[sdiy] Sallen Key caps

[rsdio at audiobanshee.com]

Thanks for the comparison, David. It's helpful to hear the experience of other designers.

In one of my recent designs, I made the PCB with dual-site placements for either through-hole or SMD. I tried the 1206 C0G/NP0 in one prototype, long enough to see that it basically worked. But the rest of the prototypes used WIMA poly caps and that's what I shipped with. I never took the time to populate the final design with C0G/NP0 to see, once and for all, whether that option would have worked as well. It's good to hear that I probably made the right choice for the shipping product.

By the way, Justin, I placed the WIMA caps as close together as possible, and overlapped the SMD pads in between. There didn't seem to be much of a space difference between the two options, since I was using 1206, but perhaps I could have gotten things a little tighter if I'd only wanted the SMD option. I designed with the 5mm lead WIMA caps. The height is certainly different, but the PCB area seems basically the same.

Brian


On Apr 24, 2015, at 10:54 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
> I have found that poly caps in filters lead to more stable self-oscillation
> at low frequencies than C0G/NP0 ceramics.  That's why we put WIMA poly caps
> in the Doc Oc.
> 
>> -----Original Message-----
>> From: Justin Owen
>> Sent: Thursday, April 23, 2015 4:59 PM
>> 
>> Hello,
>> 
>> I'm using a Sallen Key LPF as a DAC output reconstruction 
>> filter with cutoff set at approx. 500Hz.
>> 
>> I've been using these PET through-hole caps:
>> 
>> http://uk.farnell.com/kemet/r82dc3100dq50k/cap-film-pet-0-1uf-63v-rad/dp/2429344?ost=242-9344
>> 
>> Filter works as expected - but I was wondering if these PEN 
>> 1210 SMD might be a viable space-saving alternative?
>> 
>> http://uk.farnell.com/avx/cb027c0104jba/cap-film-pen-100nf-25v-smd/dp/1100602
>> 
>> Haven't used a cap like this in surface mount before - so any 
>> info would be appreciated.
>> 
>> Thanks,
>> 
>> - J
>