On Thu, 15 Dec 2011 16:48:25 -0800, Joerg <invalid@invalid.invalid> wrote:>John Larkin wrote: >> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <invalid@invalid.invalid> >> wrote: >> >>> John Larkin wrote: >>> >>> [SPICE netlist] >>> >>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >>>> resistor value is way below the 1K that ADI suggests for a follower. >>>> >>>> I had to use .lib instead of .include to make LT Spice happy. The >>>> default pin order was ok. >>>> >>>> If the opamp model is accurate (namely, it doesn't oscillate with the >>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >>>> L1=56n) was terrible. I'll try it in real life next. >>>> >>> Doesn't look bad at all. For snappier corners you have to pick an amp >>> with a lot more bandwidth. Like this little dude: >>> >>> http://www.ti.com/lit/ds/symlink/ths4303.pdf >> >> I need unity gain here. We have used the 4303 before, and it's very >> nice, probably because the feedback resistors are internal. >> > >Why unity gain? Can't you divide down the ramp? Or use 1/5th of the >charge current?Well, it's partly that I need unity gain for the bootstrap current source. And partly that the board is laid out. ftp://jjlarkin.lmi.net/TEM2_CTRL.jpg John
LT Spice question
Started by ●December 15, 2011
Reply by ●December 15, 20112011-12-15
Reply by ●December 15, 20112011-12-15
On Dec 15, 8:23=A0pm, Joerg <inva...@invalid.invalid> wrote:> Jim Thompson wrote: > > On Thu, 15 Dec 2011 19:19:21 -0500, Jamie > > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: > > >> John Larkin wrote: > > >>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <inva...@invalid.invalid> > >>> wrote: > > >>>> John Larkin wrote: > > >>>> [SPICE netlist] > > >>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts =in> >>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback > >>>>> resistor value is way below the 1K that ADI suggests for a follower=.> > >>>>> I had to use .lib instead of .include to make LT Spice happy. The > >>>>> default pin order was ok. > > >>>>> If the opamp model is accurate (namely, it doesn't oscillate with t=he> >>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=3D1=K,> >>>>> L1=3D56n) was terrible. I'll try it in real life next. > > >>>> Doesn't look bad at all. For snappier corners you have to pick an am=p> >>>> with a lot more bandwidth. Like this little dude: > > >>>>http://www.ti.com/lit/ds/symlink/ths4303.pdf > > >>> I need unity gain here. We have used the 4303 before, and it's very > >>> nice, probably because the feedback resistors are internal. > > >>>> However, the AD8014 is a CFB and they really do not like this > >>>> configuration with just Rf and a cap from IN- to ground. Might put t=hem> >>>> close to oscillation even if SPICE says they are ok. > > >>> That cap just sims PCB parasitics. Things don't change much from 0 to > >>> 2 pF, and I doubt I even have 1 pF there. > > >>> The bad news is that there's some ringing at the early part of the > >>> ramp in real life, > > >>>ftp://jjlarkin.lmi.net/AD8014_ramp.JPG > > >>> not on the sim, which I ascribe to a rotten Spice model of the opamp. > >>> I can set Rf to zero ohms, and it still simulates nicely, which > >>> doesn't sound right to me. These current-mode amps usually go > >>> bezerkers with zero ohms of Rf. > > >>> I guess I'll defy the sim and change Rf some, or try another amp, an > >>> AD8009 maybe. > > >>> John > > >> If I didn't know any better, that looks like it maybe a standing wave > >> you're seeing. Have you tried a R load on the output instead of a Cap? > > >> Jamie > > > Good point, the device probably needs some DC load. =A0And it seems > > rather squirrely at G=3D1, but nice at G=3D2. > > CFBs are usually ok at G=3D-1OK maybe just invert it then? Or is there a power supply issue. but at G=3D+1 they can be like a Lamborghini> on black ice. Lots of gain peaking at the high end, can ring like heck. > Although (assuming the scope is on 20nsec/div) the ringing is almost a > factor of three too low in frequency for this part. That's a bit > strange, something else must be going on.Yes or throw away some signal and add some gain. (as JT suggested) A little resistance here and there can be a good thing. George H.> > -- > Regards, Joerg > > http://www.analogconsultants.com/- Hide quoted text - > > - Show quoted text -
