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LT Spice question

Started by John Larkin December 15, 2011
John Larkin wrote:

> 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 >
Like a lot of things I've seen in spice. Of course, it's always possible you're introducing conditions at the bench that is not being placed in the spice simulation. Btw, I used one of the LT current amps and was able to reproduce what you're getting. I was also able to remove it by increasing the (-) feed back R to 900 ohms, instead of the 249 you have.. For what ever that is worth. Jamie
On Sat, 17 Dec 2011 23:07:31 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

>John Larkin wrote: > >> 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 >> > > Like a lot of things I've seen in spice. Of course, it's always >possible you're introducing conditions at the bench that is not being >placed in the spice simulation. > > Btw, I used one of the LT current amps and was able to reproduce what >you're getting. I was also able to remove it by increasing the (-) feed >back R to 900 ohms, instead of the 249 you have.. For what ever that is >worth. > > >Jamie > >
With a 1K resistor, I get an initial jump in the output ramp. At 250 ohms, it rings. That makes sense, but the sim shows neither effect. The AD8014 model is useless for optimizing the ramp linearity, or even demonstrating gross behavior, so it's back to soldering. The board is in a big rackmount chassis, and there are a jillion cables to disconnect and re-hook up, and JTAG stuff to reload, to change a resistor. Well, just like the olden days. 470 ohms seem about right. I still get a small jump and a hint of ringing, but it's over before I enter my active zone at +1 volt. I'm getting around 60 ps equivalent RMS time error on the ramp, good enough. This board is just one of six in the box! And the customer is having kittens to get this installed. So good enough will have to do. The argument had been made, in certain circles, that we should lay out a proto pcb and seed it with all the new/unknown/risky circuits on a big thing like this, and check them individually before we gerber the main board. Not a bad idea. John
John Larkin wrote:

> On Sat, 17 Dec 2011 23:07:31 -0500, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > > >>John Larkin wrote: >> >> >>>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 >>> >> >> Like a lot of things I've seen in spice. Of course, it's always >>possible you're introducing conditions at the bench that is not being >>placed in the spice simulation. >> >> Btw, I used one of the LT current amps and was able to reproduce what >>you're getting. I was also able to remove it by increasing the (-) feed >>back R to 900 ohms, instead of the 249 you have.. For what ever that is >>worth. >> >> >>Jamie >> >> > > > With a 1K resistor, I get an initial jump in the output ramp. At 250 > ohms, it rings. That makes sense, but the sim shows neither effect. > The AD8014 model is useless for optimizing the ramp linearity, or even > demonstrating gross behavior, so it's back to soldering. The board is > in a big rackmount chassis, and there are a jillion cables to > disconnect and re-hook up, and JTAG stuff to reload, to change a > resistor. Well, just like the olden days. > > 470 ohms seem about right. I still get a small jump and a hint of > ringing, but it's over before I enter my active zone at +1 volt. I'm > getting around 60 ps equivalent RMS time error on the ramp, good > enough. > > This board is just one of six in the box! And the customer is having > kittens to get this installed. So good enough will have to do. > > The argument had been made, in certain circles, that we should lay out > a proto pcb and seed it with all the new/unknown/risky circuits on a > big thing like this, and check them individually before we gerber the > main board. Not a bad idea. > > John >
