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LT Spice question

Started by John Larkin December 15, 2011
Fred Bartoli wrote:
> Joerg a �crit : >> 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 >> >> 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. >> > > Ahem, CFB opamp, for the same FB resistor, do tolerate more parasitics > than VFB opamps. > Because the additional parasitic pole frequency is Rfb Cp for the VFB > and is Rin Cp for the CFB opamp, with Rin being roughly between 50R and > 100R. > What CFB opamps don't like much is parasitic inductance in series with > their minus input. >
http://cds.linear.com/docs/Design%20Note/dn46fa.pdf Quote "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE" Yes, they put it all in capital letters, and if LTC does that they have their reasons :-) -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> Fred Bartoli wrote: > >>Joerg a �crit : >> >>>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 >>> >>>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. >>> >> >>Ahem, CFB opamp, for the same FB resistor, do tolerate more parasitics >>than VFB opamps. >>Because the additional parasitic pole frequency is Rfb Cp for the VFB >>and is Rin Cp for the CFB opamp, with Rin being roughly between 50R and >>100R. >>What CFB opamps don't like much is parasitic inductance in series with >>their minus input. >> > > > http://cds.linear.com/docs/Design%20Note/dn46fa.pdf > > Quote "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT > OF A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE" > > Yes, they put it all in capital letters, and if LTC does that they have > their reasons :-) >
Why not? doesn't every one want a relaxation oscillator? Jamie
Jamie wrote:
> Joerg wrote: > >> Fred Bartoli wrote: >> >>> Joerg a �crit : >>> >>>> 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 >>>> >>>> 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. >>>> >>> >>> Ahem, CFB opamp, for the same FB resistor, do tolerate more parasitics >>> than VFB opamps. >>> Because the additional parasitic pole frequency is Rfb Cp for the VFB >>> and is Rin Cp for the CFB opamp, with Rin being roughly between 50R and >>> 100R. >>> What CFB opamps don't like much is parasitic inductance in series with >>> their minus input. >>> >> >> >> http://cds.linear.com/docs/Design%20Note/dn46fa.pdf >> >> Quote "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT >> OF A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE" >> >> Yes, they put it all in capital letters, and if LTC does that they have >> their reasons :-) >> > Why not? doesn't every one want a relaxation oscillator? >
I could imagine better things for relaxation :-) Thing is, with VFB opamps you can add a little capacitance across Rf in order to compensate for the capacitance of whatever hangs off of IN-. With CFB amps all hell can break loose if you do that. Or maybe it doesn't on the bench but then it does in production. -- Regards, Joerg http://www.analogconsultants.com/
On Thu, 15 Dec 2011 23:53:34 +0100, Fred Bartoli <" "> wrote:

>Joerg a &#2013265929;crit : >> 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 >> >> 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. >> > >Ahem, CFB opamp, for the same FB resistor, do tolerate more parasitics >than VFB opamps. >Because the additional parasitic pole frequency is Rfb Cp for the VFB >and is Rin Cp for the CFB opamp, with Rin being roughly between 50R and >100R. >What CFB opamps don't like much is parasitic inductance in series with >their minus input.
Since April ?:-) ...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 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
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
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?
>> 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. >
That looks too low in frequency to be the opamp itself. Sure it's not some cable like Jamie hinted?
> I guess I'll defy the sim and change Rf some, or try another amp, an > AD8009 maybe. >
And maybe touch things, see what makes the 100Mhz or so rinbing change in frequency. -- Regards, Joerg http://www.analogconsultants.com/
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 >
Good point, the device probably needs some DC load. And it seems rather squirrely at G=1, but nice at G=2. ...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.
Jim Thompson 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 >> > > 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 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. -- Regards, Joerg http://www.analogconsultants.com/
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