Forums

signal pickoff

Started by John Larkin November 13, 2015
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
On Sunday, 15 November 2015 02:55:54 UTC, Phil Hobbs  wrote:
> On 11/14/2015 09:20 PM, 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: > >>>>> You'll want the top resistor to be a chain of smaller ones, or at most, > >>>>> one large power resistor. I'm guessing SMTs would be easier and cheaper > >>>>> though. > >>>> > >>>> I've spent a fair amount of sweat trying to reduce the effect of > >>>> capacitance on large resistors. Fixing the frequency response isn't > >>>> hard--you split it into multiple sections, with each set of > >>>> interconnected pads sitting over a small pour on the adjacent level, and > >>>> slots cut under the resistor bodies to reduce capacitance and get the > >>>> flux out. Then you drive the pours with a lower impedance resistor string. > >>>> > >>>> From my usual noise-obsessed POV that doesn't fix the problem, because > >>>> the current noise of the first resistor couples through the pad > >>>> capacitance to the low-Z string, dumping the noise into the summing > >>>> junction. So it doesn't help the SNR except at low frequency. > >>>> > >>>> It also isn't so easy to do at 1200V. > >>>> > >>>> > >>>>> > >>>>> All those intermediate pads/traces/bodies will have parasitic > >>>>> capacitance. Shunt that with several pF per resistor. The ratio Cshunt > >>>>> / Cstray determines how much improvement in flatness you get (against > >>>>> slight peaks and dips at middle frequencies, what Tektronix called > >>>>> 'hook', I belive?). For 1/x times better flatness, you need a ratio of, > >>>>> at most, x. > >>>> > >>>> A gigohm and a picofarad makes a zero at 160 Hz. The circuit would have > >>>> to be pretty slow for that not to dominate. > >>> > >>> I'm guessing that the 2010 resistor will have enough capacitance that > >>> I don't really need more. The compensating cap will be 500x that > >>> value, so 0.05 pF across R1 needs about 25 pF below to compensate it. > >>> I can get a reasonable spread of caps around 25p, like 16, 18, 22, 27, > >>> 33... > >>> > >>> Maybe I should use two caps, to allow fine tuning. I could use a > >>> trimmer cap, but that's one more thing the test folks would have to > >>> deal with. > >>> > >>> > >>> > >>>> > >>>>> > >>>>> Needless to say, cut out ground plane and surrounding copper, on all > >>>>> layers, around the divider resistors. The characteristic impedance > >>>>> needs to be high. Of course, you won't get it into the megs, but your > >>>>> cap divider will have some impedance at HF. Keep that in mind where the > >>>>> probe part connects to the pulse part: if it has to be a very stable > >>>>> characteristic impedance (say 50 ohms, out into the 100s of MHz), you'll > >>>>> have to account for the probe capacitance too. > >>>>> > >>>>> The divider then has to have a pretty large capacitor at the bottom, > >>>>> because of the large ratio. Maybe the total probe capacitance is 1pF, > >>>>> so you need 500pF at the bottom. That's a sucky varicap. So don't > >>>>> bother at all. Let the AC gain be what it's going to be, then buffer > >>>>> it, and put a trimpot there. You still need compensation, but it's > >>>>> easier done at DC instead. So instead of a fixed bottom-of-divider R, > >>>>> use a trimpot. Cal process: adjust Rbottom until flat (no leading edge > >>>>> over/undershoot), adjust output gain until AC/DC gain is right. > >>>> > >>>> 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. > > > > > Plus it would drift around with temperature and humidity. FR4 is a > horrible dielectric.