The LT1206 seems to show what you're getting. I would be willing to bet that the LT1206 model with some minor input values changed, could be used for for the AD version you're trying to use. Jamie
On Sun, 18 Dec 2011 00:36:45 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

>John Larkin wrote: > >> On Sat, 17 Dec 2011 23:07:31 -0500, Jamie >> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >> >>>John Larkin wrote: >>> >>> >>>>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 >>>> >>> >>> Like a lot of things I've seen in spice. Of course, it's always >>>possible you're introducing conditions at the bench that is not being >>>placed in the spice simulation. >>> >>> Btw, I used one of the LT current amps and was able to reproduce what >>>you're getting. I was also able to remove it by increasing the (-) feed >>>back R to 900 ohms, instead of the 249 you have.. For what ever that is >>>worth. >>> >>> >>>Jamie >>> >>> >> >> >> With a 1K resistor, I get an initial jump in the output ramp. At 250 >> ohms, it rings. That makes sense, but the sim shows neither effect. >> The AD8014 model is useless for optimizing the ramp linearity, or even >> demonstrating gross behavior, so it's back to soldering. The board is >> in a big rackmount chassis, and there are a jillion cables to >> disconnect and re-hook up, and JTAG stuff to reload, to change a >> resistor. Well, just like the olden days. >> >> 470 ohms seem about right. I still get a small jump and a hint of >> ringing, but it's over before I enter my active zone at +1 volt. I'm >> getting around 60 ps equivalent RMS time error on the ramp, good >> enough. >> >> This board is just one of six in the box! And the customer is having >> kittens to get this installed. So good enough will have to do. >> >> The argument had been made, in certain circles, that we should lay out >> a proto pcb and seed it with all the new/unknown/risky circuits on a >> big thing like this, and check them individually before we gerber the >> main board. Not a bad idea. >> >> John >> >The LT1206 seems to show what you're getting. I would be willing to >bet that the LT1206 model with some minor input values changed, could be >used for for the AD version you're trying to use. > > Jamie > >
I've talked to AD from time-to-time about modeling. Even showed them how to properly model GBW in an OpAmp. When I quoted doing a rather complex A2D at 4 days, they thought that too expensive... they're cheap-ass ;-) And they wouldn't, even under NDA, let me see an actual schematic or netlist. For my IBIS modeling customers I've demonstrated to them that my IBIS representations dead-on overlay the Spice results from their own netlist... they were very happy! ...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 | Annoy a Liberal, Stand Them Up to a Blackboard and Ask Them to Show Their Math That Balances the Budget, Even After Taxing/Taking Everything the Rich Make... Then Punish Their Ignorance :-)
Joerg a &#2013265929;crit :
> 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. >> > > 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" >
Don't clip the end of sentence please. So: "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." Now the next sentence is somewhat incomplete. "The capacitor on the inverting input will cause peaking or oscillations." should be more like : "A capacitor between the inverting input and GND will cause peaking while a capacitor between the inverting input and the output will cause oscillations."
> Yes, they put it all in capital letters, and if LTC does that they have > their reasons :-)
