Forums

CV measurement

Started by George Herold May 8, 2014
Hi all, I want to do some CV (capacitance vs voltage) measurements 
On some diodes and things. 
I drew up this,
https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG

Pretty much brute force.  
(I sometimes get the feeling that opamps are the only tool I know.)
I don't really like putting the bias and modulation through the same opamp.
Any other ideas?  
(I don't want to go the transformer route.)

Thanks
George H.
On 05/08/2014 12:44 PM, George Herold wrote:
> > Hi all, I want to do some CV (capacitance vs voltage) measurements > On some diodes and things. > I drew up this, > https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG > > Pretty much brute force. > (I sometimes get the feeling that opamps are the only tool I know.) > I don't really like putting the bias and modulation through the same opamp. > Any other ideas? > (I don't want to go the transformer route.) > > Thanks > George H. >
Get a Boonton 72 off eBay for ~$100. They have external DC bias inputs, and work really really well. 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 Thu, 08 May 2014 12:48:29 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 05/08/2014 12:44 PM, George Herold wrote: >> >> Hi all, I want to do some CV (capacitance vs voltage) measurements >> On some diodes and things. >> I drew up this, >> https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG >> >> Pretty much brute force. >> (I sometimes get the feeling that opamps are the only tool I know.) >> I don't really like putting the bias and modulation through the same opamp. >> Any other ideas? >> (I don't want to go the transformer route.) >> >> Thanks >> George H. >> > >Get a Boonton 72 off eBay for ~$100. They have external DC bias inputs, >and work really really well. > >Cheers > >Phil Hobbs
They have a demodulated analog output, so you could automate it with a LabJack or some such. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Thursday, May 8, 2014 12:48:29 PM UTC-4, Phil Hobbs wrote:
> On 05/08/2014 12:44 PM, George Herold wrote: > > > > > > Hi all, I want to do some CV (capacitance vs voltage) measurements > > > On some diodes and things. > > > I drew up this, > > > https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG > > > > > > Pretty much brute force. > > > (I sometimes get the feeling that opamps are the only tool I know.) > > > I don't really like putting the bias and modulation through the same opamp. > > > Any other ideas? > > > (I don't want to go the transformer route.) > > > > > > Thanks > > > George H. > > > > > > > Get a Boonton 72 off eBay for ~$100. They have external DC bias inputs, > and work really really well.
Hmm Maybe I should get one. But I was thinking of a circuit that might go into something we sell. I notice the Boonton has synchronous detection. http://exodus.poly.edu/~kurt/manuals/manuals/Other/BOONTON%2072B%20Instruction.pdf Is that how they take care of the phase angle? (maybe I can just implement that in my labview software, ducking :^) George H.
> > 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
"George Herold" <gherold@teachspin.com> wrote in message 
news:5a9d7273-c971-42ba-8b79-623471898a6a@googlegroups.com...
> > Hi all, I want to do some CV (capacitance vs voltage) measurements > On some diodes and things. > I drew up this, > https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG > > Pretty much brute force. > (I sometimes get the feeling that opamps are the only tool I know.) > I don't really like putting the bias and modulation through the same > opamp. > Any other ideas? > (I don't want to go the transformer route.) > > Thanks > George H.
Last time I did that, I used a CCS and measured dV/dt. The CCS was verified to have good compliance and stability, and the rig was tested for intrinsic capacitance (over the voltage range, it was pretty stable in the 60-70pF range). It was also verified that switching was complete by the time data collection began (base/gate voltage at zero, charge injection died out), so that the data is not affected by Miller effect of the switching device. Here's a calculated result (jig capacitance not subtracted out), overlaid with the datasheet graph, which I suspect contains a graphical error. http://seventransistorlabs.com/Images/STP19NM50N_Cdss_Overlay.jpg Curiously, the data best fit a two part piecewise exponential. The actual "best fit" function I used resembles JT's obsession with hyperbolic functions, though I believe I did this work before he started babbling about them here. Oh well, it's just functions, whatever works best. Raw data follows below as an example. This works nice for things with integrated switching capability (like MOSFETs..). A very low-C rig could be made, or bootstrapped in some way perhaps, for use with low capacitance transistors and diodes. Even then, the accuracy gets questionable down in the single digit pF. It's probably not so great around hysteresis (ceramic caps) either. In these cases, a more traditional "bias tee" approach would probably be better. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com STP19NM50N Current 0.883 mA Time (us) Volts (V) 0 0 1 0.5 2 0.5 3 0.4 4 0.3 5 0.2 6 0 7 0.2 8 0.3 9 0.5 10 0.6 11 0.8 12 1 13 1.3 14 1.5 15 1.8 16 2 17 2.3 18 2.5 19 2.9 20 3.2 21 3.4 22 3.7 23 4.1 24 4.6 25 4.9 26 5.3 27 5.7 28 6.1 29 6.6 30 6.9 31 7.4 32 7.8 33 8.3 34 8.8 35 9.3 36 9.8 37 10.3 38 10.8 39 11.4 40 11.9 45 14.9 50 18.6 55 23.9 60 31.8 65 44 70 69 75 101.2 80 0
On Thu, 8 May 2014 10:54:50 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Thursday, May 8, 2014 12:48:29 PM UTC-4, Phil Hobbs wrote: >> On 05/08/2014 12:44 PM, George Herold wrote: >> >> > >> >> > Hi all, I want to do some CV (capacitance vs voltage) measurements >> >> > On some diodes and things. >> >> > I drew up this, >> >> > https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG >> >> > >> >> > Pretty much brute force. >> >> > (I sometimes get the feeling that opamps are the only tool I know.) >> >> > I don't really like putting the bias and modulation through the same opamp. >> >> > Any other ideas? >> >> > (I don't want to go the transformer route.) >> >> > >> >> > Thanks >> >> > George H. >> >> > >> >> >> >> Get a Boonton 72 off eBay for ~$100. They have external DC bias inputs, >> and work really really well. > >Hmm Maybe I should get one.
