Forums

filters

Started by John Larkin March 5, 2012
This cool. Click on a box on one of the surfaces.

http://tinyurl.com/3jyxpke

Click "filter expert" then "dual supplies." Then click on a box on one
of the surfaces.




I came up with a new-to-me filter architecture that *almost* works for
me. It's a degenerated MFB lowpass, where the opamp itself replaces
the usual opamp-r-c integrator. 

http://dl.dropbox.com/u/53724080/Circuits/MFB_hack.JPG

This is freaky because it's weird to see a cap from an opamp inverting
output to ground. But the opamp is acting like an integrator, not an
opamp. Its input is *not* a summing point.

Unfortunately, this circuit has too few parts, so not enough degrees
of freedom to give me the gain I need (gotta have -5) and a decent
filter response: I seem to be stuck with a +14 dB peak. Cute, though.


-- 

John Larkin, President
Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation
On Mar 5, 6:54=A0pm, John Larkin <jlar...@highlandtechnology.com> wrote:
> This cool. Click on a box on one of the surfaces. > > http://tinyurl.com/3jyxpke > > Click "filter expert" then "dual supplies." Then click on a box on one > of the surfaces. > > I came up with a new-to-me filter architecture that *almost* works for > me. It's a degenerated MFB lowpass, where the opamp itself replaces > the usual opamp-r-c integrator. > > http://dl.dropbox.com/u/53724080/Circuits/MFB_hack.JPG
That top one looks like a low pass, with a bit coming back from the output.
> > This is freaky because it's weird to see a cap from an opamp inverting > output to ground. But the opamp is acting like an integrator, not an > opamp. Its input is *not* a summing point. > > Unfortunately, this circuit has too few parts, so not enough degrees > of freedom to give me the gain I need (gotta have -5) and a decent > filter response: I seem to be stuck with a +14 dB peak. Cute, though.
DC gain or just at the peak? Maybe some DC gain will add to the peak? George H.
> > -- > > John Larkin, President > Highland Technology, Inc > > jlarkin at highlandtechnology dot comhttp://www.highlandtechnology.com > > Precision electronic instrumentation > Picosecond-resolution Digital Delay and Pulse generators > Custom laser controllers > Photonics and fiberoptic TTL data links > VME thermocouple, LVDT, synchro =A0 acquisition and simulation
On Mon, 5 Mar 2012 20:04:22 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Mar 5, 6:54&#2013266080;pm, John Larkin <jlar...@highlandtechnology.com> wrote: >> This cool. Click on a box on one of the surfaces. >> >> http://tinyurl.com/3jyxpke >> >> Click "filter expert" then "dual supplies." Then click on a box on one >> of the surfaces. >> >> I came up with a new-to-me filter architecture that *almost* works for >> me. It's a degenerated MFB lowpass, where the opamp itself replaces >> the usual opamp-r-c integrator. >> >> http://dl.dropbox.com/u/53724080/Circuits/MFB_hack.JPG > >That top one looks like a low pass, with a bit coming back from the >output.
It's a textbook 2-pole MFB lowpass.
> >> >> This is freaky because it's weird to see a cap from an opamp inverting >> output to ground. But the opamp is acting like an integrator, not an >> opamp. Its input is *not* a summing point. >> >> Unfortunately, this circuit has too few parts, so not enough degrees >> of freedom to give me the gain I need (gotta have -5) and a decent >> filter response: I seem to be stuck with a +14 dB peak. Cute, though. > >DC gain or just at the peak? > >Maybe some DC gain will add to the peak? >George H.
DC gain has to be -5. The lower circuit has an additional gain bump, 14 dB roughly, at the high and of the passband. (I posted this to s.e.b. by mistake, should have been s.e.d.) -- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Mar 6, 11:48=A0am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 5 Mar 2012 20:04:22 -0800 (PST), George Herold > > > > > > <gher...@teachspin.com> wrote: > >On Mar 5, 6:54=A0pm, John Larkin <jlar...@highlandtechnology.com> wrote: > >> This cool. Click on a box on one of the surfaces. > > >>http://tinyurl.com/3jyxpke > > >> Click "filter expert" then "dual supplies." Then click on a box on one > >> of the surfaces. > > >> I came up with a new-to-me filter architecture that *almost* works for > >> me. It's a degenerated MFB lowpass, where the opamp itself replaces > >> the usual opamp-r-c integrator. > > >>http://dl.dropbox.com/u/53724080/Circuits/MFB_hack.JPG > > >That top one looks like a low pass, with a bit coming back from the > >output. > > It's a textbook 2-pole MFB lowpass. > > > > >> This is freaky because it's weird to see a cap from an opamp inverting > >> output to ground. But the opamp is acting like an integrator, not an > >> opamp. Its input is *not* a summing point. > > >> Unfortunately, this circuit has too few parts, so not enough degrees > >> of freedom to give me the gain I need (gotta have -5) and a decent > >> filter response: I seem to be stuck with a +14 dB peak. Cute, though. > > >DC gain or just at the peak? > > >Maybe some DC gain will add to the peak? > >George H. > > DC gain has to be -5. The lower circuit has an additional gain bump, > 14 dB roughly, at the high and of the passband.
OK... the bottom one is like a PD TIA... you can hang a bit of C on the inverting input. Is the gain peak at the mean of the opamp GBW and the Rf/Cin frequency? Would it help to split the input R in half and put the C in the middle?
> > (I posted this to s.e.b. by mistake, should have been s.e.d.)
Oh... well too late now. George H.
> > -- > > John Larkin, President =A0 =A0 =A0 Highland Technology Incwww.highlandtec=
hnology.com=A0 jlarkin at highlandtechnology dot com
> > Precision electronic instrumentation > Picosecond-resolution Digital Delay and Pulse generators > Custom timing and laser controllers > Photonics and fiberoptic TTL data links > VME =A0analog, thermocouple, LVDT, synchro, tachometer > Multichannel arbitrary waveform generators- Hide quoted text - > > - Show quoted text -