While if I'm saying that I do have mine :-) See the netlist bellow. The pencil & paper demonstration left as an exercise to the student, to parrot some people here :-) Version 4 SHEET 1 1884 680 WIRE 304 -224 112 -224 WIRE 704 -224 304 -224 WIRE 1088 -224 704 -224 WIRE 1488 -224 1088 -224 WIRE 304 -208 304 -224 WIRE 704 -208 704 -224 WIRE 1088 -208 1088 -224 WIRE 1488 -208 1488 -224 WIRE 112 -192 112 -224 WIRE 304 -96 304 -128 WIRE 304 -96 272 -96 WIRE 352 -96 304 -96 WIRE 480 -96 432 -96 WIRE 704 -96 704 -128 WIRE 704 -96 672 -96 WIRE 752 -96 704 -96 WIRE 880 -96 832 -96 WIRE 1088 -96 1088 -128 WIRE 1088 -96 1072 -96 WIRE 1136 -96 1088 -96 WIRE 1264 -96 1216 -96 WIRE 1488 -96 1488 -128 WIRE 1488 -96 1472 -96 WIRE 1504 -96 1488 -96 WIRE 1664 -96 1584 -96 WIRE 112 -80 112 -112 WIRE 1088 -48 1088 -96 WIRE 384 -16 -32 -16 WIRE 784 -16 384 -16 WIRE 1168 -16 784 -16 WIRE 1568 -16 1168 -16 WIRE -32 32 -32 -16 WIRE 480 32 480 -96 WIRE 880 32 880 -96 WIRE 1264 32 1264 -96 WIRE 1664 32 1664 -96 WIRE 384 48 384 -16 WIRE 784 48 784 -16 WIRE 1168 48 1168 -16 WIRE 1568 48 1568 -16 WIRE 304 64 304 -96 WIRE 352 64 304 64 WIRE 704 64 704 -96 WIRE 752 64 704 64 WIRE 1088 64 1088 32 WIRE 1136 64 1088 64 WIRE 1488 64 1488 -96 WIRE 1536 64 1488 64 WIRE 480 80 480 32 WIRE 480 80 416 80 WIRE 880 80 880 32 WIRE 880 80 816 80 WIRE 1264 80 1264 32 WIRE 1264 80 1200 80 WIRE 1664 80 1664 32 WIRE 1664 80 1600 80 WIRE 352 96 320 96 WIRE 752 96 720 96 WIRE 1136 96 1104 96 WIRE 1536 96 1504 96 WIRE 320 112 320 96 WIRE 720 112 720 96 WIRE 1104 112 1104 96 WIRE 1504 112 1504 96 WIRE -32 128 -32 112 WIRE -32 128 -96 128 WIRE -96 144 -96 128 WIRE -32 144 -32 128 WIRE -32 288 -32 224 WIRE 384 288 384 112 WIRE 384 288 -32 288 WIRE 784 288 784 112 WIRE 784 288 384 288 WIRE 1168 288 1168 112 WIRE 1168 288 784 288 WIRE 1568 288 1568 112 WIRE 1568 288 1168 288 FLAG -96 144 0 FLAG 320 112 0 FLAG 1504 112 0 FLAG 112 -80 0 FLAG 208 -96 0 FLAG 1408 -96 0 FLAG 1104 112 0 FLAG 1008 -96 0 FLAG 720 112 0 FLAG 608 -96 0 FLAG 480 32 Out1 IOPIN 480 32 Out FLAG 880 32 Out2 IOPIN 880 32 Out FLAG 1264 32 Out3 IOPIN 1264 32 Out FLAG 1664 32 Out4 IOPIN 1664 32 Out SYMBOL Opamps\\LT1395 384 16 R0 SYMATTR InstName U1 SYMBOL voltage -32 16 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5V SYMBOL voltage -32 128 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 5V SYMBOL res 448 -112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 250 SYMBOL res 1600 -112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 250 SYMBOL res 288 -224 R0 SYMATTR InstName R3 SYMATTR Value 250 SYMBOL res 1472 -224 R0 SYMATTR InstName R4 SYMATTR Value 250 SYMBOL voltage 112 -208 R0 WINDOW 0 -78 17 Left 2 WINDOW 3 -239 100 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value PULSE(0 2 10n 1n) SYMBOL cap 272 -112 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 50p SYMBOL cap 1472 -112 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 20p SYMBOL res 1232 -112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 250 SYMBOL res 1072 -224 R0 SYMATTR InstName R6 SYMATTR Value 250 SYMBOL ind 1072 -64 R0 SYMATTR InstName L1 SYMATTR Value 10n SYMBOL cap 1072 -112 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 50p SYMBOL res 848 -112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 250 SYMBOL res 688 -224 R0 SYMATTR InstName R8 SYMATTR Value 250 SYMBOL cap 672 -112 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C4 SYMATTR Value 100p SYMBOL Opamps\\LT1395 784 16 R0 SYMATTR InstName U2 SYMBOL Opamps\\LT1395 1168 16 R0 SYMATTR InstName U3 SYMBOL Opamps\\LT1818 1568 80 R0 SYMATTR InstName U4 TEXT -24 -264 Left 2 !.tran 0 100n 0 1n TEXT 616 -328 Left 2 ;LT1395: CFB opamp BW = 400MHz TEXT 1408 -336 Left 2 ;LT1818: .VFB opamp GBW = 400MHz -- Thanks, Fred.