If the upper capacitance is partly made with FR4 dielectric you want the lower one to be likewise. Getting them to the same degree should be fun. NT
On 11/15/2015 06:33 AM, tabbypurr@gmail.com wrote:
> On Sunday, 15 November 2015 02:55:54 UTC, Phil Hobbs wrote: >> On 11/14/2015 09:20 PM, 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: >>>>>>> You'll want the top resistor to be a chain of smaller >>>>>>> ones, or at most, one large power resistor. I'm guessing >>>>>>> SMTs would be easier and cheaper though. >>>>>> >>>>>> I've spent a fair amount of sweat trying to reduce the >>>>>> effect of capacitance on large resistors. Fixing the >>>>>> frequency response isn't hard--you split it into multiple >>>>>> sections, with each set of interconnected pads sitting over >>>>>> a small pour on the adjacent level, and slots cut under the >>>>>> resistor bodies to reduce capacitance and get the flux out. >>>>>> Then you drive the pours with a lower impedance resistor >>>>>> string. >>>>>> >>>>>> From my usual noise-obsessed POV that doesn't fix the >>>>>> problem, because the current noise of the first resistor >>>>>> couples through the pad capacitance to the low-Z string, >>>>>> dumping the noise into the summing junction. So it doesn't >>>>>> help the SNR except at low frequency. >>>>>> >>>>>> It also isn't so easy to do at 1200V. >>>>>> >>>>>> >>>>>>> >>>>>>> All those intermediate pads/traces/bodies will have >>>>>>> parasitic capacitance. Shunt that with several pF per >>>>>>> resistor. The ratio Cshunt / Cstray determines how much >>>>>>> improvement in flatness you get (against slight peaks and >>>>>>> dips at middle frequencies, what Tektronix called 'hook', >>>>>>> I belive?). For 1/x times better flatness, you need a >>>>>>> ratio of, at most, x. >>>>>> >>>>>> A gigohm and a picofarad makes a zero at 160 Hz. The >>>>>> circuit would have to be pretty slow for that not to >>>>>> dominate. >>>>> >>>>> I'm guessing that the 2010 resistor will have enough >>>>> capacitance that I don't really need more. The compensating >>>>> cap will be 500x that value, so 0.05 pF across R1 needs about >>>>> 25 pF below to compensate it. I can get a reasonable spread >>>>> of caps around 25p, like 16, 18, 22, 27, 33... >>>>> >>>>> Maybe I should use two caps, to allow fine tuning. I could >>>>> use a trimmer cap, but that's one more thing the test folks >>>>> would have to deal with. >>>>> >>>>> >>>>> >>>>>> >>>>>>> >>>>>>> Needless to say, cut out ground plane and surrounding >>>>>>> copper, on all layers, around the divider resistors. The >>>>>>> characteristic impedance needs to be high. Of course, >>>>>>> you won't get it into the megs, but your cap divider will >>>>>>> have some impedance at HF. Keep that in mind where the >>>>>>> probe part connects to the pulse part: if it has to be a >>>>>>> very stable characteristic impedance (say 50 ohms, out >>>>>>> into the 100s of MHz), you'll have to account for the >>>>>>> probe capacitance too. >>>>>>> >>>>>>> The divider then has to have a pretty large capacitor at >>>>>>> the bottom, because of the large ratio. Maybe the total >>>>>>> probe capacitance is 1pF, so you need 500pF at the >>>>>>> bottom. That's a sucky varicap. So don't bother at all. >>>>>>> Let the AC gain be what it's going to be, then buffer it, >>>>>>> and put a trimpot there. You still need compensation, >>>>>>> but it's easier done at DC instead. So instead of a >>>>>>> fixed bottom-of-divider R, use a trimpot. Cal process: >>>>>>> adjust Rbottom until flat (no leading edge >>>>>>> over/undershoot), adjust output gain until AC/DC gain is >>>>>>> right. >>>>>> >>>>>> 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. >>> >>> >> Plus it would drift around with temperature and humidity. FR4 is >> a horrible dielectric. > > If the upper capacitance is partly made with FR4 dielectric you want > the lower one to be likewise. Getting them to the same degree should > be fun. >
You can do a pretty good job of getting rid of the FR4 contribution by extending the pads and putting big slots under the top resistor. (Extra credit for plating the slots without shorting the resistor.) That helps the leakage, too. It's pretty hard to get all the flux out from under the resistor without using a slot. 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
tabbypurr@gmail.com writes:

> On Sunday, 15 November 2015 02:19:14 UTC, John Larkin wrote: >> On Sat, 14 Nov 2015 16:04:44 -0800 (PST), nt wrote: >> >On Saturday, 14 November 2015 22:02:42 UTC, John Larkin wrote: >> >> On Sat, 14 Nov 2015 13:30:40 -0800 (PST), Phil Hobbs >> >> <pcdhobbs@gmail.com> wrote: >> >> >> >> >The major imponderable will probably be the poorly-controlled >> >> > epsilon of the board. If you make the air gaps big enough, you >> >> > can probably control that well enough. >> > >> >> A pot or a trimmer cap makes sense. I get pushback from young things >> >> that are prejudiced against pots. But really, when testing is making >> >> pulses, it would only take them a few seconds to turn a pot to make >> >> the pulses look right. >> > >> >Preset pots are a long term reliability disaster. >> >> Why? > > I'm not sure why, but experience shows they are. It's why they've > fallen out of favour.
