opamp input capacitance

Sure, you probably don't have Genesee out there.

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.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
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luted

I'm not sure how polluted it can be, (the genesee river) it mostly runs tho ugh agricultural land. It 'makes' Letchworth state park (A must see when ' dressed' in the Autumn foliage.) It ehn goes through Rochester NY where I g uess there would be some industrial pollution, more so in the past.

Hey.. (well this is getting way OT.) looking at the map reminded me that th ere's a spot in Pennsylvania where three watersheds meet, the Genesee runs up and dumps into the great lakes, the Allegheny goes across and down to th e Mississippi and the Susquehanna which flows into Chesapeake Bay.

George H.

ly,

The Monongahela (watershed in West Virginia) joins the Allegheny River to form the Ohio River at Pittsburgh, PA.

(The Ohio River forming the western boundary of West Virginia, between WV and Ohio. I grew up in Huntington, WV :-) ...Jim Thompson

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I love to cook with wine.     Sometimes I even put it in the food.

Grin, they don't call it "Three River Stadium" for nothing. I really love watershed maps.

George H.

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 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.

Gennie cream ale is a bit of an acquired taste, IMHO.

Tried Wexford? It's delish.

--

John Larkin         Highland Technology, Inc 

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

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

Le Tue, 17 Sep 2013 10:12:03 -0700, miso a écrit:

I was just telling what the Ccm value was.

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.

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Thanks, 
Fred.

Don't go on Monday, though. ;-)

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.

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.

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