Forums

opamp input capacitance

Started by John Larkin September 16, 2013
On Tue, 17 Sep 2013 07:55:04 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Tue, 17 Sep 2013 07:10:09 -0700 (PDT), George Herold <gherold@teachspin.com> >wrote: > >>On Monday, September 16, 2013 9:41:12 PM UTC-4, John Larkin wrote: >>> On Mon, 16 Sep 2013 17:36:21 -0700 (PDT), George Herold >> >><snap> >>> >> I do. I'd better measure it, I guess. Grumble. >>> >>> > >>> >>> >Great, I hope it is 0.75pF! >>> >>> >If it's >2pF then you have to buy me a beer if I'm every in SF. >>> >>> >(I get buy yours at the same time.) >>> >>> >George H. >> >>> Bud Light? >>Sure, you probably don't have Genesee out there. >>http://beeradvocate.com/beer/profile/302/865 >> >>George (there's no such thing as bad beer) Herold >> > >I've had Genesee. It's made from Genesee river water, one of the more polluted >waterways on the planet. Pretty tasty beer. > >I could take you to Zeitgeist a few blocks away. Al fresco dining (actually, >picnic tables under a freeway ramp) and 44 beers on tap. Biker bar, but >bicycles. Joerg likes it. > >Or The Monk's Kettle, with beers ranging from $6 a glass up to $80.
Don't go on Monday, though. ;-)
On 9/17/2013 1:26 PM, Fred Bartoli wrote:
> Le Tue, 17 Sep 2013 10:12:03 -0700, miso a &eacute;crit: > >> For simulation purposes, they lump the common mode capacitance to >> ground. But in real life, the common mode capacitance could be to any AC >> ground. > > I was just telling what the Ccm value was. > >> I can't see this being significant unless the device was driven >> with a very high impedance source. If the source was very high >> impedance, there could be power supply coupling issues. >> Again it depends on the circuit, but common mode capacitance in real >> life is more likely to be to one of the power supply rails, or even more >> than one rail. If you visualize the input stage, the common mode >> capacitance is really a combination of parasitics. > > > Yup, but not necessary so high impedance. I was once working on a sub ppm > 50kHz LPF (MFB so as to avoid CM distorsion) and found THD to be > unexpectedly high. It was just the output stage class B action that > modulated the supply rails which in turn sneaked into the opamp virtual > ground node through the supply to input parasitics. > >
I understood you were just giving the ccm from the spice file. Your example shows the pitfalls of a model just lumping the common mode capacitance to ground. Some CMOS op amps put the long tail pair in a well. I can see that kind of parasitic screwing up some designs if they just lumped it to ground.
On Tuesday, September 17, 2013 1:44:29 PM UTC-4, Spehro Pefhany wrote:
> On Tue, 17 Sep 2013 07:10:09 -0700 (PDT), George Herold >
<snap>
> >> >If it's >2pF then you have to buy me a beer if I'm every in SF. > >> >(I get buy yours at the same time.) > >> > >> >George H. > >> Bud Light? > > >Sure, you probably don't have Genesee out there. > > >http://beeradvocate.com/beer/profile/302/865 > >George (there's no such thing as bad beer) Herold > > Gennie cream ale is a bit of an acquired taste, IMHO.
Grin, beer seems to be an acquired taste. As a youth growing up in Western NY it was almost a requirement that the first beer you had was Genny Cream. I'm not sure why... I guess it's a bit sweeter than 'normal' beer, and still very cheap. I haven't had a Genny Cream in ages. George H.
On Wed, 18 Sep 2013 07:14:25 -0700 (PDT), George Herold <gherold@teachspin.com>
wrote:

>On Tuesday, September 17, 2013 1:44:29 PM UTC-4, Spehro Pefhany wrote: >> On Tue, 17 Sep 2013 07:10:09 -0700 (PDT), George Herold >> ><snap> > >> >> >If it's >2pF then you have to buy me a beer if I'm every in SF. >> >> >(I get buy yours at the same time.) >> >> >> >> >George H. >> >> Bud Light? >> >> >Sure, you probably don't have Genesee out there. >> >> >http://beeradvocate.com/beer/profile/302/865 >> >George (there's no such thing as bad beer) Herold >> >> Gennie cream ale is a bit of an acquired taste, IMHO. > >Grin, beer seems to be an acquired taste.
Like oysters. -- John Larkin 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