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# Notch Filter - AOE

E

#### ehsjr

Jan 1, 1970
0
According to Art of Electronics, there are prefab Twin-T based
notch filters available from 1 Hz to 50 kHz. Anybody know who
supplies/manufactures these?

Thanks,
Ed

J

#### Joerg

Jan 1, 1970
0
Hello Ed,

According to Art of Electronics, there are prefab Twin-T based
notch filters available from 1 Hz to 50 kHz. Anybody know who
supplies/manufactures these?

I doubt that those products would still be around. Maybe the odd one for
50/60Hz or so. It's done digitally these days. Selecting capacitors to
get to within a percent or less of tolerance is cost prohibitive these
days. Switched capacitor filters are a modern option but even most of
those chips are beginning to fade away.

J

Jan 1, 1970
0
E

#### ehsjr

Jan 1, 1970
0
Joerg said:
Hello Jorgen,

Wow, didn't know they still made these. But Ed needs to hurry up,
Digikey has a mere 5 left. $12.58 a pop. That's a nice chip, & the software does the design work. I was hoping to find a drop in 3-legged device, from what I read in AOE. I hoped maybe they got around the precision cap problem by growing them on silicon. As you said, everything's gone active and moving toward DSP these days. Thanks to both you & Jorgen for your answers. Ed J #### Joerg Jan 1, 1970 0 Hello Ed, That's a nice chip, & the software does the design work. I was hoping to find a drop in 3-legged device, from what I read in AOE. I hoped maybe they got around the precision cap problem by growing them on silicon. Caps on silicon are plagued by very large tolerances. Easily 30%. That would require expensive laser trimming. Another method would be sectioning and connecting islands by FET switches during final test and then storing the settings in flash. Both rather expensive propositions. What you can do on silicon is to build up several caps where the absolute values are not precise but where their ratios are very precise. That is what made switched capacitor filters popular which can also be nicely used for a notch filter. However, even those seem to have fallen from grace a bit given the small number of remaining offerings. My take is that they kept their prices too high, maybe trying to ride the "Oh look at this cool stuff" wave for too long. That's why I used them in only one design. H #### Hal Murray Jan 1, 1970 0 What you can do on silicon is to build up several caps where the absolute values are not precise but where their ratios are very precise. That is what made switched capacitor filters popular which can also be nicely used for a notch filter. However, even those seem to have fallen from grace a bit given the small number of remaining offerings. My take is that they kept their prices too high, maybe trying to ride the "Oh look at this cool stuff" wave for too long. That's why I used them in only one design. I wonder how much of that market area is now done in software? What was the upper limit on the useful frequency of switched cap filters? J #### Joerg Jan 1, 1970 0 Hello Hal, I wonder how much of that market area is now done in software? What was the upper limit on the useful frequency of switched cap filters? AFAIK the LTC1068 can be clocked to above 5MHz and if you'd design a filter with a clock/CF ratio of 25 you could manage 200kHz. Not impossible with a digital system but it does get expensive. However, the LTC1068 would also cost you over$5/1k.

So nowadays it often looks like this: Audio is done in firmware while
things above 100kHz are handled by ye olde LC circuitry, clever mixing
and so on. Active filters have crept up into those regions as well but
power consumption can become an issue.

It all depends on what you want to do. There really is no way around the
fact that discrete inductors show around 10% tolerance and capacitors
with tolerances under 5% are expensive. Many times we analog guys first
take a look around, see what's there in filters for consumer gear.
Ceramic IF filters and so on. Then we try to come with a mixing scheme
that lets us use those filters since the generation of carrier
frequencies is inexpensive and precise.

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