Yes. They really rock. the low range is 1 pF full-scale. I like the analog-meter versions.
>But I was thinking of a circuit that might go into something we sell. > >I notice the Boonton has synchronous detection. > >http://exodus.poly.edu/~kurt/manuals/manuals/Other/BOONTON%2072B%20Instruction.pdf > >Is that how they take care of the phase angle? >(maybe I can just implement that in my labview software, >ducking :^)
Yes. They use a low-level sinewave drive (20 or 100 mV, I think, on different versions, semiconductor-friendly), tuned circuit amp, and synchronous detector. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Thursday, May 8, 2014 2:54:43 PM UTC-4, Tim Williams wrote:
> "George Herold" <gherold@teachspin.com> wrote in message > > news:5a9d7273-c971-42ba-8b79-623471898a6a@googlegroups.com... > > > > > > Hi all, I want to do some CV (capacitance vs voltage) measurements > > > On some diodes and things. > > > I drew up this, > > > https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG > > > > > > Pretty much brute force. > > > (I sometimes get the feeling that opamps are the only tool I know.) > > > I don't really like putting the bias and modulation through the same > > > opamp. > > > Any other ideas? > > > (I don't want to go the transformer route.) > > > > > > Thanks > > > George H. > > > > Last time I did that, I used a CCS and measured dV/dt. The CCS was > verified to have good compliance and stability, and the rig was tested for > intrinsic capacitance (over the voltage range, it was pretty stable in the > 60-70pF range).
So you did a square pulse or some such of current and then measured the final voltage? (after some known pulse time.) That's not going to work for me (I don't think.) There could be some series or parallel resistance. I think they even take data with the diode slightly forward biased.. like 100mV or so.
> It was also verified that switching was complete by the > time data collection began (base/gate voltage at zero, charge injection > died out), so that the data is not affected by Miller effect of the > switching device. > > > Here's a calculated result (jig capacitance not subtracted out), overlaid > with the datasheet graph, which I suspect contains a graphical error. > http://seventransistorlabs.com/Images/STP19NM50N_Cdss_Overlay.jpg
OK.. data looks a little noisy. (no offense intended.)
> > > Curiously, the data best fit a two part piecewise exponential. The actual > "best fit" function I used resembles JT's obsession with hyperbolic > functions, though I believe I did this work before he started babbling > about them here. Oh well, it's just functions, whatever works best. >
For the diode's it goes as 1/V**2 or something like that. I can pull out the carrier density if I know the area.
> > Raw data follows below as an example. > > > > This works nice for things with integrated switching capability (like > > MOSFETs..). A very low-C rig could be made, or bootstrapped in some way > > perhaps, for use with low capacitance transistors and diodes. > > > > Even then, the accuracy gets questionable down in the single digit pF. > It's probably not so great around hysteresis (ceramic caps) either. In > these cases, a more traditional "bias tee" approach would probably be > better. >
Thanks for the response. George H.
> > > Tim > > > > -- > > Seven Transistor Labs > > Electrical Engineering Consultation > > Website: http://seventransistorlabs.com > > > > > > STP19NM50N > > Current 0.883 mA > > Time (us) Volts (V) > > 0 0 > > 1 0.5 > > 2 0.5 > > 3 0.4 > > 4 0.3 > > 5 0.2 > > 6 0 > > 7 0.2 > > 8 0.3 > > 9 0.5 > > 10 0.6 > > 11 0.8 > > 12 1 > > 13 1.3 > > 14 1.5 > > 15 1.8 > > 16 2 > > 17 2.3 > > 18 2.5 > > 19 2.9 > > 20 3.2 > > 21 3.4 > > 22 3.7 > > 23 4.1 > > 24 4.6 > > 25 4.9 > > 26 5.3 > > 27 5.7 > > 28 6.1 > > 29 6.6 > > 30 6.9 > > 31 7.4 > > 32 7.8 > > 33 8.3 > > 34 8.8 > > 35 9.3 > > 36 9.8 > > 37 10.3 > > 38 10.8 > > 39 11.4 > > 40 11.9 > > 45 14.9 > > 50 18.6 > > 55 23.9 > > 60 31.8 > > 65 44 > > 70 69 > > 75 101.2 > > 80 0
On Thursday, May 8, 2014 3:01:04 PM UTC-4, John Larkin wrote:
> On Thu, 8 May 2014 10:54:50 -0700 (PDT), George Herold
> >Hmm Maybe I should get one. > > Yes. They really rock. the low range is 1 pF full-scale. > > I like the analog-meter versions.