Reply by ●December 16, 20112011-12-16
John Larkin wrote:>>> Speaking of which, we have more ideas and stuff to do than we have >>> time and energy. It would be great to have someone who could do Spice >>> setups and simulations and parts research and maybe a little >>> breadboarding for us occasionally, for pay of course.I can do all that stuff and more. Trouble is, I have a regular job in Livermore. Until the government goes broke of course, which will probably come in 1-3 years. If we get shut down I'll stop by your shop in SF and show you my CV. Or perhaps if they ask me to inspect 2000 pieces of equipment again.>> I don't know anyone in the S.F. area. But I do know someone who is >> experienced in fast FPGA stuff if that ever comes up, about a mile from >> you guys. > > Connect us up! We're constantly overloaded on FPGA design. There's a > beer in it for you. > > We are working with a really good guy in San Diego, but he's in San > Diego. > > http://www.amazon.com/Blaine-C.-Readler/e/B001K7PSRM/ref=sr_ntt_srch_lnk_7?qid=1323968214&sr=1-7 > > Check out his books.That's an interesting author. Oh joy, another "Verilog by Example" book. Maybe I'll buy it on Monday. Have you seen it already and can offer a "mini-review?" I have these two already: http://www.amazon.com/FPGA-Prototyping-Verilog-Examples-Spartan-3/dp/0470185325/ref=sr_1_1?ie=UTF8&qid=1317921000&sr=8-1 http://www.lbebooks.com/booksandkits-HWDesignVerilog.htm#verilogbasys http://www.lbebooks.com/booksandkits-HWDesignVerilog.htm#verilognexys2 I forget which one tells the reader to divide down the clock using a counter, then to clock other experiments with the logic derived clocks. Needless to say, this is not a popular technique in the real world. I tend to have to get my work done before I have time to finish reading my books, so I haven't had a chance to study them in depth. One wonders why I have them at all? Well, I still have a lot to learn in order to tackle some upcoming projects, so a few weeks of serious book study is in the cards once I get my F2812 and Spartan 3E PCBs done. -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
Reply by ●December 16, 20112011-12-16
On Thu, 15 Dec 2011 18:47:39 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Dec 15, 8:23�pm, Joerg <inva...@invalid.invalid> wrote: >> Jim Thompson wrote: >> > On Thu, 15 Dec 2011 19:19:21 -0500, Jamie >> > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: >> >> >> John Larkin wrote: >> >> >>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <inva...@invalid.invalid> >> >>> wrote: >> >> >>>> John Larkin wrote: >> >> >>>> [SPICE netlist] >> >> >>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >> >>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >> >>>>> resistor value is way below the 1K that ADI suggests for a follower. >> >> >>>>> I had to use .lib instead of .include to make LT Spice happy. The >> >>>>> default pin order was ok. >> >> >>>>> If the opamp model is accurate (namely, it doesn't oscillate with the >> >>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >> >>>>> L1=56n) was terrible. I'll try it in real life next. >> >> >>>> Doesn't look bad at all. For snappier corners you have to pick an amp >> >>>> with a lot more bandwidth. Like this little dude: >> >> >>>>http://www.ti.com/lit/ds/symlink/ths4303.pdf >> >> >>> I need unity gain here. We have used the 4303 before, and it's very >> >>> nice, probably because the feedback resistors are internal. >> >> >>>> However, the AD8014 is a CFB and they really do not like this >> >>>> configuration with just Rf and a cap from IN- to ground. Might put them >> >>>> close to oscillation even if SPICE says they are ok. >> >> >>> That cap just sims PCB parasitics. Things don't change much from 0 to >> >>> 2 pF, and I doubt I even have 1 pF there. >> >> >>> The bad news is that there's some ringing at the early part of the >> >>> ramp in real life, >> >> >>>ftp://jjlarkin.lmi.net/AD8014_ramp.JPG >> >> >>> not on the sim, which I ascribe to a rotten