Because customers just *love* to twiddle them IME.
> >> >Perhaps a set of 4 wire links that you cut to get the right C. >> >> And uncut if you overshoot. > > I expect all units would want much the same setting > > > NT
-- John Devereux
On Sun, 15 Nov 2015 02:47:06 -0800 (PST), tabbypurr@gmail.com wrote:

>On Sunday, 15 November 2015 02:19:14 UTC, John Larkin wrote: >> On Sat, 14 Nov 2015 16:04:44 -0800 (PST), nt wrote: >> >On Saturday, 14 November 2015 22:02:42 UTC, John Larkin wrote: >> >> On Sat, 14 Nov 2015 13:30:40 -0800 (PST), Phil Hobbs >> >> <pcdhobbs@gmail.com> wrote: >> >> >> >> >The major imponderable will probably be the poorly-controlled epsilon of the board. If you make the air gaps big enough, you can probably control that well enough. >> > >> >> A pot or a trimmer cap makes sense. I get pushback from young things >> >> that are prejudiced against pots. But really, when testing is making >> >> pulses, it would only take them a few seconds to turn a pot to make >> >> the pulses look right. >> > >> >Preset pots are a long term reliability disaster. >> >> Why? > >I'm not sure why, but experience shows they are. It's why they've fallen out of favour.
Pots are fine as long as you don't expect more than about 0.25% longterm stability. In my current situation, I'm just trying to make a pulse look good on a scope, so a few per cent of the pot range would be close enough.
> >> >Perhaps a set of 4 wire links that you cut to get the right C. >> >> And uncut if you overshoot. > >I expect all units would want much the same setting
If that were the case, I could just pick the compensation cap value once. Which might be good enough.
On Saturday, November 14, 2015 at 11:30:34 AM UTC-5, John Larkin wrote:
> On Sat, 14 Nov 2015 03:03:27 -0800 (PST), dagmargoodboat@yahoo.com > wrote: > > >On Friday, November 13, 2015 at 10:34:07 PM UTC-5, John Larkin wrote: > >> On Fri, 13 Nov 2015 16:54:29 -0800 (PST), Lasse Langwadt Christensen > >> <langwadt@fonz.dk> wrote: > >> > >> >Den l&#2013266168;rdag den 14. november 2015 kl. 01.22.28 UTC+1 skrev John Larkin: > >> >> I'm designing a high-voltage pulser, 1200 volts or so, and I'd like to > >> >> pick off a divided signal for the customer to monitor. > >> >> > >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/Pickoff.JPG > >> >> > >> >> Rise/fall times will be a few us maybe. > >> >> > >> >> 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. These have to > >> >> be balanced at 500:1 to match the resistors. The cap across R2 can be > >> >> chosen to work, but I don't want a discrete cap across R1. > >> >> > >> >> Maybe the 1G resistor has enough self-capacitance that I can pick the > >> >> lower cap to work. Or maybe I should add some PCB traces to add > >> >> capacitance across R1. I need about 0.02 pF if, say, I pad the bottom > >> >> up to 10 pF. > >> >> > >> >> This will be a 4-layer board with L2 ground plane, but I can cut out > >> >> some plane if that would help. > >> >> > >> >> Has anyone done something like this? > >> >> > >> > > >> >why such big resistors? > >> > > >> >a standard 500:1 scope probe is 10M afaict > >> > > >> >why not a string of say 2M resistors and capacitors > >> > >> I have rational reasons for wanting a 1G resistor. Even more would be > >> good, but I can get a 1G 3KV 2010. > > > >Wouldn't a trans impedance amp be faster? Then you wouldn't have to move the > >summing node. You might have to make an ultra-low capacitance buffer, lest the > >input capacitance make the thing unstable. > > > >Cheers, > >James > > That's about a wash, and I'd need to re-invert the TIA output with > another amp. > > The 1G resistor has some (currently unknown) shunt capacitance, and > that makes a tau that any amplifier has to deal with. If the shunt C > were zero, an ideal TIA would work.
