Forums

Creating SPICE models

Started by Phil Hobbs October 26, 2016
So I've been designing a Joerg-esque super cheap single diode sampler.
With vaguely realistic parasitics, LTspice says it should give a
sampling FWHM of 120 ps or a bit faster, all for well under a buck
excluding the timing generation part, but including the pulse speedup part.

In the spirit of keeping things cheap and small, I'm using a single JFET
switch and a hold capacitor to store the sampled voltage until  the ADC
gets round to coverting it.

Since it's all on a single 5-V supply, I need a small JFET with super
low pinchoff voltage.  The best one I can get for cheap seems to be the
2SK3796-3, which is about 9 cents in reels from distributors.

Unfortunately, there's no SPICE model that I can find.

So since I'm not going to buy PSPICE just for this one part, what else
do you folks use for model creation?

Thanks

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 Wed, 26 Oct 2016 11:41:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So I've been designing a Joerg-esque super cheap single diode sampler. >With vaguely realistic parasitics, LTspice says it should give a >sampling FWHM of 120 ps or a bit faster, all for well under a buck >excluding the timing generation part, but including the pulse speedup part. > >In the spirit of keeping things cheap and small, I'm using a single JFET >switch and a hold capacitor to store the sampled voltage until the ADC >gets round to coverting it.
Are you overt while you are coverting ?>:-}
> >Since it's all on a single 5-V supply, I need a small JFET with super >low pinchoff voltage. The best one I can get for cheap seems to be the >2SK3796-3, which is about 9 cents in reels from distributors. > >Unfortunately, there's no SPICE model that I can find. > >So since I'm not going to buy PSPICE just for this one part, what else >do you folks use for model creation? > >Thanks > >Phil Hobbs
System modeling is easy, device models that are accurate require expensive parameter extraction test equipment. If you can find a model with the same parasitics, but wrong threshold, you can just twiddle the threshold. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
On Wed, 26 Oct 2016 11:41:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So I've been designing a Joerg-esque super cheap single diode sampler. >With vaguely realistic parasitics, LTspice says it should give a >sampling FWHM of 120 ps or a bit faster, all for well under a buck >excluding the timing generation part, but including the pulse speedup part. > >In the spirit of keeping things cheap and small, I'm using a single JFET >switch and a hold capacitor to store the sampled voltage until the ADC >gets round to coverting it. > >Since it's all on a single 5-V supply, I need a small JFET with super >low pinchoff voltage. The best one I can get for cheap seems to be the >2SK3796-3, which is about 9 cents in reels from distributors. > >Unfortunately, there's no SPICE model that I can find. > >So since I'm not going to buy PSPICE just for this one part, what else >do you folks use for model creation? > >Thanks > >Phil Hobbs
Why not use a cmos opamp? What are you using to make the sampling pulse? Transistor? SRD? A dual-diode sampler isn't much more expensive. There are SRDs available for under a dollar, and dual diodes are cheap. One of the Tek plugins, the 1S2 I think, had a 2-diode sampler dumping into a big capacitor, which did the "smoothing" function and eliminated all sorts of concerns. 1-bit samplers are interesting too. Use a flipflop or a strobed comparator for the sampler. Everything is low impedance, and you don't need a narrow sampling impulse. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 10/26/2016 12:18 PM, John Larkin wrote:
> On Wed, 26 Oct 2016 11:41:52 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> So I've been designing a Joerg-esque super cheap single diode sampler. >> With vaguely realistic parasitics, LTspice says it should give a >> sampling FWHM of 120 ps or a bit faster, all for well under a buck >> excluding the timing generation part, but including the pulse speedup part. >> >> In the spirit of keeping things cheap and small, I'm using a single JFET >> switch and a hold capacitor to store the sampled voltage until the ADC >> gets round to coverting it. >> >> Since it's all on a single 5-V supply, I need a small JFET with super >> low pinchoff voltage. The best one I can get for cheap seems to be the >> 2SK3796-3, which is about 9 cents in reels from distributors. >> >> Unfortunately, there's no SPICE model that I can find. >> >> So since I'm not going to buy PSPICE just for this one part, what else >> do you folks use for model creation? >> >> Thanks >> >> Phil Hobbs > > Why not use a cmos opamp?
Too slow. The front-end sample cap is only a few pF, so I have to do the sample-and-hold in a couple hundred ns at most. I have a discrete BJT amp for the first stage--its base current supplies most of the reverse bias for the sampling diode.
> > What are you using to make the sampling pulse? Transistor? SRD?
One of your LVDS receivers running nearly unloaded, followed by a practically infinitely fast transistor. Its voltage gain is what sharpens up the edge.
