Reply by Phil Hobbs November 17, 20152015-11-17
>>>I saw an appnote recently, somewhere, for reducing resistor shunt >>>capacitance, by a lot, by filling under the resistor with some topside >>>ground. But that would wreck my high voltage clearance. > >>That's basically the three-terminal capacitance measurement trick. Like the Hi/Lo Z >>trick, it helps the frequency response but not the noise, and the C loading to ground may be a >>problem in itself. I much prefer to make the capacitance stable and predictable using air. > >Given a 1200 volt signal, noise is not a concern here!
Of course. However, as you know, I build a lot of low-level stuff where the noise of high- value resistors is as inconvenient as their slowness. Finding ways round it is a fun problem, even better than Fields's 555s. ;) Cheers Phil Hobbs
Reply by John Devereux November 17, 20152015-11-17
John Larkin <jjlarkin@highlandtechnology.com> writes:

> On Sat, 14 Nov 2015 13:19:21 -0800, John Larkin > <jjlarkin@highlandtechnology.com> wrote: > >>On Sat, 14 Nov 2015 11:24:14 -0800 (PST), whit3rd <whit3rd@gmail.com> >>wrote: >> >>>On Friday, November 13, 2015 at 4:22:28 PM UTC-8, John Larkin wrote: >>>> I'm designing a high-voltage pulser, 1200 volts or so, and I'd like to >>>> pick off a divided signal... >>>> I'd like to have the pulse response be pretty good, so the >>>> capacitances matter. The OPA171 opamp and R2 have capacitance, roughly >>>> 4 pF total, and I guess the 1G 2010 resistor will, too. >>> >>>You probably want a guard at the divider node, because that 1G >>>resistor ought only to be driving a resistive load. That means a >>>guard/shield driven by the buffer amplifier, of course, and that's a >>>bit of positive feedback. The alternative is to use inverting buffer, >>>making the node pseudo-ground (and grounded shield is adequate). >>> >>>Two guards are penciled in; the HV side of the 1G doesn't really require one, does it? >> >>A guard will reduce my high-voltage clearance. Besides, I *want* some >>capacitance across the 1G resistor. I think. >> >>The Welyn 2512 1G resistor (oops, not 2010) is rated for 3KV, which is >>pretty scary as regards surface creepage, even at 1200 volts. >> >>http://tinyurl.com/qaa94r3 >> >> > > Note that this is a 1Gohm, 1 watt resistor.
Good for 30kV then! So your 1200 volts is no problem :) -- John Devereux
Reply by John Larkin November 16, 20152015-11-16
On Mon, 16 Nov 2015 13:31:09 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Monday, November 16, 2015 at 12:19:48 PM UTC-5, John Larkin wrote: >> On Mon, 16 Nov 2015 08:31:45 -0800 (PST), dagmargoo...@yahoo.com >> wrote: >> >> >On Sunday, November 15, 2015 at 7:27:18 PM UTC-5, legg wrote: >> >> Effect of Gigohm >> >> and 1/10th environmental stray C >> >> >> >> RL >> > >> ><snip sim file> >> > >> >That was fun. The strays have horrible time constants if they're isolated >> >by high resistances. >> > >> >In the Dept. of Silly Walks department, dividers made entirely of a single >> >resistor type are inherently perfectly compensated. >> > >> > Vin >> > --- >> > | >> > [R1] >> > | >> > [R2] >> > | >> > +-----+--> Vin/5 >> > | | >> > [R3] [R4] >> > | | >> > === === >> > >> > >> > Vin >> > --- >> > | >> > [R1] >> > | >> > [R2] >> > | >> > [R3] >> > | >> > +-----+-----+--> Vin/10 >> > | | | >> > [R4] [R5] [R6] >> > | | | >> > === === === >> > >> > >> > Vin >> > --- >> > | >> > [R1] >> > | >> > +------. >> > | | >> > [R2] [R3] >> > | | >> > === +-----. >> > | | >> > [R4] [R5] >> > | | >> > === +-----. >> > | | >> > [R6] [R7] >> > | | >> > === +----> >> > | >> > [R8] >> > | >> > === >> > >> >> I have files of all the resistor values and all the dividers that you >> can make from a quad resistor pack. >> >> Here's one of them >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/R-pak_values.JPG > >That's fun. Now we need all the divider combos for an eight-pack. :-) > >Here's a start: 'n' in series feeding 'n' in parallel yields a (n^2+1) : 1 >divider ratio. > >Cheers, >James