You're right, the problem here is peaking, the opposite of the usual TI amp problem.
> If I have a reasonable capacitance across the 1G, then the lowside > compensating cap (either amp config) gets up into the tens or hundreds > of pF, where I can get a reasonable selection of compensation cap > choices. > > This is just a freebie current monitor so the customer can check that > the pulse is working. It's not worth extreme measures.
Then why not change the 2M divider resistor to 200k? 200K * 4pF = 800nS, problem solved. Make the op-amp stage x10. Or 100k and x20.
> > There are PCB techniques for reducing the effective capacitance across > resistors. I want to increase it!
Constructing the divider from 500 instances of the same resistor would automatically balance their shunt capacitances according to the divider ratio (and look very silly!). Cheers, James Arthur
On Sunday, 15 November 2015 16:27:39 UTC, John Larkin  wrote:
> On Sun, 15 Nov 2015 02:47:06 -0800 (PST), nt wrote: > > >On Sunday, 15 November 2015 02:19:14 UTC, John Larkin wrote: > >> On Sat, 14 Nov 2015 16:04:44 -0800 (PST), nt wrote: > >> >On Saturday, 14 November 2015 22:02:42 UTC, John Larkin wrote: > >> >> On Sat, 14 Nov 2015 13:30:40 -0800 (PST), Phil Hobbs > >> >> <pcdhobbs@gmail.com> wrote: > >> >> > >> >> >The major imponderable will probably be the poorly-controlled epsilon of the board. If you make the air gaps big enough, you can probably control that well enough. > >> > > >> >> A pot or a trimmer cap makes sense. I get pushback from young things > >> >> that are prejudiced against pots. But really, when testing is making > >> >> pulses, it would only take them a few seconds to turn a pot to make > >> >> the pulses look right. > >> > > >> >Preset pots are a long term reliability disaster. > >> > >> Why? > > > >I'm not sure why, but experience shows they are. It's why they've fallen out of favour. > > Pots are fine as long as you don't expect more than about 0.25% > longterm stability. In my current situation, I'm just trying to make a > pulse look good on a scope, so a few per cent of the pot range would > be close enough.
Pots aren't too bad, little presets are NT
> >> >Perhaps a set of 4 wire links that you cut to get the right C. > >> > >> And uncut if you overshoot. > > > >I expect all units would want much the same setting > > If that were the case, I could just pick the compensation cap value > once. Which might be good enough.
On Sun, 15 Nov 2015 09:24:39 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Saturday, November 14, 2015 at 11:30:34 AM UTC-5, John Larkin wrote: >> On Sat, 14 Nov 2015 03:03:27 -0800 (PST), dagmargoodboat@yahoo.com >> wrote: >> >> >On Friday, November 13, 2015 at 10:34:07 PM UTC-5, John Larkin wrote: >> >> On Fri, 13 Nov 2015 16:54:29 -0800 (PST), Lasse Langwadt Christensen >> >> <langwadt@fonz.dk> wrote: >> >> >> >> >Den l&#2013266168;rdag den 14. november 2015 kl. 01.22.28 UTC+1 skrev John Larkin: >> >> >> I'm designing a high-voltage pulser, 1200 volts or so, and I'd like to >> >> >> pick off a divided signal for the customer to monitor. >> >> >> >> >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/Pickoff.JPG >> >> >> >> >> >> Rise/fall times will be a few us maybe. >> >> >> >> >> >> 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. These have to >> >> >> be balanced at 500:1 to match the resistors. The cap across R2 can be >> >> >> chosen to work, but I don't want a discrete cap across R1. >> >> >> >> >> >> Maybe the 1G resistor has enough self-capacitance that I can pick the >> >> >> lower cap to work. Or maybe I should add some PCB traces to add >> >> >> capacitance across R1. I need about 0.02 pF if, say, I pad the bottom >> >> >> up to 10 pF. >> >> >> >> >> >> This will be a 4-layer board with L2 ground plane, but I can cut out >> >> >> some plane if that would help. >> >> >> >> >> >> Has anyone done something like this? >> >> >> >> >> > >> >> >why