> > A dual-diode sampler isn't much more expensive. There are SRDs > available for under a dollar, and dual diodes are cheap. One of the > Tek plugins, the 1S2 I think, had a 2-diode sampler dumping into a big > capacitor, which did the "smoothing" function and eliminated all sorts > of concerns.
A buck is the budget for the whole thing. ;)
> > 1-bit samplers are interesting too. Use a flipflop or a strobed > comparator for the sampler. Everything is low impedance, and you don't > need a narrow sampling impulse.
I'm differentiating the output of the fast transistor to make the sampling pulse. So does anybody have a free/cheap way of making models from datasheet curves? PSPICE has a model editor. I can probably cruft together a test bench in LTspice, but I'd much rather learn from other people's mistakes. ;) 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 10/26/2016 11:57 AM, Jim Thompson wrote:
> On Wed, 26 Oct 2016 11:41:52 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> So I've been designing a Joerg-esque super cheap single diode sampler. >> With vaguely realistic parasitics, LTspice says it should give a >> sampling FWHM of 120 ps or a bit faster, all for well under a buck >> excluding the timing generation part, but including the pulse speedup part. >> >> In the spirit of keeping things cheap and small, I'm using a single JFET >> switch and a hold capacitor to store the sampled voltage until the ADC >> gets round to coverting it. > > Are you overt while you are coverting ?>:-}
It's a hush-hush project. ;)
> >> >> Since it's all on a single 5-V supply, I need a small JFET with super >> low pinchoff voltage. The best one I can get for cheap seems to be the >> 2SK3796-3, which is about 9 cents in reels from distributors. >> >> Unfortunately, there's no SPICE model that I can find. >> >> So since I'm not going to buy PSPICE just for this one part, what else >> do you folks use for model creation? >> >> Thanks >> >> Phil Hobbs > > System modeling is easy, device models that are accurate require > expensive parameter extraction test equipment. > > If you can find a model with the same parasitics, but wrong threshold, > you can just twiddle the threshold.
I've done that before, but it's kind of hit-or-miss. Ideally I'm looking for something like the PSPICE model editor without PSPICE attached. 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 Wed, 26 Oct 2016 12:39:11 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 10/26/2016 12:18 PM, John Larkin wrote: >> On Wed, 26 Oct 2016 11:41:52 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> So I've been designing a Joerg-esque super cheap single diode sampler. >>> With vaguely realistic parasitics, LTspice says it should give a >>> sampling FWHM of 120 ps or a bit faster, all for well under a buck >>> excluding the timing generation part, but including the pulse speedup part. >>> >>> In the spirit of keeping things cheap and small, I'm using a single JFET >>> switch and a hold capacitor to store the sampled voltage until the ADC >>> gets round to coverting it. >>> >>> Since it's all on a single 5-V supply, I need a small JFET with super >>> low pinchoff voltage. The best one I can get for cheap seems to be the >>> 2SK3796-3, which is about 9 cents in reels from distributors. >>> >>> Unfortunately, there's no SPICE model that I can find. >>> >>> So since I'm not going to buy PSPICE just for this one part, what else >>> do you folks use for model creation? >>> >>> Thanks >>> >>> Phil Hobbs >> >> Why not use a cmos opamp? > >Too slow. The front-end sample cap is only a few pF, so I have to do >the sample-and-hold in a couple hundred ns at most. I have a discrete >BJT amp for the first stage--its base current supplies most of the >reverse bias for the sampling diode. > >> >> What are you using to make the sampling pulse? Transistor? SRD? > >One of your LVDS receivers running nearly unloaded, followed by a >practically infinitely fast transistor. Its voltage gain is what >sharpens up the edge. > >> >> A dual-diode sampler isn't much more expensive. There are SRDs >> available for under a dollar, and dual diodes are cheap. One of the >> Tek plugins, the 1S2 I think, had a 2-diode sampler dumping into a big >> capacitor, which did the "smoothing" function and eliminated all sorts >> of concerns. > >A buck is the budget for the whole thing. ;) >> >> 1-bit samplers are interesting too. Use a flipflop or a strobed >> comparator for the sampler. Everything is low impedance, and you don't >> need a narrow sampling impulse. > >I'm differentiating the output of the fast transistor to make the >sampling pulse.
Shorted transmission lines work too.
> >So does anybody have a free/cheap way of making models from datasheet >curves? PSPICE has a model editor. I can probably cruft together a >test bench in LTspice, but I'd much rather learn from other people's >mistakes. ;) > >Cheers > >Phil Hobbs
I just breadboard stuff like this. It's faster, more fun, and you'll need to try it out in real life anyhow. Spice can run 12 orders of magnitude slower than real life. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Am 26.10.2016 um 18:39 schrieb Phil Hobbs:

> So does anybody have a free/cheap way of making models from datasheet > curves? PSPICE has a model editor. I can probably cruft together a > test bench in LTspice, but I'd much rather learn from other people's > mistakes. ;)
It used to be well known that if a BIP model featured a certain value for Rbb (50 Ohm or so), it was made by Pspice model generator. No matter how fat or tiny the transistor was. Maybe they have improved in the last 20 years. regards, Gerhard
On Wed, 26 Oct 2016 12:41:08 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 10/26/2016 11:57 AM, Jim Thompson wrote: >> On Wed, 26 Oct 2016 11:41:52 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> So I've been designing a Joerg-esque super cheap single diode sampler. >>> With vaguely realistic parasitics, LTspice says it should give a >>> sampling FWHM of 120 ps or a bit faster, all for well under a buck >>> excluding the timing generation part, but including the pulse speedup part. >>> >>> In the spirit of keeping things cheap and small, I'm using a single JFET >>> switch and a hold capacitor to store the sampled voltage until the ADC >>> gets round to coverting it. >> >> Are you overt while you are coverting ?>:-} > >It's a hush-hush project. ;) > >> >>> >>> Since it's all on a single 5-V supply, I need a small JFET with super >>> low pinchoff voltage. The best one I can get for cheap seems to be the >>> 2SK3796-3, which is about 9 cents in reels from distributors. >>> >>> Unfortunately, there's no SPICE model that I can find. >>> >>> So since I'm not going to buy PSPICE just for this one part, what else >>> do you folks use for model creation? >>> >>> Thanks >>> >>> Phil Hobbs >> >> System modeling is easy, device models that are accurate require >> expensive parameter extraction test equipment. >> >> If you can find a model with the same parasitics, but wrong threshold, >> you can just twiddle the threshold. > >I've done that before, but it's kind of hit-or-miss. Ideally I'm >looking for something like the PSPICE model editor without PSPICE attached. > >Cheers > >Phil Hobbs
The PSpice Model Editor is junk. I've input data, generated a model, then tested the model and it doesn't return a match to the input data. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
>The PSpice Model Editor is junk. &nbsp;I've input data, generated a model, >then tested the model and it doesn't return a match to the input data.
Well, since you're PSpice's #1 fanboi, I guess that's pretty definitive. Cheers Phil Hobbs
>I just breadboard stuff like this. It's faster, more fun, and you'll >need to try it out in real life anyhow. Spice can run 12 orders of >magnitude slower than real life.
As long as you don't have to stare at it, it seems like only 11 orders. ;) The SPICEing was to see how good it ought to be. Once we get client approval, the next stage is a board with a bunch of test structures and a best guess at the final configuration. We ought to zero in on the final final configuration pretty easily if the test structures are well thought out. Fun. Cheers Phil Hobbs