Don't stop now; you're on a roll. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by jeroen Belleman November 16, 20152015-11-16
On 16/11/15 23:23, Carl Ijames wrote:
> "John Larkin" wrote in message > news:ll3k4bt278r660jf2f4fr4h2rdlj5omj38@4ax.com... > > On Mon, 16 Nov 2015 08:23:35 -0800 (PST), dagmargoodboat@yahoo.com > wrote: > >> On Monday, November 16, 2015 at 3:51:22 AM UTC-5, Jeroen Belleman wrote: >>> On 2015-11-15 23:34, John Larkin wrote: >>>> On Sun, 15 Nov 2015 14:11:07 -0800 (PST), dagmargoodboat@yahoo.com >>>> wrote: >>>> >>> [snip! >>>> >>>> >>>> The waveform at B is good if C4 is 50 pF. >>>> >>>> C is fine, but it's only 120 millivolts. >>>> >>>> I just have no idea if C1 is really 0.1 pF. >>>> >>> >>> I got a fair estimate of the parasitic capacitance >>> of a 1206 SMD resistor by just assuming it's a flat >>> plate capacitor with all the field confined to the >>> SiO2 substrate. I calculated 30fF and subsequent >>> measurement indicated 50fF. >>> >>> For what it's worth.. >>> >>> Jeroen Belleman >> >> That's a useful notion, despite Phil's catch about the substrate >> material. >> >> k(Al2O3)=9.7, vs. k(SiO2)=3.9, FWIW. >> >> John's 2010 and most of the other form factors all have the same >> 2:1 aspect ratio, so the same capacitance per that approximation. >> >> Too bad. >> >> Cheers, >> James Arthur > > Right. Most surface mount resistors are made on 20 mil alumina, with > about the same aspect ratio, so their capacitance (and their thermal > resistance, and power dissipation capability [1]) is independent of > size. > > > [1] assuming you heat sink the end caps > ============================================================ > > For completeness, was the mistake made when doing the calculation, or when > typing up the post? Is the estimate for a 1206 SMD resistor 30 fF and you > just happened to type SiO2 instead of Al2O3, or really 9.7/3.9 times that, > 75 fF? Interesting that one is about 50% below the measured value, and the > other about 50% high; I would predict that the estimate would be low due to > field leakage around the sides. Anyway, just pedantically curious. > > ----- > Regards, > Carl Ijames > >
I meant Al2O3, honest. The capacitance values are right. I measured several resistor values. Details if how I went about it are at: <http://cern.ch/jeroen/resistor/shuntC.html>. Gosh, that's five years ago, already. Jeroen Belleman
Reply by Carl Ijames November 16, 20152015-11-16
"John Larkin"  wrote in message 
news:ll3k4bt278r660jf2f4fr4h2rdlj5omj38@4ax.com...

On Mon, 16 Nov 2015 08:23:35 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Monday, November 16, 2015 at 3:51:22 AM UTC-5, Jeroen Belleman wrote: >> On 2015-11-15 23:34, John Larkin wrote: >> > On Sun, 15 Nov 2015 14:11:07 -0800 (PST), dagmargoodboat@yahoo.com >> > wrote: >> > >> [snip! >> > >> > >> > The waveform at B is good if C4 is 50 pF. >> > >> > C is fine, but it's only 120 millivolts. >> > >> > I just have no idea if C1 is really 0.1 pF. >> > >> >> I got a fair estimate of the parasitic capacitance >> of a 1206 SMD resistor by just assuming it's a flat >> plate capacitor with all the field confined to the >> SiO2 substrate. I calculated 30fF and subsequent >> measurement indicated 50fF. >> >> For what it's worth.. >> >> Jeroen Belleman > >That's a useful notion, despite Phil's catch about the substrate >material. > >k(Al2O3)=9.7, vs. k(SiO2)=3.9, FWIW. > >John's 2010 and most of the other form factors all have the same >2:1 aspect ratio, so the same capacitance per that approximation. > >Too bad. > >Cheers, >James Arthur
