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LT Spice question

Started by John Larkin December 15, 2011
On 16 Dec., 18:26, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 16 Dec 2011 07:59:10 -0800, Joerg <inva...@invalid.invalid> > wrote: > > > > > > > > > > >John Larkin wrote: > >> On Thu, 15 Dec 2011 16:48:25 -0800, Joerg <inva...@invalid.invalid> > >> 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. > > >>> 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
is it just the way the picture is taken that makes it look like there's no flooding on the top copper layer? or is there a reason for it -Lasse
On Fri, 16 Dec 2011 09:37:20 -0800, Joerg <invalid@invalid.invalid>
wrote:

>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. > >[...]
It still has to drive the bootstrap, which would load a fet and cause new linearity errors. So I'd have to go to an active current source. Too much like work. The ramp at the cap looks great, so the opamp loading isn't a problem. It's making the opamp follow accurately that's the issue. John
On Fri, 16 Dec 2011 09:52:37 -0800 (PST), "langwadt@fonz.dk"
<langwadt@fonz.dk> wrote:

>On 16 Dec., 18:26, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Fri, 16 Dec 2011 07:59:10 -0800, Joerg <inva...@invalid.invalid> >> wrote: >> >> >> >> >> >> >> >> >> >> >John Larkin wrote: >> >> On Thu, 15 Dec 2011 16:48:25 -0800, Joerg <inva...@invalid.invalid> >> >> 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. >> >> >>> 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 > > >is it just the way the picture is taken that makes it look like >there's >no flooding on the top copper layer? >or is there a reason for it > >-Lasse
No layer 1 flooding. It's an 8-layer board, with a power plane on 2, ground on 4. John
John Larkin wrote:
> On Fri, 16 Dec 2011 09:37:20 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> 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. >> >> [...] > > It still has to drive the bootstrap, which would load a fet and cause > new linearity errors. ...
I meant still bootstrap it from the end.
> ... So I'd have to go to an active current source. > Too much like work. >
Hmm, I thought this is exactly the kind of work you like, all analog :-)
> The ramp at the cap looks great, so the opamp loading isn't a problem. > It's making the opamp follow accurately that's the issue. >
I know, that's what your sim shows. This is why I think it is important to get the most clean high-BW amp solution possible. And I just don't think that's easily possible with a CFB running at G=+1. They are like a car with worn shocks. But I think first the root cause for the weird 100MHz ring-a-ding-ding has to be found. Can't imagine an AD8014 doing that on its own. -- Regards, Joerg http://www.analogconsultants.com/
On Fri, 16 Dec 2011 12:13:20 -0800, Joerg <invalid@invalid.invalid>
wrote:

>John Larkin wrote: >> On Fri, 16 Dec 2011 09:37:20 -0800, Joerg <invalid@invalid.invalid> >> wrote: >> >>> 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. >>> >>> [...] >> >> It still has to drive the bootstrap, which would load a fet and cause >> new linearity errors. ... > > >I meant still bootstrap it from the end. > > >> ... So I'd have to go to an active current source. >> Too much like work. >> > >Hmm, I thought this is exactly the kind of work you like, all analog :-) > > >> The ramp at the cap looks great, so the opamp loading isn't a problem. >> It's making the opamp follow accurately that's the issue. >> > >I know, that's what your sim shows. This is why I think it is important >to get the most clean high-BW amp solution possible. And I just don't >think that's easily possible with a CFB running at G=+1. They are like a >car with worn shocks. > >But I think first the root cause for the weird 100MHz ring-a-ding-ding >has to be found. Can't imagine an AD8014 doing that on its own.
With 1K, the recommended datasheet value, it doesn't ring, but it has a high slope from zero to about +1.3 volts or so, then settles down to follow the cap. The initial slope is about twice the correct value. I tried 470 ohms, and it's a compromise: a little ringing, a little pre-slope. The equivalent ramp error is about 75 ps RMS, so I may live with that. Still ugly, though. John
John Larkin wrote:
> On Fri, 16 Dec 2011 12:13:20 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Fri, 16 Dec 2011 09:37:20 -0800, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> 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. >>>> >>>> [...] >>> It still has to drive the bootstrap, which would load a fet and cause >>> new linearity errors. ... >> >> I meant still bootstrap it from the end. >> >> >>> ... So I'd have to go to an active current source. >>> Too much like work. >>> >> Hmm, I thought this is exactly the kind of work you like, all analog :-) >> >> >>> The ramp at the cap looks great, so the opamp loading isn't a problem. >>> It's making the opamp follow accurately that's the issue. >>> >> I know, that's what your sim shows. This is why I think it is important >> to get the most clean high-BW amp solution possible. And I just don't >> think that's easily possible with a CFB running at G=+1. They are like a >> car with worn shocks. >> >> But I think first the root cause for the weird 100MHz ring-a-ding-ding >> has to be found. Can't imagine an AD8014 doing that on its own. > > > With 1K, the recommended datasheet value, it doesn't ring, but it has > a high slope from zero to about +1.3 volts or so, then settles down to > follow the cap. The initial slope is about twice the correct value. >
In your sim file it stops doing the overshoot when the gain is above 1.3 but who knows, SPICE models only go so far.
> I tried 470 ohms, and it's a compromise: a little ringing, a little > pre-slope. The equivalent ramp error is about 75 ps RMS, so I may live > with that. Still ugly, though. >
Looks like you need a faster amp. How about this one? http://www.ti.com/lit/gpn/ths3202 Assuming cost is not of prime concern here. -- Regards, Joerg http://www.analogconsultants.com/
On Fri, 16 Dec 2011 14:10:39 -0800, Joerg <invalid@invalid.invalid>
wrote:

>John Larkin wrote: >> On Fri, 16 Dec 2011 12:13:20 -0800, Joerg <invalid@invalid.invalid> >> wrote: >> >>> John Larkin wrote: >>>> On Fri, 16 Dec 2011 09:37:20 -0800, Joerg <invalid@invalid.invalid> >>>> wrote: >>>> >>>>> 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. >>>>> >>>>> [...] >>>> It still has to drive the bootstrap, which would load a fet and cause >>>> new linearity errors. ... >>> >>> I meant still bootstrap it from the end. >>> >>> >>>> ... So I'd have to go to an active current source. >>>> Too much like work. >>>> >>> Hmm, I thought this is exactly the kind of work you like, all analog :-) >>> >>> >>>> The ramp at the cap looks great, so the opamp loading isn't a problem. >>>> It's making the opamp follow accurately that's the issue. >>>> >>> I know, that's what your sim shows. This is why I think it is important >>> to get the most clean high-BW amp solution possible. And I just don't >>> think that's easily possible with a CFB running at G=+1. They are like a >>> car with worn shocks. >>> >>> But I think first the root cause for the weird 100MHz ring-a-ding-ding >>> has to be found. Can't imagine an AD8014 doing that on its own. >> >> >> With 1K, the recommended datasheet value, it doesn't ring, but it has >> a high slope from zero to about +1.3 volts or so, then settles down to >> follow the cap. The initial slope is about twice the correct value. >> > >In your sim file it stops doing the overshoot when the gain is above 1.3 >but who knows, SPICE models only go so far. > > >> I tried 470 ohms, and it's a compromise: a little ringing, a little >> pre-slope. The equivalent ramp error is about 75 ps RMS, so I may live >> with that. Still ugly, though. >> > >Looks like you need a faster amp. How about this one? > >http://www.ti.com/lit/gpn/ths3202 > >Assuming cost is not of prime concern here.
We have THS3201s (1.8 GHz) and AD8009s (1 GHz) in stock, so I'll have to try them some day. But I'm down to 75 ps with the AD8014 and 470 ohms, so I think I'll rest there for a while. Too bad about the sims. John
John Larkin wrote:

> On Thu, 15 Dec 2011 19:19:21 -0500, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> 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. >>> >>> >>> >>>>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 >> > > > That's the load I have to drive! > > John >
Yes, ok but still, have you tried a load R directly on the CA output ? Jamie
On Thu, 15 Dec 2011 08:21:17 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>Hi, > >I have the AD8014 Spice model from Analog Devices, and I have LT >Spice. > >The model file AD8014.cir starts with... > > >AD8014 SPICE model > >* Node assignments >* non-inverting input >* | inverting input >* | | positive supply >* | | | negative supply >* | | | | output >* | | | | | >.SUBCKT AD8014 1 2 99 50 28 > > >So, how do I draw an LT Spice schematic, with the usual opamp symbol, >and plug this model into it? > >I'm having a small problem with my ramp circuit > >ftp://jjlarkin.lmi.net/Ramp.JPG > >and it would be more convenient, just now, to tweak it by simulating >instead of soldering. > >Yes, yes, I should know this, but I don't use Spice often enough to >remember all the mechanics. > >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. > >John
The AD8014 Spice model is crap... pure behavioral. Even the NPN and PNP used in the model are Spice defaults. ...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.
On Fri, 16 Dec 2011 16:53:26 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Thu, 15 Dec 2011 08:21:17 -0800, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>Hi, >> >>I have the AD8014 Spice model from Analog Devices, and I have LT >>Spice. >> >>The model file AD8014.cir starts with... >> >> >>AD8014 SPICE model >> >>* Node assignments >>* non-inverting input >>* | inverting input >>* | | positive supply >>* | | | negative supply >>* | | | | output >>* | | | | | >>.SUBCKT AD8014 1 2 99 50 28 >> >> >>So, how do I draw an LT Spice schematic, with the usual opamp symbol, >>and plug this model into it? >> >>I'm having a small problem with my ramp circuit >> >>ftp://jjlarkin.lmi.net/Ramp.JPG >> >>and it would be more convenient, just now, to tweak it by simulating >>instead of soldering. >> >>Yes, yes, I should know this, but I don't use Spice often enough to >>remember all the mechanics. >> >>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. >> >>John > >The AD8014 Spice model is crap... pure behavioral.
But it doesn't behave right! John