains
>>>>> is
>>>>> lowest/highest!  Like in that paper you referenced about varying SVF
>>>>> gain by
>>>>> manipulating the integrator gains.
>>>> 
>>>> True!
>>>> This is intriguing. Does this indicate that the loss of resonance
>>>> towards the bottom end is actually caused by another effect than the
>>>> gain mismatch? Possibly capacitor leakage (e.g., through zeners) or
>>>> something else?
>>> 
>>> 
>>> There are three potential causes we've identified:
>>> 
>>> 1) Phase lag
>>> 2) Gain mismatch being worse at greater attenuation
>>> 3) Zener current robbing the integrators.
>>> 
>>> The trouble is *all* of these effects operate in the same direction,
>>> making the filter less likely to resonate at lower frequencies.
>>> 
>>> How much of each is causing the effect we're seeing is still yet to be
>>> determined! It's quite possible that it's a mixture of all three and that
>>> sorting out any one might give an improvement but won't fix the problem on
>>> its own.
>>> 
>>> Any ideas for experiments we could try to find out which is the worst
>>> offender?
>>> 
>>> Tom
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>> 
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