Spice model of the opamp. >> >>> I can set Rf to zero ohms, and it still simulates nicely, which >> >>> doesn't sound right to me. These current-mode amps usually go >> >>> bezerkers with zero ohms of Rf. >> >> >>> I guess I'll defy the sim and change Rf some, or try another amp, an >> >>> AD8009 maybe. >> >> >>> John >> >> >> If I didn't know any better, that looks like it maybe a standing wave >> >> you're seeing. Have you tried a R load on the output instead of a Cap? >> >> >> Jamie >> >> > Good point, the device probably needs some DC load. �And it seems >> > rather squirrely at G=1, but nice at G=2. >> >> CFBs are usually ok at G=-1 > >OK maybe just invert it then? Or is there a power supply issue.Larkin has been trying (since April) to make a boot-strapped ramp, so he needs +1> > > but at G=+1 they can be like a Lamborghini >> on black ice. Lots of gain peaking at the high end, can ring like heck. >> Although (assuming the scope is on 20nsec/div) the ringing is almost a >> factor of three too low in frequency for this part. That's a bit >> strange, something else must be going on. > >Yes or throw away some signal and add some gain. (as JT suggested) >A little resistance here and there can be a good thing. > >George H. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/- Hide quoted text - >> >> - Show quoted text -...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●December 16, 20112011-12-16
John Larkin wrote:> On Thu, 15 Dec 2011 16:48:25 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> John Larkin wrote: >>>> >>>> [SPICE netlist] >>>> >>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >>>>> resistor value is way below the 1K that ADI suggests for a follower. >>>>> >>>>> I had to use .lib instead of .include to make LT Spice happy. The >>>>> default pin order was ok. >>>>> >>>>> If the opamp model is accurate (namely, it doesn't oscillate with the >>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >>>>> L1=56n) was terrible. I'll try it in real life next. >>>>> >>>> Doesn't look bad at all. For snappier corners you have to pick an amp >>>> with a lot more bandwidth. Like this little dude: >>>> >>>> http://www.ti.com/lit/ds/symlink/ths4303.pdf >>> I need unity gain here. We have used the 4303 before, and it's very >>> nice, probably because the feedback resistors are internal. >>> >> Why unity gain? Can't you divide down the ramp? Or use 1/5th of the >> charge current? > > Well, it's partly that I need unity gain for the bootstrap current > source. And partly that the board is laid out. >The brute force would be to use two unity gain inverting amps. If the non-linearity is the big concern maybe you could try a "gooser", 100ohms in series with 2.2pF from IN- to GND. You'd have to check phase margins. But that 100MHz ringing you have there is suspicious, that could hardly come out of the amp.> ftp://jjlarkin.lmi.net/TEM2_CTRL.jpg >That is a cool color. I have never seen such a deep blue offered for FR4 material. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●December 16, 20112011-12-16
On Dec 16, 10:20=A0am, Jim Thompson <To-Email-Use-The-Envelope-I...@On- My-Web-Site.com> wrote:> On Thu, 15 Dec 2011 18:47:39 -0800 (PST), George Herold > > > > > > <gher...@teachspin.com> wrote: > >On Dec 15, 8:23=A0pm, Joerg <inva...@invalid.invalid> wrote: > >> Jim Thompson wrote: > >> > On Thu, 15 Dec 2011 19:19:21 -0500, Jamie > >> > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: > > >> >> John Larkin wrote: > > >> >>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <inva...@invalid.invalid= > > >> >>> wrote: > > >> >>>> John Larkin wrote: > > >> >>>> [SPICE netlist] > > >> >>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 vol=ts in> >> >>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback > >> >>>>> resistor value is way below the 1K that ADI suggests for a follo=wer.