such big resistors? >> >> > >> >> >a standard 500:1 scope probe is 10M afaict >> >> > >> >> >why not a string of say 2M resistors and capacitors >> >> >> >> I have rational reasons for wanting a 1G resistor. Even more would be >> >> good, but I can get a 1G 3KV 2010. >> > >> >Wouldn't a trans impedance amp be faster? Then you wouldn't have to move the >> >summing node. You might have to make an ultra-low capacitance buffer, lest the >> >input capacitance make the thing unstable. >> > >> >Cheers, >> >James >> >> That's about a wash, and I'd need to re-invert the TIA output with >> another amp. >> >> The 1G resistor has some (currently unknown) shunt capacitance, and >> that makes a tau that any amplifier has to deal with. If the shunt C >> were zero, an ideal TIA would work. > >You're right, the problem here is peaking, the opposite of the usual TI >amp problem. > >> If I have a reasonable capacitance across the 1G, then the lowside >> compensating cap (either amp config) gets up into the tens or hundreds >> of pF, where I can get a reasonable selection of compensation cap >> choices. >> >> This is just a freebie current monitor so the customer can check that >> the pulse is working. It's not worth extreme measures. > >Then why not change the 2M divider resistor to 200k? 200K * 4pF = 800nS, >problem solved. Make the op-amp stage x10. Or 100k and x20.
The parasitic shunt cap across the 1G resistor will differentiate the HV rising edge and still make a pulse overshoot. 0.1 pF and 1 Gohms has a tau of 100 usec
> >> >> There are PCB techniques for reducing the effective capacitance across >> resistors. I want to increase it! > >Constructing the divider from 500 instances of the same resistor would >automatically balance their shunt capacitances according to the divider >ratio (and look very silly!).
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. I think some people do that trick with multiple series resistors, too. The worst way to tune a design is by re-etching PC boards. I don't want to do that. A variable cap or an equivalent pot-controlled capacitance would let my test people tweak the step response, independent of PCB variations. People who don't approve of trimmers will whine, but I am The President, after all. Caddock makes axial thickfilm resistors that have essentially no L or C. One of them can make a high voltage divider, into a 50 ohm scope, that's flat to something like 6 GHz. But I don't think they can do that at 1G ohms. Or in surface mount.
On Sun, 15 Nov 2015 10:43:05 -0800 (PST), tabbypurr@gmail.com wrote:

>On Sunday, 15 November 2015 16:27:39 UTC, John Larkin wrote: >> On Sun, 15 Nov 2015 02:47:06 -0800 (PST), nt wrote: >> >> >On Sunday, 15 November 2015 02:19:14 UTC, John Larkin wrote: >> >> On Sat, 14 Nov 2015 16:04:44 -0800 (PST), nt wrote: >> >> >On Saturday, 14 November 2015 22:02:42 UTC, John Larkin wrote: >> >> >> On Sat, 14 Nov 2015 13:30:40 -0800 (PST), Phil Hobbs >> >> >> <pcdhobbs@gmail.com> wrote: >> >> >> >> >> >> >The major imponderable will probably be the poorly-controlled epsilon of the board. If you make the air gaps big enough, you can probably control that well enough. >> >> > >> >> >> A pot or a trimmer cap makes sense. I get pushback from young things >> >> >> that are prejudiced against pots. But really, when testing is making >> >> >> pulses, it would only take them a few seconds to turn a pot to make >> >> >> the pulses look right. >> >> > >> >> >Preset pots are a long term reliability disaster. >> >> >> >> Why? >> > >> >I'm not sure why, but experience shows they are. It's why they've fallen out of favour. >> >> Pots are fine as long as you don't expect more than about 0.25% >> longterm stability. In my current situation, I'm just trying to make a >> pulse look good on a scope, so a few per cent of the pot range would >> be close enough. > >Pots aren't too bad, little presets are
What is a "little preset"?
