Forums

opamp input capacitance

Started by John Larkin September 16, 2013

I see opamps that specify "differential" and "common-mode" input capacitance,
but don't say what that really means. An extreme case is ADA4817-1, where Ccm is
1.3 pF and Cdiff is 0.1!

Any ideas? If I use this as a classic inverting amp, with the + input grounded,
what would the capacitance look like at the - input?

(I'm being pressured to get a proposal out, with specs and pricing, so I don't
have time right now to measure this... not that it would be easy.)



-- 

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
On Mon, 16 Sep 2013 09:39:47 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

> > >I see opamps that specify "differential" and "common-mode" input capacitance, >but don't say what that really means. An extreme case is ADA4817-1, where Ccm is >1.3 pF and Cdiff is 0.1! > >Any ideas? If I use this as a classic inverting amp, with the + input grounded, >what would the capacitance look like at the - input?
Guess: 0.75? -- 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
On Monday, September 16, 2013 12:39:47 PM UTC-4, John Larkin wrote:
> I see opamps that specify "differential" and "common-mode" input capacitance, > > but don't say what that really means. An extreme case is ADA4817-1, where Ccm is > > 1.3 pF and Cdiff is 0.1! > > > > Any ideas? If I use this as a classic inverting amp, with the + input grounded, > > what would the capacitance look like at the - input?
1.3 + 0.1 or 1.4pF. (As a guess.) (It's always worse than you think :^) Unfortunately it often seems to be the common mode capacitance that one cares about. George H.
> > > > (I'm being pressured to get a proposal out, with specs and pricing, so I don't > > have time right now to measure this... not that it would be easy.) > > > > > > > > -- > > > > 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
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:ijce39dn1p0dojss56tr1uto50jhe102tm@4ax.com...
> > > I see opamps that specify "differential" and "common-mode" input > capacitance, > but don't say what that really means. An extreme case is ADA4817-1, where > Ccm is > 1.3 pF and Cdiff is 0.1! > > Any ideas? If I use this as a classic inverting amp, with the + input > grounded, > what would the capacitance look like at the - input? > > (I'm being pressured to get a proposal out, with specs and pricing, so I > don't > have time right now to measure this... not that it would be easy.) > > > > -- > > John Larkin Highland Technology Inc
Take a look at fig 50 in the datasheet. tm
On Mon, 16 Sep 2013 13:05:18 -0400, "tm" <No_one_home@white-house.gov> wrote:

> >"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >news:ijce39dn1p0dojss56tr1uto50jhe102tm@4ax.com... >> >> >> I see opamps that specify "differential" and "common-mode" input >> capacitance, >> but don't say what that really means. An extreme case is ADA4817-1, where >> Ccm is >> 1.3 pF and Cdiff is 0.1! >> >> Any ideas? If I use this as a classic inverting amp, with the + input >> grounded, >> what would the capacitance look like at the - input? >> >> (I'm being pressured to get a proposal out, with specs and pricing, so I >> don't >> have time right now to measure this... not that it would be easy.) >> >> >> >> -- >> >> John Larkin Highland Technology Inc > >Take a look at fig 50 in the datasheet. > > >tm
Well, duh! It's not in the test circuit figures where I expected it. Next question: is Ccm = 2 * Cm? So an inverting amp, with IN+ grounded, will see 0.75 pF at IN- ??? -- 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
On Mon, 16 Sep 2013 10:03:36 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Monday, September 16, 2013 12:39:47 PM UTC-4, John Larkin wrote: >> I see opamps that specify "differential" and "common-mode" input capacitance, >> >> but don't say what that really means. An extreme case is ADA4817-1, where Ccm is >> >> 1.3 pF and Cdiff is 0.1! >> >> >> >> Any ideas? If I use this as a classic inverting amp, with the + input grounded, >> >> what would the capacitance look like at the - input? > >1.3 + 0.1 or 1.4pF. (As a guess.) >(It's always worse than you think :^) > >Unfortunately it often seems to be the common mode capacitance that one cares about. > >George H.
I think your guess is correct. CCM is mostly due to the package pin to ground. That differential number is flaky, given that the inputs are adjacent.
>> >> >> >> (I'm being pressured to get a proposal out, with specs and pricing, so I don't >> >> have time right now to measure this... not that it would be easy.) >> >> >> >> >> >> >> >> -- >> >> >> >> 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
...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Mon, 16 Sep 2013 10:16:28 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Mon, 16 Sep 2013 13:05:18 -0400, "tm" <No_one_home@white-house.gov> wrote: > >> >>"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >>news:ijce39dn1p0dojss56tr1uto50jhe102tm@4ax.com... >>> >>> >>> I see opamps that specify "differential" and "common-mode" input >>> capacitance, >>> but don't say what that really means. An extreme case is ADA4817-1, where >>> Ccm is >>> 1.3 pF and Cdiff is 0.1! >>> >>> Any ideas? If I use this as a classic inverting amp, with the + input >>> grounded, >>> what would the capacitance look like at the - input? >>> >>> (I'm being pressured to get a proposal out, with specs and pricing, so I >>> don't >>> have time right now to measure this... not that it would be easy.) >>> >>> >>> >>> -- >>> >>> John Larkin Highland Technology Inc >> >>Take a look at fig 50 in the datasheet. >> >> >>tm > >Well, duh! It's not in the test circuit figures where I expected it. > >Next question: is Ccm = 2 * Cm? > >So an inverting amp, with IN+ grounded, will see 0.75 pF at IN- ???
They call Cm the "common mode capacitance of the amplifier" (between equations 14 and 15) so I would guess 1.4pF, as George's not-unduly-pessimistic reading of the ds.
On 09/16/2013 12:50 PM, John Larkin wrote:
> On Mon, 16 Sep 2013 09:39:47 -0700, John Larkin > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> >> I see opamps that specify "differential" and "common-mode" input capacitance, >> but don't say what that really means. An extreme case is ADA4817-1, where Ccm is >> 1.3 pF and Cdiff is 0.1! >> >> Any ideas? If I use this as a classic inverting amp, with the + input grounded, >> what would the capacitance look like at the - input? > > Guess: 0.75? > > >
Different makers do it differently, unfortunately. And it's never done under power, AFAICT. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Mon, 16 Sep 2013 10:03:36 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Monday, September 16, 2013 12:39:47 PM UTC-4, John Larkin wrote: >> I see opamps that specify "differential" and "common-mode" input capacitance, >> >> but don't say what that really means. An extreme case is ADA4817-1, where Ccm is >> >> 1.3 pF and Cdiff is 0.1! >> >> >> >> Any ideas? If I use this as a classic inverting amp, with the + input grounded, >> >> what would the capacitance look like at the - input? > >1.3 + 0.1 or 1.4pF. (As a guess.) >(It's always worse than you think :^)
If Ccm is the sum of both pin capacitances to ground, and Cdiff is the 3-terminal capacitance of one pin to the other, the capacitance at the inverting output might be Ccm/2 + Cdiff. Like 0.75 pF maybe. I *want* it to be 0.75. -- 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
On Monday, September 16, 2013 3:08:17 PM UTC-4, Spehro Pefhany wrote:
> On Mon, 16 Sep 2013 10:16:28 -0700, John Larkin > > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > > > > >On Mon, 16 Sep 2013 13:05:18 -0400, "tm" <No_one_home@white-house.gov> wrote:
<snip>
> >>Take a look at fig 50 in the datasheet. > >> > > >>tm > >Well, duh! It's not in the test circuit figures where I expected it. > > > > > >Next question: is Ccm = 2 * Cm? > > > > > >So an inverting amp, with IN+ grounded, will see 0.75 pF at IN- ??? > > > > They call Cm the "common mode capacitance of the amplifier" (between > > equations 14 and 15) so I would guess 1.4pF, as George's > > not-unduly-pessimistic reading of the ds.
I was moving wood after work today, (and thinking), it's not the most 'pessimistic number'. I'm not sure how they measure the CM Cap. but they could tie the inputs together, measure that, and then divide by 2. In which case most of the CM cap could be in one input or the other. So my pessimistic number would be 2.5, (someone may have measured it.) George H.