[sdiy] Filter problems

Pete Hartman pete.hartman at gmail.com
Sun Jan 28 05:10:02 CET 2018


Many thanks for that link Nathan.  I was looking for info about Ib a few
weeks ago in a completely different context, and while I found a few things
then that helped explain, I don't think any of them were as clear as this
is.

Pete

On Sat, Jan 27, 2018 at 10:02 PM, Nathan Trites <nathan at idmclassics.net>
wrote:

> Thanks to Tim for bringing this up and everyone's advice on the input bias
> current of the 5532.
>
> This most likely explains a similar issue I couldn't figure out on a
> handful of filters I made 10+ years ago using vintage LM301s as
> integrators, probably near or over max spec for input bias current. Some
> would fail after being in use for a while by drifting to the positive rail.
> Hadn't thought of that in ages and at the time didn't know enough to figure
> that out - in retrospect this makes sense.
>
> AD note MT-038 deals with input bias current: http://www.analog.com/media/
> en/training-seminars/tutorials/MT-038.pdf
>
> Nathan
>
> On Sat, Jan 27, 2018 at 6:34 PM, Neil Johnson <neil.johnson71 at gmail.com>
> wrote:
>
>> Tim,
>>
>> Don't use NE5532 for the integrators (U3).  The input bias current
>> will integrate over time and push the integrator opamps to the
>> positive rail.  This will happen once the gain of the 2164s is low
>> enough that the bias current dominates.  Switch to TL072 in that
>> position.  I would also suggest a TL072 for the other op-amp as with
>> the resistor values you're using and taking worst-case bias current
>> you could be seeing 10s of mV (especially U2A).
>>
>> Also I wouldn't recommend putting the zeners across the integrator
>> capacitor.
>>
>> Neil
>>
>>
>> On 27 January 2018 at 16:28, Tim Ressel <timr at circuitabbey.com> wrote:
>> > Hi,
>> >
>> > Filters can be a pain. I've got one that is misbehaving. Here is a link
>> to
>> > the schematic:
>> >
>> > https://www.dropbox.com/s/acjvwb17nmuok1n/PrismFilter.pdf?dl=0
>> >
>> > At low CVs it filters fine, but as the freq cv hits around 2 volts the
>> > behavior gets odd. I start seeing a rising DC bias on all the outputs.
>> At
>> > about 3 volts all the outputs rail positive.
>> >
>> > Any help is appreciated. Thanks for playing!
>> >
>> > --
>> > --Tim Ressel
>> > Circuit Abbey
>> > timr at circuitabbey.com
>> >
>> > _______________________________________________
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>>
>>
>>
>> --
>> http://www.njohnson.co.uk
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>
>
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