On Sunday, November 15, 2015 at 3:41:06 PM UTC-5, John Larkin wrote:
> On Sun, 15 Nov 2015 09:24:39 -0800 (PST), dagmargoodboat@yahoo.com > wrote: > > >On Saturday, November 14, 2015 at 11:30:34 AM UTC-5, John Larkin wrote: > >> On Sat, 14 Nov 2015 03:03:27 -0800 (PST), dagmargoodboat@yahoo.com > >> wrote: > >> > >> >On Friday, November 13, 2015 at 10:34:07 PM UTC-5, John Larkin wrote: > >> >> On Fri, 13 Nov 2015 16:54:29 -0800 (PST), Lasse Langwadt Christensen > >> >> <langwadt@fonz.dk> wrote: > >> >> > >> >> >Den l&#2013266168;rdag den 14. november 2015 kl. 01.22.28 UTC+1 skrev John Larkin: > >> >> >> I'm designing a high-voltage pulser, 1200 volts or so, and I'd like to > >> >> >> pick off a divided signal for the customer to monitor. > >> >> >> > >> >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Resistors/Pickoff.JPG > >> >> >> > >> >> >> Rise/fall times will be a few us maybe. > >> >> >> > >> >> >> 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. These have to > >> >> >> be balanced at 500:1 to match the resistors. The cap across R2 can be > >> >> >> chosen to work, but I don't want a discrete cap across R1. > >> >> >> > >> >> >> Maybe the 1G resistor has enough self-capacitance that I can pick the > >> >> >> lower cap to work. Or maybe I should add some PCB traces to add > >> >> >> capacitance across R1. I need about 0.02 pF if, say, I pad the bottom > >> >> >> up to 10 pF. > >> >> >> > >> >> >> This will be a 4-layer board with L2 ground plane, but I can cut out > >> >> >> some plane if that would help. > >> >> >> > >> >> >> Has anyone done something like this? > >> >> >> > >> >> > > >> >> >why such big resistors? > >> >> > > >> >> >a standard 500:1 scope probe is 10M afaict > >> >> > > >> >> >why not a string of say 2M resistors and capacitors > >> >> > >> >> I have rational reasons for wanting a 1G resistor. Even more would be > >> >> good, but I can get a 1G 3KV 2010. > >> > > >> >Wouldn't a trans impedance amp be faster? Then you wouldn't have to move the > >> >summing node. You might have to make an ultra-low capacitance buffer, lest the > >> >input capacitance make the thing unstable. > >> > > >> >Cheers, > >> >James > >> > >> That's about a wash, and I'd need to re-invert the TIA output with > >> another amp. > >> > >> The 1G resistor has some (currently unknown) shunt capacitance, and > >> that makes a tau that any amplifier has to deal with. If the shunt C > >> were zero, an ideal TIA would work. > > > >You're right, the problem here is peaking, the opposite of the usual TI > >amp problem. > > > >> If I have a reasonable capacitance across the 1G, then the lowside > >> compensating cap (either amp config) gets up into the tens or hundreds > >> of pF, where I can get a reasonable selection of compensation cap > >> choices. > >> > >> This is just a freebie current monitor so the customer can check that > >> the pulse is working. It's not worth extreme measures. > > > >Then why not change the 2M divider resistor to 200k? 200K * 4pF = 800nS, > >problem solved. Make the op-amp stage x10. Or 100k and x20. > > The parasitic shunt cap across the 1G resistor will differentiate the > HV rising edge and still make a pulse overshoot. 0.1 pF and 1 Gohms > has a tau of 100 usec
1) Yes, it's differentiated and yes it'll overshoot. But the time constant is .1pF and (1G in parallel with 200k), not 1G, so the overshoot is short-lived, and maybe even too small to notice, depending on your slew rate. (The pulse / overshoot is also, technically, not 1,200V, but 1,200V * the capacitive divider ratio between .1pF and ~4pF, or about 1/40th * 1,200V). But we're talking past each other--I've attached a sim file. Look at the settling time difference between my suggestion V(A) and the original, V(B). V(B_scaled) lets you see the two on the same scale. 2) If it's really .1pF that's horrible; different measures are called for, like a diff amp where a 2nd 1G resistor is a.c.-coupled, and subtracts the 1G resistor's capacitively-coupled current from the output.