On Sun, 18 Dec 2011 23:09:18 +0100, Fred Bartoli <" "> wrote:

>Joerg a &#2013265929;crit : >> 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. >>> >> >> 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" >> > >Don't clip the end of sentence please. So: >"DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT >FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." > >Now the next sentence is somewhat incomplete. >"The capacitor on the inverting input will cause peaking or >oscillations." should be more like : >"A capacitor between the inverting input and GND will cause peaking >while a capacitor between the inverting input and the output will cause >oscillations." > >> Yes, they put it all in capital letters, and if LTC does that they have >> their reasons :-) > >While if I'm saying that I do have mine :-) > >See the netlist bellow. >The pencil & paper demonstration left as an exercise to the student, to >parrot some people here :-) > > >Version 4 >SHEET 1 1884 680 >WIRE 304 -224 112 -224 >WIRE 704 -224 304 -224 >WIRE 1088 -224 704 -224 >WIRE 1488 -224 1088 -224 >WIRE 304 -208 304 -224 >WIRE 704 -208 704 -224 >WIRE 1088 -208 1088 -224 >WIRE 1488 -208 1488 -224 >WIRE 112 -192 112 -224 >WIRE 304 -96 304 -128 >WIRE 304 -96 272 -96 >WIRE 352 -96 304 -96 >WIRE 480 -96 432 -96 >WIRE 704 -96 704 -128 >WIRE 704 -96 672 -96 >WIRE 752 -96 704 -96 >WIRE 880 -96 832 -96 >WIRE 1088 -96 1088 -128 >WIRE 1088 -96 1072 -96 >WIRE 1136 -96 1088 -96 >WIRE 1264 -96 1216 -96 >WIRE 1488 -96 1488 -128 >WIRE 1488 -96 1472 -96 >WIRE 1504 -96 1488 -96 >WIRE 1664 -96 1584 -96 >WIRE 112 -80 112 -112 >WIRE 1088 -48 1088 -96 >WIRE 384 -16 -32 -16 >WIRE 784 -16 384 -16 >WIRE 1168 -16 784 -16 >WIRE 1568 -16 1168 -16 >WIRE -32 32 -32 -16 >WIRE 480 32 480 -96 >WIRE 880 32 880 -96 >WIRE 1264 32 1264 -96 >WIRE 1664 32 1664 -96 >WIRE 384 48 384 -16 >WIRE 784 48 784 -16 >WIRE 1168 48 1168 -16 >WIRE 1568 48 1568 -16 >WIRE 304 64 304 -96 >WIRE 352 64 304 64 >WIRE 704 64 704 -96 >WIRE 752 64 704 64 >WIRE 1088 64 1088 32 >WIRE 1136 64 1088 64 >WIRE 1488 64 1488 -96 >WIRE 1536 64 1488 64 >WIRE 480 80 480 32 >WIRE 480 80 416 80 >WIRE 880 80 880 32 >WIRE 880 80 816 80 >WIRE 1264 80 1264 32 >WIRE 1264 80 1200 80 >WIRE 1664 80 1664 32 >WIRE 1664 80 1600 80 >WIRE 352 96 320 96 >WIRE 752 96 720 96 >WIRE 1136 96 1104 96 >WIRE 1536 96 1504 96 >WIRE 320 112 320 96 >WIRE 720 112 720 96 >WIRE 1104 112 1104 96 >WIRE 1504 112 1504 96 >WIRE -32 128 -32 112 >WIRE -32 128 -96 128 >WIRE -96 144 -96 128 >WIRE -32 144 -32 128 >WIRE -32 288 -32 224 >WIRE 384 288 384 112 >WIRE 384 288 -32 288 >WIRE 784 288 784 112 >WIRE 784 288 384 288 >WIRE 1168 288 1168 112 >WIRE 1168 288 784 288 >WIRE 1568 288 1568 112 >WIRE 1568 288 1168 288 >FLAG -96 144 0 >FLAG 320 112 0 >FLAG 1504 112 0 >FLAG 112 -80 0 >FLAG 208 -96 0 >FLAG 1408 -96 0 >FLAG 1104 112 0 >FLAG 1008 -96 0 >FLAG 720 112 0 >FLAG 608 -96 0 >FLAG 480 32 Out1 >IOPIN 480 32 Out >FLAG 880 32 Out2 >IOPIN 880 32 Out >FLAG 1264 32 Out3 >IOPIN 1264 32 Out >FLAG 1664 32 Out4 >IOPIN 1664 32 Out >SYMBOL Opamps\\LT1395 384 16 R0 >SYMATTR InstName U1 >SYMBOL voltage -32 16 R0 >WINDOW 123 0 0 Left 2 >WINDOW 39 0 0 Left 2 >SYMATTR InstName V1 >SYMATTR Value 5V >SYMBOL voltage -32 128 R0 >WINDOW 123 0 0 Left 2 >WINDOW 39 0 0 Left 2 >SYMATTR InstName V2 >SYMATTR Value 5V >SYMBOL res 448 -112 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R1 >SYMATTR Value 250 >SYMBOL res 1600 -112 