Right. Most surface mount resistors are made on 20 mil alumina, with about the same aspect ratio, so their capacitance (and their thermal resistance, and power dissipation capability [1]) is independent of size. [1] assuming you heat sink the end caps ============================================================ For completeness, was the mistake made when doing the calculation, or when typing up the post? Is the estimate for a 1206 SMD resistor 30 fF and you just happened to type SiO2 instead of Al2O3, or really 9.7/3.9 times that, 75 fF? Interesting that one is about 50% below the measured value, and the other about 50% high; I would predict that the estimate would be low due to field leakage around the sides. Anyway, just pedantically curious. ----- Regards, Carl Ijames
Reply by November 16, 20152015-11-16
On Monday, November 16, 2015 at 12:19:48 PM UTC-5, John Larkin wrote:
> On Mon, 16 Nov 2015 08:31:45 -0800 (PST), dagmargoo...@yahoo.com > wrote: > > >On Sunday, November 15, 2015 at 7:27:18 PM UTC-5, legg wrote: > >> Effect of Gigohm > >> and 1/10th environmental stray C > >> > >> RL > > > ><snip sim file> > > > >That was fun. The strays have horrible time constants if they're isolated > >by high resistances. > > > >In the Dept. of Silly Walks department, dividers made entirely of a single > >resistor type are inherently perfectly compensated. > > > > Vin > > --- > > | > > [R1] > > | > > [R2] > > | > > +-----+--> Vin/5 > > | | > > [R3] [R4] > > | | > > === === > > > > > > Vin > > --- > > | > > [R1] > > | > > [R2] > > | > > [R3] > > | > > +-----+-----+--> Vin/10 > > | | | > > [R4] [R5] [R6] > > | | | > > === === === > > > > > > Vin > > --- > > | > > [R1] > > | > > +------. > > | | > > [R2] [R3] > > | | > > === +-----. > > | | > > [R4] [R5] > > | | > > === +-----. > > | | > > [R6] [R7] > > | | > > === +----> > > | > > [R8] > > | > > === > > > > I have files of all the resistor values and all the dividers that you > can make from a quad resistor pack. > > Here's one of them > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/R-pak_values.JPG
That's fun. Now we need all the divider combos for an eight-pack. :-) Here's a start: 'n' in series feeding 'n' in parallel yields a (n^2+1) : 1 divider ratio. Cheers, James
Reply by John Larkin November 16, 20152015-11-16
On Sat, 14 Nov 2015 13:19:21 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>On Sat, 14 Nov 2015 11:24:14 -0800 (PST), whit3rd <whit3rd@gmail.com> >wrote: > >>On Friday, November 13, 2015 at 4:22:28 PM UTC-8, John Larkin wrote: >>> I'm designing a high-voltage pulser, 1200 volts or so, and I'd like to >>> pick off a divided signal... >>> I'd like to have the pulse response be pretty good, so the >>> capacitances matter. The OPA171 opamp and R2 have capacitance, roughly >>> 4 pF total, and I guess the 1G 2010 resistor will, too. >> >>You probably want a guard at the divider node, because that 1G >>resistor ought only to be driving a resistive load. That means a >>guard/shield driven by the buffer amplifier, of course, and that's a >>bit of positive feedback. The alternative is to use inverting buffer, >>making the node pseudo-ground (and grounded shield is adequate). >> >>Two guards are penciled in; the HV side of the 1G doesn't really require one, does it? > >A guard will reduce my high-voltage clearance. Besides, I *want* some >capacitance across the 1G resistor. I think. > >The Welyn 2512 1G resistor (oops, not 2010) is rated for 3KV, which is >pretty scary as regards surface creepage, even at 1200 volts. > >http://tinyurl.com/qaa94r3 > >
Note that this is a 1Gohm, 1 watt resistor. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by John Larkin November 16, 20152015-11-16
On Sun, 15 Nov 2015 02:50:08 -0800 (PST), tabbypurr@gmail.com wrote:

>On Sunday, 15 November 2015 02:20:14 UTC, John Larkin wrote: >> On Sat, 14 Nov 2015 16:03:03 -0800 (PST), nt wrote: >> >On Saturday, 14 November 2015 21:10:35 UTC, John Larkin wrote: >> >> On Sat, 14 Nov 2015 12:07:36 -0500, Phil Hobbs >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 11/13/2015 08:09 PM, Tim Williams wrote: > > >> >> >Good idea if the resistance tempco is tolerable. Good cermet pots come >> >> >in at around +-100 ppm, iirc. John has been talking about 1 ppm/V >> >> >high-ohm resistors for awhile, so the requirements might be tighter than >> >> >that. Getting the untrimmed capacitances fairly close to right would >> >> >allow the pot to be a small part of the total resistance, which would >> >> >help a lot. Putting a slot in the PCB under the big resistor would make >> >> >its capacitance more predictable, so the pot could be smaller. >> >> > >> >> >I suggest a variable bootstrap. Drive the cold end of the bottom >> >> >capacitor from a small-value pot hung off the output amp. That would >> >> >make the capacitance adjustable, and the tempco of the pot would be >> >> >ratioed out. >> >> >> >> A pot in a c-bootstrap is better than a variable cap, but still needs >> >> adjustment in production. >> >> >> >> How about a capacitance that's tuned by two resistors? That will have >> >> better resolution than selecting from caps that we have in stock. >> >> >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/Pickoff_2.jpg >> >> >> >> >> >> I still don't have any idea what the stray C might be. I suppose I >> >> could measure it. Grumble. >> > >> >Or use PCB fingers/tracks. Add solder or wire link to connect a finger to give more capacitance. Or cut wire links maybe. >> > >> >> Testing would kill me, and manufacturing would jump on the bones. > >Why? It's standard practice. > > >NT
Not around here. Testing people are not expected to solder. If I wanted to adjust capacitance in steps, I'd use a dip switch, or one of those IC multi-step capacitors. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by John Larkin November 16, 20152015-11-16
On Mon, 16 Nov 2015 08:31:45 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Sunday, November 15, 2015 at 7:27:18 PM UTC-5, legg wrote: >> Effect of Gigohm >> and 1/10th environmental stray C >> >> RL > ><snip sim file> > >That was fun. The strays have horrible time constants if they're isolated >by high resistances. > >In the Dept. of Silly Walks department, dividers made entirely of a single >resistor type are inherently perfectly compensated. > > Vin > --- > | > [R1] > | > [R2] > | > +-----+--> Vin/5 > | | > [R3] [R4] > | | > === === > > > Vin > --- > | > [R1] > | > [R2] > | > [R3] > | > +-----+-----+--> Vin/10 > | | | > [R4] [R5] [R6] > | | | > === === === > > > Vin > --- > | > [R1] > | > +------. > | | > [R2] [R3] > | | > === +-----. > | | > [R4] [R5] > | | > === +-----. > | | > [R6] [R7] > | | > === +----> > | > [R8] > | > === > >Cheers, >James Arthur
I have files of all the resistor values and all the dividers that you can make from a quad resistor pack. Here's one of them https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/R-pak_values.JPG
Reply by John Larkin November 16, 20152015-11-16
On Mon, 16 Nov 2015 08:23:35 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Monday, November 16, 2015 at 3:51:22 AM UTC-5, Jeroen Belleman wrote: >> On 2015-11-15 23:34, John Larkin wrote: >> > On Sun, 15 Nov 2015 14:11:07 -0800 (PST), dagmargoodboat@yahoo.com >> > wrote: >> > >> [snip! >> > >> > >> > The waveform at B is good if C4 is 50 pF. >> > >> > C is fine, but it's only 120 millivolts. >> > >> > I just have no idea if C1 is really 0.1 pF. >> > >> >> I got a fair estimate of the parasitic capacitance >> of a 1206 SMD resistor by just assuming it's a flat >> plate capacitor with all the field confined to the >> SiO2 substrate. I calculated 30fF and subsequent >> measurement indicated 50fF. >> >> For what it's worth.. >> >> Jeroen Belleman > >That's a useful notion, despite Phil's catch about the substrate >material. > >k(Al2O3)=9.7, vs. k(SiO2)=3.9, FWIW. > >John's 2010 and most of the other form factors all have the same >2:1 aspect ratio, so the same capacitance per that approximation. > >Too bad. > >Cheers, >James Arthur
Right. Most surface mount resistors are made on 20 mil alumina, with about the same aspect ratio, so their capacitance (and their thermal resistance, and power dissipation capability [1]) is independent of size. [1] assuming you heat sink the end caps