> >> There are PCB techniques for reducing the effective capacitance across > >> resistors. I want to increase it! > > > >Constructing the divider from 500 instances of the same resistor would > >automatically balance their shunt capacitances according to the divider > >ratio (and look very silly!). > > 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. > > I think some people do that trick with multiple series resistors, too. > > The worst way to tune a design is by re-etching PC boards. I don't > want to do that. A variable cap or an equivalent pot-controlled > capacitance would let my test people tweak the step response, > independent of PCB variations. People who don't approve of trimmers > will whine, but I am The President, after all. > > Caddock makes axial thickfilm resistors that have essentially no L or > C. One of them can make a high voltage divider, into a 50 ohm scope, > that's flat to something like 6 GHz. But I don't think they can do > that at 1G ohms. Or in surface mount.
Cheers, James Arthur Version 4 SHEET 1 1032 680 WIRE 192 0 -48 0 WIRE 272 0 192 0 WIRE 368 0 272 0 WIRE 448 0 368 0 WIRE 800 0 448 0 WIRE 880 0 800 0 WIRE 272 16 272 0 WIRE 880 16 880 0 WIRE 192 32 192 0 WIRE 368 32 368 0 WIRE 448 32 448 0 WIRE 800 32 800 0 WIRE -48 96 -48 0 WIRE 192 128 192 96 WIRE 272 128 272 96 WIRE 272 128 192 128 WIRE 368 128 368 96 WIRE 448 128 448 112 WIRE 448 128 368 128 WIRE 800 128 800 96 WIRE 880 128 880 96 WIRE 880 128 800 128 WIRE 272 160 272 128 WIRE 272 160 192 160 WIRE 640 160 272 160 WIRE 880 160 880 128 WIRE 880 160 800 160 WIRE 992 160 880 160 WIRE 272 176 272 160 WIRE 880 176 880 160 WIRE 192 192 192 160 WIRE 448 192 448 128 WIRE 448 192 368 192 WIRE 640 192 448 192 WIRE 800 192 800 160 WIRE -48 208 -48 176 WIRE 448 208 448 192 WIRE 368 224 368 192 WIRE 192 272 192 256 WIRE 800 272 800 256 WIRE 272 288 272 256 WIRE 880 288 880 256 WIRE 448 304 448 288 WIRE 640 304 448 304 WIRE 368 320 368 288 WIRE 448 320 448 304 WIRE 448 432 448 400 FLAG -48 208 0 FLAG 272 288 0 FLAG 192 272 0 FLAG 448 432 0 FLAG 368 320 0 FLAG 640 160 A FLAG 640 304 B_scaled FLAG 640 192 B FLAG 880 288 0 FLAG 800 272 0 FLAG 992 160 C SYMBOL voltage -48 80 R0 WINDOW 3 -154 169 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 1200 0 2uS 2uS 50uS) SYMBOL res 256 0 R0 SYMATTR InstName R1 SYMATTR Value 1G SYMBOL res 256 160 R0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL cap 176 32 R0 SYMATTR InstName C1 SYMATTR Value .1pF SYMBOL cap 176 192 R0 SYMATTR InstName C2 SYMATTR Value 4pF SYMBOL res 432 16 R0 SYMATTR InstName R3 SYMATTR Value 1G SYMBOL res 432 192 R0 SYMATTR InstName R4 SYMATTR Value 1.9meg SYMBOL cap 352 32 R0 SYMATTR InstName C3 SYMATTR Value .1pF SYMBOL cap 352 224 R0 SYMATTR InstName C4 SYMATTR Value 4pF SYMBOL res 432 304 R0 SYMATTR InstName R5 SYMATTR Value 100k SYMBOL res 864 0 R0 SYMATTR InstName R6 SYMATTR Value 1G SYMBOL res 864 160 R0 SYMATTR InstName R7 SYMATTR Value 100k SYMBOL cap 784 32 R0 SYMATTR InstName C5 SYMATTR Value .1pF SYMBOL cap 784 192 R0 SYMATTR InstName C6 SYMATTR Value 1nF TEXT 30 360 Left 2 !.tran 150uS TEXT 608 384 Left 2 ;15-Nov-2015 jda