> > >> >>>>> I had to use .lib instead of .include to make LT Spice happy. Th=e> >> >>>>> default pin order was ok. > > >> >>>>> If the opamp model is accurate (namely, it doesn't oscillate wit=h the> >> >>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2==3D1K,> >> >>>>> L1=3D56n) was terrible. I'll try it in real life next. > > >> >>>> Doesn't look bad at all. For snappier corners you have to pick an=amp> >> >>>> with a lot more bandwidth. Like this little dude: > > >> >>>>http://www.ti.com/lit/ds/symlink/ths4303.pdf > > >> >>> I need unity gain here. We have used the 4303 before, and it's ver=y> >> >>> nice, probably because the feedback resistors are internal. > > >> >>>> However, the AD8014 is a CFB and they really do not like this > >> >>>> configuration with just Rf and a cap from IN- to ground. Might pu=t them> >> >>>> close to oscillation even if SPICE says they are ok. > > >> >>> That cap just sims PCB parasitics. Things don't change much from 0=to> >> >>> 2 pF, and I doubt I even have 1 pF there. > > >> >>> The bad news is that there's some ringing at the early part of the > >> >>> ramp in real life, > > >> >>>ftp://jjlarkin.lmi.net/AD8014_ramp.JPG > > >> >>> not on the sim, which I ascribe to a rotten Spice model of the opa=mp.> >> >>> I can set Rf to zero ohms, and it still simulates nicely, which > >> >>> doesn't sound right to me. These current-mode amps usually go > >> >>> bezerkers with zero ohms of Rf. > > >> >>> I guess I'll defy the sim and change Rf some, or try another amp, =an> >> >>> AD8009 maybe. > > >> >>> John > > >> >> If I didn't know any better, that looks like it maybe a standing wa=ve> >> >> you're seeing. Have you tried a R load on the output instead of a C=ap?> > >> >> Jamie > > >> > Good point, the device probably needs some DC load. =A0And it seems > >> > rather squirrely at G=3D1, but nice at G=3D2. > > >> CFBs are usually ok at G=3D-1 > > >OK maybe just invert it then? =A0Or is there a power supply issue. > > Larkin has been trying (since April) to make a boot-strapped ramp, so > he needs +1 > > > > > > > > > but at G=3D+1 they can be like a Lamborghini > >> on black ice. Lots of gain peaking at the high end, can ring like heck=.> >> Although (assuming the scope is on 20nsec/div) the ringing is almost a > >> factor of three too low in frequency for this part. That's a bit > >> strange, something else must be going on. > > >Yes or throw away some signal and add some gain. =A0(as JT suggested) > >A little resistance here and there can be a good thing. > > >George H. > > >> -- > >> Regards, Joerg > > >>http://www.analogconsultants.com/-Hide quoted text - > > >> - Show quoted text - > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 ...Jim Thompson> -- > | James E.Thompson, CTO =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0| =A0 =A0mens =A0 =A0 |> | Analog Innovations, Inc. =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 | =A0 =A0 et =A0 =A0 =A0|> | Analog/Mixed-Signal ASIC's and Discrete Systems =A0| =A0 =A0manus =A0 ==A0|> | Phoenix, Arizona =A085048 =A0 =A0Skype: Contacts Only =A0| =A0 =A0 =A0 ==A0 =A0 =A0 |> | Voice:(480)460-2350 =A0Fax: Available upon request | =A0Brass Rat =A0| > | E-mail Icon athttp://www.analog-innovations.com| =A0 =A01962 =A0 =A0 | > > I love to cook with wine. =A0 =A0 Sometimes I even put it in the food.- H=ide quoted text -> > - Show quoted text -- Hide quoted text - > > - Show quoted text -Hmm OK, I've barely done any bootstrapping. But can't you put the same amount of gain in the bootstrap circiut too? George H.
Reply by ●December 16, 20112011-12-16
On Fri, 16 Dec 2011 08:04:14 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Dec 16, 10:20�am, Jim Thompson <To-Email-Use-The-Envelope-I...@On- >My-Web-Site.com> wrote: >> On Thu, 15 Dec 2011 18:47:39 -0800 (PST), George Herold >> >> >> >> >> >> <gher...@teachspin.com> wrote: >> >On Dec 15, 8:23�pm, Joerg <inva...@invalid.invalid> wrote: >> >> Jim Thompson wrote: >> >> > On Thu, 15 Dec 2011 19:19:21 -0500, Jamie >> >> > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: >> >> >> >> John Larkin wrote: >> >> >> >>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <inva...