LT Spice question

y

supply

put

include the file, or paste all of it as a spice statement on the schematic that way it is all in one file

put an opamp2 on the schematic and right-click to change its 'value' to AD8014

check the pin order

-Lasse

It's been very long ago that I crammed in another opamp but it's mostly about the correct order of nodes. This seems like a good step-by-step descritpion:

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.

--
Regards, Joerg

http://www.analogconsultants.com/

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.

Check out his books.

John

.include AD8014.CIR

F2->OpAmp2->set value to AD8014

Oh, and make sure the pins are in the right order in the .SUBCKT line--check by comparison to a model that you know works.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

He writes books about FPGAs and monsters in the attic. I see the common thread. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Done.

He's got some really interesting books :-)

It is great when engineers also do other totally non-EE stuff, else they become nerds. The most drastic career change I saw is an EE who used to design 750kV equipment and transmission lines, then decided to start a care home for Alzheimer's patients. And he is amazingly good at that.

--
Regards, Joerg

http://www.analogconsultants.com/

On Thu, 15 Dec 2011 12:07:04 -0500, Phil Hobbs wrote:

OK, this works:

Version 4 SHEET 1 916 680 WIRE 656 -304 608 -304 WIRE 704 -304 656 -304 WIRE 608 -256 608 -304 WIRE 560 -240 -272 -240 WIRE 192 -176 96 -176 WIRE 96 -128 96 -176 WIRE 608 -128 608 -176 WIRE 192 -112 192 -176 WIRE 560 -16 560 -192 WIRE -128 16 -192 16 WIRE 16 16 -64 16 WIRE 144 16 16 16 WIRE 272 16 224 16 WIRE -192 48 -192 16 WIRE 96 64 96 -48 WIRE 16 80 16 16 WIRE 64 80 16 80 WIRE 272 96 272 16 WIRE 272 96 128 96 WIRE 336 96 272 96 WIRE 448 96 416 96 WIRE 560 96 560 -16 WIRE 560 96 528 96 WIRE -352 112 -416 112 WIRE -272 112 -272 -240 WIRE -272 112 -352 112 WIRE 64 112 -272 112 WIRE 560 128 560 96 WIRE -416 144 -416 112 WIRE -272 144 -272 112 WIRE 560 224 560 192 WIRE -416 240 -416 224 WIRE -272 240 -272 208 WIRE 96 240 96 128 WIRE 96 368 96 320 FLAG -272 240 0 FLAG 560 224 0 FLAG -416 240 0 FLAG 192 -112 0 FLAG 96 368 0 FLAG -192 48 0 FLAG 608 -128 0 FLAG 656 -304 ERROR FLAG -352 112 RAMP FLAG 272 16 AMP FLAG 560 -16 OUT SYMBOL cap -288 144 R0 WINDOW 0 71 15 Left 2 WINDOW 3 64 50 Left 2 SYMATTR InstName C1 SYMATTR Value 47p SYMBOL cap 544 128 R0 WINDOW 0 71 11 Left 2 WINDOW 3 65 47 Left 2 SYMATTR InstName C2 SYMATTR Value 47p SYMBOL ind 432 112 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 1n SYMBOL res 432 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL current -416 224 R180 WINDOW 0 24 88 Left 2 WINDOW 3 -93 202 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value PULSE(0 0.006 0 1n 1n 30n) SYMBOL Opamps\\opamp2 96 32 R0 WINDOW 0 45 101 Left 2 WINDOW 3 30 139 Left 2 SYMATTR InstName U1 SYMATTR Value AD8014 SYMBOL voltage 96 -32 R180 WINDOW 0 53 71 Left 2 WINDOW 3 61 36 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage 96 336 R180 WINDOW 0 69 70 Left 2 WINDOW 3 75 33 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 5 SYMBOL res 240 0 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 249 SYMBOL cap -64 0 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 1p SYMBOL e 608 -272 R0 WINDOW 0 66 42 Left 2 WINDOW 3 64 76 Left 2 SYMATTR InstName E1 SYMATTR Value 10 TEXT -184 -176 Left 2 !.tran 0 30n 0 TEXT -200 -120 Left 2 !.lib AD8014.CIR

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.

Thanks.

John

[SPICE netlist]

Doesn't look bad at all. For snappier corners you have to pick an amp with a lot more bandwidth. Like this little dude:

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.

--
Regards, Joerg

http://www.analogconsultants.com/

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.

--
Thanks,
Fred.

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/

Why not? doesn't every one want a relaxation oscillator?

Jamie

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/

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.

I need unity gain here. We have used the 4303 before, and it's very nice, probably because the feedback resistors are internal.

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

Why unity gain? Can't you divide down the ramp? Or use 1/5th of the charge current?

That looks too low in frequency to be the opamp itself. Sure it's not some cable like Jamie hinted?

And maybe touch things, see what makes the 100Mhz or so rinbing change in frequency.

--
Regards, Joerg

http://www.analogconsultants.com/

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.

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/

That's the load I have to drive!

John