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R2 >SYMATTR Value 250 >SYMBOL res 288 -224 R0 >SYMATTR InstName R3 >SYMATTR Value 250 >SYMBOL res 1472 -224 R0 >SYMATTR InstName R4 >SYMATTR Value 250 >SYMBOL voltage 112 -208 R0 >WINDOW 0 -78 17 Left 2 >WINDOW 3 -239 100 Left 2 >WINDOW 123 0 0 Left 2 >WINDOW 39 0 0 Left 2 >SYMATTR InstName V3 >SYMATTR Value PULSE(0 2 10n 1n) >SYMBOL cap 272 -112 R90 >WINDOW 0 0 32 VBottom 2 >WINDOW 3 32 32 VTop 2 >SYMATTR InstName C1 >SYMATTR Value 50p >SYMBOL cap 1472 -112 R90 >WINDOW 0 0 32 VBottom 2 >WINDOW 3 32 32 VTop 2 >SYMATTR InstName C2 >SYMATTR Value 20p >SYMBOL res 1232 -112 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R5 >SYMATTR Value 250 >SYMBOL res 1072 -224 R0 >SYMATTR InstName R6 >SYMATTR Value 250 >SYMBOL ind 1072 -64 R0 >SYMATTR InstName L1 >SYMATTR Value 10n >SYMBOL cap 1072 -112 R90 >WINDOW 0 0 32 VBottom 2 >WINDOW 3 32 32 VTop 2 >SYMATTR InstName C3 >SYMATTR Value 50p >SYMBOL res 848 -112 R90 >WINDOW 0 0 56 VBottom 2 >WINDOW 3 32 56 VTop 2 >SYMATTR InstName R7 >SYMATTR Value 250 >SYMBOL res 688 -224 R0 >SYMATTR InstName R8 >SYMATTR Value 250 >SYMBOL cap 672 -112 R90 >WINDOW 0 0 32 VBottom 2 >WINDOW 3 32 32 VTop 2 >SYMATTR InstName C4 >SYMATTR Value 100p >SYMBOL Opamps\\LT1395 784 16 R0 >SYMATTR InstName U2 >SYMBOL Opamps\\LT1395 1168 16 R0 >SYMATTR InstName U3 >SYMBOL Opamps\\LT1818 1568 80 R0 >SYMATTR InstName U4 >TEXT -24 -264 Left 2 !.tran 0 100n 0 1n >TEXT 616 -328 Left 2 ;LT1395: CFB opamp BW = 400MHz >TEXT 1408 -336 Left 2 ;LT1818: .VFB opamp GBW = 400MHz
If you move C1 to across R1 it's beautifully stable, for most any value of C1. So much for theory! Maybe all CFA models are bad. John
Fred Bartoli wrote:
> Joerg a &#2013265929;crit : >> 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. >>> >> >> 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" >> > > Don't clip the end of sentence please. So: > "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT > FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." > > Now the next sentence is somewhat incomplete. > "The capacitor on the inverting input will cause peaking or > oscillations." should be more like : > "A capacitor between the inverting input and GND will cause peaking > while a capacitor between the inverting input and the output will cause > oscillations." >
In my experience both can cause oscillation.
>> Yes, they put it all in capital letters, and if LTC does that they have >> their reasons :-) > > While if I'm saying that I do have mine :-) >
As long as our wives are around only their reasons really count :-)
> See the netlist bellow. > The pencil & paper demonstration left as an exercise to the student, to > parrot some people here :-) >
Ok, now move C1 across R1 instead of ground -> Total stability. And that ain't happ'nin in real life. I do not trust behavioral models for this kind of stuff. Does anyone have a SPICE model for a CFB amp that is 100% down-to-the-bone tried and true and has zero behavioral elements in there? Even then it's risky. CFB amps aren't all the same. Some really do not like G=+1 operation while others are ok in that configuration. [...] -- Regards, Joerg http://www.analogconsultants.com/
John Larkin wrote:

[...]

> > If you move C1 to across R1 it's beautifully stable, for most any > value of C1. So much for theory! >
Strange, I tried exactly the same thing after loading Fred's file. And I was not surprised about the result.