@invalid.invalid> >> >> >>> wrote: >> >> >> >>>> John Larkin wrote: >> >> >> >>>> [SPICE netlist] >> >> >> >>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >> >> >>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >> >> >>>>> resistor value is way below the 1K that ADI suggests for a follower. >> >> >> >>>>> I had to use .lib instead of .include to make LT Spice happy. The >> >> >>>>> default pin order was ok. >> >> >> >>>>> If the opamp model is accurate (namely, it doesn't oscillate with the >> >> >>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >> >> >>>>> L1=56n) was terrible. I'll try it in real life next. >> >> >> >>>> Doesn't look bad at all. For snappier corners you have to pick an amp >> >> >>>> with a lot more bandwidth. Like this little dude: >> >> >> >>>>http://www.ti.com/lit/ds/symlink/ths4303.pdf >> >> >> >>> I need unity gain here. We have used the 4303 before, and it's very >> >> >>> nice, probably because the feedback resistors are internal. >> >> >> >>>> However, the AD8014 is a CFB and they really do not like this >> >> >>>> configuration with just Rf and a cap from IN- to ground. Might put them >> >> >>>> close to oscillation even if SPICE says they are ok. >> >> >> >>> That cap just sims PCB parasitics. Things don't change much from 0 to >> >> >>> 2 pF, and I doubt I even have 1 pF there. >> >> >> >>> The bad news is that there's some ringing at the early part of the >> >> >>> ramp in real life, >> >> >> >>>ftp://jjlarkin.lmi.net/AD8014_ramp.JPG >> >> >> >>> not on the sim, which I ascribe to a rotten Spice model of the opamp. >> >> >>> I can set Rf to zero ohms, and it still simulates nicely, which >> >> >>> doesn't sound right to me. These current-mode amps usually go >> >> >>> bezerkers with zero ohms of Rf. >> >> >> >>> I guess I'll defy the sim and change Rf some, or try another amp, an >> >> >>> AD8009 maybe. >> >> >> >>> John >> >> >> >> If I didn't know any better, that looks like it maybe a standing wave >> >> >> you're seeing. Have you tried a R load on the output instead of a Cap? >> >> >> >> Jamie >> >> >> > Good point, the device probably needs some DC load. �And it seems >> >> > rather squirrely at G=1, but nice at G=2. >> >> >> CFBs are usually ok at G=-1 >> >> >OK maybe just invert it then? �Or is there a power supply issue. >> >> Larkin has been trying (since April) to make a boot-strapped ramp, so >> he needs +1 >> >> >> >> >> >> >> >> > but at G=+1 they can be like a Lamborghini >> >> on black ice. Lots of gain peaking at the high end, can ring like heck. >> >> Although (assuming the scope is on 20nsec/div) the ringing is almost a >> >> factor of three too low in frequency for this part. That's a bit >> >> strange, something else must be going on. >> >> >Yes or throw away some signal and add some gain. �(as JT suggested) >> >A little resistance here and there can be a good thing. >> >> >George H. >> >> >> -- >> >> Regards, Joerg >> >> >>http://www.analogconsultants.com/-Hide quoted text - >> >> >> - Show quoted text - >> >> � � � � � � � � � � � � � � � � � � � � ...Jim Thompson >> -- >> | James E.Thompson, CTO � � � � � � � � � � � � � �| � �mens � � | >> | Analog Innovations, Inc. � � � � � � � � � � � � | � � et � � �| >> | Analog/Mixed-Signal ASIC's and Discrete Systems �| � �manus � �| >> | Phoenix, Arizona �85048 � �Skype: Contacts Only �| � � � � � � | >> | Voice:(480)460-2350 �Fax: Available upon request | �Brass Rat �| >> | E-mail Icon athttp://www.analog-innovations.com| � �1962 � � | >> >> I love to cook with wine. � � Sometimes I even put it in the food.- Hide quoted text - >> >> - Show quoted text -- Hide quoted text - >> >> - Show quoted text - > >Hmm OK, I've barely done any bootstrapping. But can't you put the same >amount of gain in the bootstrap circiut too? > >George H.Last April I suggested a way to insert a current to produce the level shift, but I was clucked :-( ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●December 16, 20112011-12-16