Yup. I like meters too.
> > > > > > > >But I was thinking of a circuit that might go into something we sell. > > > > > >I notice the Boonton has synchronous detection. > > > > > >http://exodus.poly.edu/~kurt/manuals/manuals/Other/BOONTON%2072B%20Instruction.pdf > > > >Is that how they take care of the phase angle? > >(maybe I can just implement that in my labview software, > >ducking :^) > > Yes. They use a low-level sinewave drive (20 or 100 mV, I think, on > different versions, semiconductor-friendly), tuned circuit amp, and > synchronous detector.
Hmm, Yeah I'll have to keep the drive low, 100 mV might be a lot if I'm looking at a diode near zero bias. A little lock-in thing might be OK.. Are there any newer lock-in chips besides the AD630? Or does everyone do this in software these days? (For now I can use my digital scope as a lockin.) George H.
> > > > > > -- > > > > John Larkin Highland Technology, Inc > > > > jlarkin att highlandtechnology dott com > > http://www.highlandtechnology.com
On 05/08/2014 01:54 PM, George Herold wrote:
> On Thursday, May 8, 2014 12:48:29 PM UTC-4, Phil Hobbs wrote: >> On 05/08/2014 12:44 PM, George Herold wrote: >> >>> >> >>> Hi all, I want to do some CV (capacitance vs voltage) measurements >> >>> On some diodes and things. >> >>> I drew up this, >> >>> https://www.dropbox.com/s/tnn1mb4o41iqno9/DSCF0008.JPG >> >>> >> >>> Pretty much brute force. >> >>> (I sometimes get the feeling that opamps are the only tool I know.) >> >>> I don't really like putting the bias and modulation through the same opamp. >> >>> Any other ideas? >> >>> (I don't want to go the transformer route.) >> >>> >> >>> Thanks >> >>> George H. >> >>> >> >> >> >> Get a Boonton 72 off eBay for ~$100. They have external DC bias inputs, >> and work really really well. > > Hmm Maybe I should get one. > But I was thinking of a circuit that might go into something we sell. > > I notice the Boonton has synchronous detection. > > http://exodus.poly.edu/~kurt/manuals/manuals/Other/BOONTON%2072B%20Instruction.pdf > > Is that how they take care of the phase angle? > (maybe I can just implement that in my labview software, > ducking :^) >
Ducking won't help. Labview is like an explosion in a sewage treatment plant. There's no place to hide. ;) Synchronous detection helps a lot, because if you're just using amplitude, your resolution falls off quadratically as you go away from omega RC = 1. 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 05/08/2014 03:40 PM, George Herold wrote:
> On Thursday, May 8, 2014 3:01:04 PM UTC-4, John Larkin wrote: >> On Thu, 8 May 2014 10:54:50 -0700 (PDT), George Herold > >>> Hmm Maybe I should get one. >> >> Yes. They really rock. the low range is 1 pF full-scale. >> >> I like the analog-meter versions. > Yup. I like meters too. >> >> >> >> >> >> >>> But I was thinking of a circuit that might go into something we >>> sell. >> >>> >> >>> I notice the Boonton has synchronous detection. >> >>> >> >>> http://exodus.poly.edu/~kurt/manuals/manuals/Other/BOONTON%2072B%20Instruction.pdf >>> >>> >>>
Is that how they take care of the phase angle?
>>> (maybe I can just implement that in my labview software, ducking >>> :^) >> >> Yes. They use a low-level sinewave drive (20 or 100 mV, I think, >> on different versions, semiconductor-friendly), tuned circuit amp, >> and synchronous detector. > > Hmm, Yeah I'll have to keep the drive low, 100 mV might be a lot if > I'm looking at a diode near zero bias. A little lock-in thing might > be OK.. Are there any newer lock-in chips besides the AD630? Or does > everyone do this in software these days? (For now I can use my > digital scope as a lockin.)
Muxes have gotten so good that there isn't that much call for dedicated lock-ins or (slow) sample/hold chips any more, at least if you can live with a single 5V supply. My little boxcar lock-in board (that Beautiful Layout Hunchback just generated Gerbers for) uses DG2042s for both. They look amazing in the datasheet, and hopefully are equally so in real life. We'll see in a week's time. 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