> Maybe all CFA models are bad. >
Behavioral ones usually are, they are no good to really ascertain stability. For switchers behavioral can be ok but for opamps I really don't like that. -- Regards, Joerg http://www.analogconsultants.com/
On Sun, 18 Dec 2011 16:19:11 -0800, Joerg <invalid@invalid.invalid>
wrote:

>Fred Bartoli wrote: >> Joerg a &#2013265929;crit : >>> 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. >>>> >>> >>> 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" >>> >> >> Don't clip the end of sentence please. So: >> "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT >> FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." >> >> Now the next sentence is somewhat incomplete. >> "The capacitor on the inverting input will cause peaking or >> oscillations." should be more like : >> "A capacitor between the inverting input and GND will cause peaking >> while a capacitor between the inverting input and the output will cause >> oscillations." >> > >In my experience both can cause oscillation. > > >>> Yes, they put it all in capital letters, and if LTC does that they have >>> their reasons :-) >> >> While if I'm saying that I do have mine :-) >> > >As long as our wives are around only their reasons really count :-) > > >> See the netlist bellow. >> The pencil & paper demonstration left as an exercise to the student, to >> parrot some people here :-) >> > >Ok, now move C1 across R1 instead of ground -> Total stability. And that >ain't happ'nin in real life. I do not trust behavioral models for this >kind of stuff. > >Does anyone have a SPICE model for a CFB amp that is 100% >down-to-the-bone tried and true and has zero behavioral elements in there?
If there were one, full of transistors and caps and such, it would probably run very, very slowly. When I sim circuits, I generally use a VCVS to simulate an opamp, with an RC on the output if bandwidth matters. Runs fast!
> >Even then it's risky. CFB amps aren't all the same. Some really do not >like G=+1 operation while others are ok in that configuration.
G=1 usually works fine if you use the right feedback resistor. I'm groveling around for 0.1% linearity on a 16 ns ramp, so that's a little delicate. The AD8014, with 1.4 mA supply current, barely qualifies as a CFB. John
John Larkin wrote:
> On Sun, 18 Dec 2011 16:19:11 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> Fred Bartoli wrote: >>> Joerg a &#2013265929;crit : >>>> 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. >>>>> >>>> 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" >>>> >>> Don't clip the end of sentence please. So: >>> "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT >>> FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." >>> >>> Now the next sentence is somewhat incomplete. >>> "The capacitor on the inverting input will cause peaking or >>> oscillations." should be more like : >>> "A capacitor between the inverting input and GND will cause peaking >>> while a capacitor between the inverting input and the output will cause >>> oscillations." >>> >> In my experience both can cause oscillation. >> >> >>>> Yes, they put it all in capital letters, and if LTC does that they have >>>> their reasons :-) >>> While if I'm saying that I do have mine :-) >>> >> As long as our wives are around only their reasons really count :-) >> >> >>> See the netlist bellow. >>> The pencil & paper demonstration left as an exercise to the student, to >>> parrot some people here :-) >>> >> Ok, now move C1 across R1 instead of ground -> Total stability. And that >> ain't happ'nin in real life. I do not trust behavioral models for this >> kind of stuff. >> >> Does anyone have a SPICE model for a CFB amp that is 100% >> down-to-the-bone tried and true and has zero behavioral elements in there? > > If there were one, full of transistors and caps and such, it would > probably run very, very slowly. >
Not in your case because you only want to look at one lone ramp. If you want to sim a switcher with start-up and all that in non-behavioral, different thing. BTDT. I used that day to repair a deck post while the sim was running inside. Why are there no aluminum deck posts that don't rot?
> When I sim circuits, I generally use a VCVS to simulate an opamp, with > an RC on the output if bandwidth matters. Runs fast! >
But it'll tell lies :-)
>> Even then it's risky. CFB amps aren't all the same. Some really do not >> like G=+1 operation while others are ok in that configuration. > > G=1 usually works fine if you use the right feedback resistor. I'm > groveling around for 0.1% linearity on a 16 ns ramp, so that's a > little delicate. > > The AD8014, with 1.4 mA supply current, barely qualifies as a CFB. >
In this case it's almost tempting to try something more unorthodox. Use a nicely linear RF amp or just a follower with enough amplitude range as a 1:1 buffer and then servo out the DC error with an el-cheapo opamp. Costs less, too, but I guess that's not a concern here. -- Regards, Joerg http://www.analogconsultants.com/