On Thu, 15 Dec 2011 18:47:39 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Dec 15, 8:23�pm, Joerg <inva...@invalid.invalid> wrote: >> Jim Thompson wrote: >> > On Thu, 15 Dec 2011 19:19:21 -0500, Jamie >> > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: >> >> >> John Larkin wrote: >> >> >>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <inva...@invalid.invalid> >> >>> wrote: >> >> >>>> John Larkin wrote: >> >> >>>> [SPICE netlist] >> >> >>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >> >>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >> >>>>> resistor value is way below the 1K that ADI suggests for a follower. >> >> >>>>> I had to use .lib instead of .include to make LT Spice happy. The >> >>>>> default pin order was ok. >> >> >>>>> If the opamp model is accurate (namely, it doesn't oscillate with the >> >>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >> >>>>> L1=56n) was terrible. I'll try it in real life next. >> >> >>>> Doesn't look bad at all. For snappier corners you have to pick an amp >> >>>> with a lot more bandwidth. Like this little dude: >> >> >>>>http://www.ti.com/lit/ds/symlink/ths4303.pdf >> >> >>> I need unity gain here. We have used the 4303 before, and it's very >> >>> nice, probably because the feedback resistors are internal. >> >> >>>> However, the AD8014 is a CFB and they really do not like this >> >>>> configuration with just Rf and a cap from IN- to ground. Might put them >> >>>> close to oscillation even if SPICE says they are ok. >> >> >>> That cap just sims PCB parasitics. Things don't change much from 0 to >> >>> 2 pF, and I doubt I even have 1 pF there. >> >> >>> The bad news is that there's some ringing at the early part of the >> >>> ramp in real life, >> >> >>>ftp://jjlarkin.lmi.net/AD8014_ramp.JPG >> >> >>> not on the sim, which I ascribe to a rotten Spice model of the opamp. >> >>> I can set Rf to zero ohms, and it still simulates nicely, which >> >>> doesn't sound right to me. These current-mode amps usually go >> >>> bezerkers with zero ohms of Rf. >> >> >>> I guess I'll defy the sim and change Rf some, or try another amp, an >> >>> AD8009 maybe. >> >> >>> John >> >> >> If I didn't know any better, that looks like it maybe a standing wave >> >> you're seeing. Have you tried a R load on the output instead of a Cap? >> >> >> Jamie >> >> > Good point, the device probably needs some DC load. �And it seems >> > rather squirrely at G=1, but nice at G=2. >> >> CFBs are usually ok at G=-1 > >OK maybe just invert it then? Or is there a power supply issue.Here's the circuit: ftp://jjlarkin.lmi.net/Ramp.JPG Inversion won't work - I need a high impedance load on the ramp capacitor - and a gain of 1 makes the bootstrap current source simple and accurate. It works, but I'm getting linearity errors equivalent to about 180 ps out of a 16 ns ramp, around 1%. That's not bad for a 16 nanosecond ramp, but the customer (unreasonably) wants better. Don't know why, there's only a few billion dollars per year affected by this error. I've done 0.1% for a 40 ns ramp before, using a real current source, so this shouldn't be all that difficult. The AD8014, while cheap and low power, wasn't the best choice, and it sure looks like the Spice model isn't very accurate. The ramp at the capacitor, seen with a fet probe, looks perfect. Sigh, I suppose I'll have to get in there and start soldering. I am waiting for some software to get done that gives me better analytics on the linearity. John
Reply by ●December 16, 20112011-12-16
On Fri, 16 Dec 2011 07:59:10 -0800, Joerg <invalid@invalid.invalid> wrote:>John Larkin wrote: >> On Thu, 15 Dec 2011 16:48:25 -0800, Joerg <invalid@invalid.invalid> >> wrote: >> >>> John Larkin wrote: >>>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <invalid@invalid.invalid> >>>> wrote: >>>> >>>>> John Larkin wrote: >>>>> >>>>> [SPICE netlist] >>>>> >>>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >>>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >>>>>> resistor value is way below the 1K that ADI suggests for a follower. >>>>>> >>>>>> I had to use .lib instead of .include to make LT Spice happy. The >>>>>> default pin order was ok. >>>>>> >>>>>> If the opamp model is accurate (namely, it doesn't oscillate with the >>>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >>>>>> L1=56n) was terrible. I'll try it in real life next. >>>>>> >>>>> Doesn't look bad at all. For snappier corners you have to pick an amp >>>>> with a lot more bandwidth. Like this little dude: >>>>> >>>>> http://www.ti.com/lit/ds/symlink/ths4303.pdf >>>> I need unity gain here. We have used the 4303 before, and it's very >>>> nice, probably because