Forums

signal pickoff

Started by John Larkin November 13, 2015

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?


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Fri, 13 Nov 2015 16:22:18 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

> > >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.
How about just a capacitor divider? Use 500:1 area over a common "plane". You can choose the dimensions such that the ratio should be fairly well controlled. It would require a buffer to drive a scope but that shouldn't be too tough.
>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?
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 -Lasse
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.

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.

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.

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:ouuc4b9o1l2d82samn5v58an6kcdiuc2ut@4ax.com...
> > > 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? > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com >
On Friday, November 13, 2015 at 7:22:28 PM UTC-5, John Larkin wrote:
> 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?
Never, but my first thought on seeing the picture was to try and put the right inductance somewhere. I think in series? I'd have to do the math. George H.
> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
On Friday, November 13, 2015 at 9:15:46 PM UTC-5, George Herold wrote:
> On Friday, November 13, 2015 at 7:22:28 PM UTC-5, John Larkin wrote: > > 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? > Never, but my first thought on seeing the picture > was to try and put the right inductance somewhere. > I think in series? > > I'd have to do the math.
What about if the inductance was in series with the 2M? That makes the first bit of current charge up the cap. GH
> > George H. > > > > > > -- > > > > John Larkin Highland Technology, Inc > > picosecond timing precision measurement > > > > jlarkin att highlandtechnology dott com > > http://www.highlandtechnology.com
On Fri, 13 Nov 2015 16:22:18 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

> > >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?
Like Tim described, but the intent is to pic a C that swamps out the upper arm stray capacitance. Maybe 200pf to 400pf, and the correct ratio on the lower R. eg; AC divider. I think HP has some appnotes on HV dividers. Cheers
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.
On Fri, 13 Nov 2015 16:22:18 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

> > >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?
It can't be more accurate than your reference, for comparison. HV probes with bandwidth are an issue, the higher the DC impedance selected, but usually just as a result of shielding effects. Without a lot of stray capacitance on the high end of the divider, it becomes easier. SOT caps on the shunt, may end up being fairly predictable. Matching tempcos and aging of Cs and Rs in the divider can be an issue. What kind of calibration procedure is intended? The source would need to be ~ insensitive to a double probe load. The effect of removal of either can be witnessed. If just a monitor, perhaps precision isn't so important? RL
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