[sdiy] SSM2164 Filter Cells

Scott Nordlund gsn10 at hotmail.com
Wed Jul 22 21:47:57 CEST 2009


OK, the thing is that these "capacitor to ground" circuits are actually integrators too.  The OTAs are current output and the buffers have high input impedance, so the current is forced through the capacitor.  I = C dv/dt, flip that around and you have an integral.  So it's an integrator, current input, voltage output, just like an op amp with a capacitor in the feedback loop.  As others have mentioned, the 2164's output has to be held at virtual ground.  Putting a capacitor to ground isn't going to work, so an op-amp integrator (which has low input impedance) is used instead.  The end result is the same (or at least the transfer function is).

Your initial comment sort of threw me as the integrator is generally the main building block of this sort of filter- put an integrator in a negative feedback loop with variable gain and you can move the pole frequency around by adjusting the gain.

----------------------------------------
> From: jays at aracnet.com
> To: Synth-diy at synth-diy.org
> Subject: RE: [sdiy] SSM2164 Filter Cells
> Date: Wed, 22 Jul 2009 09:11:16 -0700
>
> Scott,
>
> I was thinking of generic filter cells like the SSM2040, CEM3320 and LM13X00
> use. There they have a cap between the filter cell and current amp. The
> other end of the cap can be connected to ground for a low pass and all pass
> filter and used as an input for high and all pass filters. Refer to figs 11
> & 12 of the LM13700 data sheet that will show you the low and high pass
> configurations. All the SSM2164 designs I've seen have a cap in a feedback
> loop of an op-amp. I view a cap in a feedback loop as an integrator which
> should be linear and a cap between ground and a signal just charging a cap
> which is non-linear. There is a more or less linear range when charging a
> cap which is used in a lot of VCOs.
>
> I think that other posts in this thread have enlightened me with the fact
> that the SSM2164 likes to drive virtual ground and has a very low voltage
> output.
>
> No worries, just need to haul out the Active Filter Cookbook, Art of
> Electronics and Electronotes and do some research.
>
> Jay S.
>
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Scott Nordlund
>> Sent: Tuesday, July 21, 2009 7:40 PM
>> To: Synth-diy at synth-diy.org
>> Subject: RE: [sdiy] SSM2164 Filter Cells
>>
>>
>>
>>> So far I've only seen integrator based filters for the 2164,
>> nothing like
>>> the regular OTAs (cap between OTA output and output current amp). Anyone
>>> know why this is, are there issues with 2164 in this configuration?
>>
>> I may be missing something here but I don't know of any OTA-based
>> VCF that isn't made using integrators. Do you have an example?
>>
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