the feedback resistors are internal. >>>> >>> Why unity gain? Can't you divide down the ramp? Or use 1/5th of the >>> charge current? >> >> Well, it's partly that I need unity gain for the bootstrap current >> source. And partly that the board is laid out. >> > >The brute force would be to use two unity gain inverting amps. If the >non-linearity is the big concern maybe you could try a "gooser", 100ohms >in series with 2.2pF from IN- to GND. You'd have to check phase margins. >But that 100MHz ringing you have there is suspicious, that could hardly >come out of the amp. > > >> ftp://jjlarkin.lmi.net/TEM2_CTRL.jpg >> > >That is a cool color. I have never seen such a deep blue offered for FR4 >material.Our boards are yellow silk on blue solder mask. That photo was taken by skylight, north side of the building, so it looks very blue. This board is a hodge-podge of functions, and the fast ramps are just a small piece of the process. We're just now getting the FPGA coded, the ARM coded, and we're developing PC programs to talk to it and manage it and do analytics. The box this goes in has 68 connectors, 19 of them fiber optics. Beast. John
Reply by ●December 16, 20112011-12-16
John Larkin wrote:> On Thu, 15 Dec 2011 18:47:39 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > >> On Dec 15, 8:23 pm, Joerg <inva...@invalid.invalid> wrote: >>> Jim Thompson wrote: >>>> On Thu, 15 Dec 2011 19:19:21 -0500, Jamie >>>> <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: >>>>> John Larkin wrote: >>>>>> On Thu, 15 Dec 2011 13:33:06 -0800, Joerg <inva...@invalid.invalid> >>>>>> wrote: >>>>>>> John Larkin wrote: >>>>>>> [SPICE netlist] >>>>>>>> I'm trying to get the most linear ramp at OUT, from +1 to +3 volts in >>>>>>>> 16 ns. AD8014 was probably a bad choice, and the best feedback >>>>>>>> resistor value is way below the 1K that ADI suggests for a follower. >>>>>>>> I had to use .lib instead of .include to make LT Spice happy. The >>>>>>>> default pin order was ok. >>>>>>>> If the opamp model is accurate (namely, it doesn't oscillate with the >>>>>>>> 249 ohm resistor) it looks pretty good. My original circuit (R2=1K, >>>>>>>> L1=56n) was terrible. I'll try it in real life next. >>>>>>> Doesn't look bad at all. For snappier corners you have to pick an amp >>>>>>> with a lot more bandwidth. Like this little dude: >>>>>>> http://www.ti.com/lit/ds/symlink/ths4303.pdf >>>>>> I need unity gain here. We have used the 4303 before, and it's very >>>>>> nice, probably because the feedback resistors are internal. >>>>>>> However, the AD8014 is a CFB and they really do not like this >>>>>>> configuration with just Rf and a cap from IN- to ground. Might put them >>>>>>> close to oscillation even if SPICE says they are ok. >>>>>> That cap just sims PCB parasitics. Things don't change much from 0 to >>>>>> 2 pF, and I doubt I even have 1 pF there. >>>>>> The bad news is that there's some ringing at the early part of the >>>>>> ramp in real life, >>>>>> ftp://jjlarkin.lmi.net/AD8014_ramp.JPG >>>>>> not on the sim, which I ascribe to a rotten Spice model of the opamp. >>>>>> I can set Rf to zero ohms, and it still simulates nicely, which >>>>>> doesn't sound right to me. These current-mode amps usually go >>>>>> bezerkers with zero ohms of Rf. >>>>>> I guess I'll defy the sim and change Rf some, or try another amp, an >>>>>> AD8009 maybe. >>>>>> John >>>>> If I didn't know any better, that looks like it maybe a standing wave >>>>> you're seeing. Have you tried a R load on the output instead of a Cap? >>>>> Jamie >>>> Good point, the device probably needs some DC load. And it seems >>>> rather squirrely at G=1, but nice at G=2. >>> CFBs are usually ok at G=-1 >> OK maybe just invert it then? Or is there a power supply issue. > > > Here's the circuit: > > ftp://jjlarkin.lmi.net/Ramp.JPG > > Inversion won't work - I need a high impedance load on the ramp > capacitor - and a gain of 1 makes the bootstrap current source simple > and accurate. >IF a DC shift is ok or can be compensated for, how about using a fast follower? Possibly a little FET. That guarantees a high impedance. [...] -- Regards, Joerg http://www.analogconsultants.com/
