C-multiplier again

-

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nd

'd

e

lt

!"

I e " " "

That seemed to work OK, I got some error messages about the cap names, but I clikced on ignore and it loaded.

George H

I wonder what the numbers are like here. Suppose one used a front-panel type green LED as a power indicator and voltage reference. The dynamic impedance of the LED will be ohms. I'd guess that any fluorescent light induced current would be nanoamps. So we'd have nanovolts of optically-induced noise. A cap across the LED would kill the high frequency stuff, like from electronic ballasts.

Of course, the current source would have to be a lot better than your average 5-volt-supply-resistor thing.

John

The author added the resistor to prevent damage, so I assume he picked a value that worked. I'm sure the diodes inside the opamp can handle a lot more than 25 mA for a short time.

John

puts =A0and no

from

vent

It looks to me like shorting the output pulls a brief spike via C3, then about 9mA d.c. through the protection diodes, limited by R3+R5. I don't see any need to protect against hard shorting the inputs to ground--don't do that.

Walt worked at AD and knew what was inside the AD797. He's no dummy-- whatever he did, he meant it.

-- Cheers, James Arthur

George Herold wrote: [...]

Thanks! There may some problems with line wrap. LTspice doesn't like that. In most cases it is fairly easy to fix. I'll take a look later this afternoon.

Mike

Ask Joerg about exceeding the manufacturer's specs.

Mike

I meant filter-type approaches, like the C-mult. Cancellation approaches are inherently limited by the accuracy--gain and dynamic-- of the canceller function. Filter-type methods have no such limit.

-- Cheers, James Arthur

f

at,

Yeah, but I doubt very much Walt fed that reference dirty.

We knew better.

Way back, Analog Devices said switchers plus 12+ bit SAR A/Ds didn't mix, but I did all the time, carefully, with no trouble.

-- Cheers, James Arthur

What's the peak current and time constant of the current spike? That is what is doing the damage, not the 9 mA constant current after the transient has dissipated.

I agree, Walt is very good. But it is bad practise to exceed the manufacturer's spec at any time. Can you get Joerg to agree to exceed the manufacturer's spec? I don't think so.

A pair of schottky diodes back-to-back across the input pins allows you to reduce the value of R3 to 10 ohms or less, and still meet the manufacturer's maximum rated current spec.

Reducing R3 from 49.9 ohms to 10 ohms or less also reduces the Johnson noise considerably. I'll let you do the calculation.

The 49.9 ohm resistor limits any improvement you can get by changing to a better op amp. So you are stuck with 1988 performance that really doesn't meet the manufacturer's max current limit.

However, adding back-to-back schottky diodes and reducing the value of R3 from 49.9 ohms to 10 ohms or less means you can use a more modern op amp that has lower noise, and gain the advantage of improved performance.

The back-to-back schottky diodes eliminate the problem of hard shorts, and gives better noise performance, which is the whole purpose of this exercise.

Regards,

Mike

Yeah, but Walt *was* the manufacturer, designing in his own part, knowing what's inside.

I see that AD's 25mA limit for the AD797 input diodes was a recommendation, not an absolute instantaneous limit. Not knowing what's inside, withstanding a worst-case spike at 7x the recommended d.c. limit seems entirely plausible. Or maybe Walt just didn't want to give up any more noise performance.

Maybe he was just designing to withstand start up and shut down transients.

If the circuit doesn't protect against hard dead shorts, then it wasn't supposed to.

-- Cheers, James Arthur

Hi George,

The error messages are from line wrap on the capacitor parameters and strange newsreader encodings.

When you use LTspice to select a manufacturer's capacitor, it adds all kinds of crap to the entry. For example, C5 looks like the following. (I added hard carriage returns to kill the newsreader wrap):

SYMATTR InstName C5 SYMATTR Value 1µ SYMATTR SpiceLine V=25 Irms=7.25 Rser=0.004 Lser=0 mfg="KEMET" pn="C1206C105K3PAC" type="X5R"

In addition, your newsreader editor encodes some of the symbols to a different encoding. For example, the equals sign, "=", becomes "=3D", and the micro sign becomes "=B5". So a 1uF cap comes out as 1=B5 in the ascii file.

I can't tell from the header what kind of newsreader you are using, but if you want to post ascii files to the newsgroup, I'd suggest getting XNEWS. It is free, and it won't mess up your posts with strange codings.

I'd also suggest when you use a cap in LTspice, don't bother trying to get one with a vendor's model. Just find out what the typical series resistance and inductance values are for the type of cap you are using, and enter those into the standard cap symbol. Here is what it looks like in the ASC file:

SYMBOL cap -64 -256 R0 SYMATTR InstName C3 SYMATTR Value 100µf SYMATTR SpiceLine Rser=20m Lser=2.5nh

See? A lot simpler, and no line wrap or strange encodings.

Finally, after evaluating the performance of LTspice as a noise filter in the emitter follower configuration, I would not recommend using LTspice. The results are unpredictable and do not represent the real world.

I did some tests in MicroCap 8 and 9. They do a better job, but are still not close to actual data. So for these types of analysis, I'd recommend building the circuit and actually measuring the results.

That's what counts, in the final analysis.

Regards,

Mike

If you are happy with unknown fault response and 1988 noise performance, then by all means stick with that approach.

Thanks,

Mike

On Fri, 28 May 2010 07:09:46 -0700, George Herold wrote: ...

...[big snip]

Except for the line break which had to be fixed, this looked ok in the news reader, but when saved to file, displayed as

SYMATTR SpiceLine V=3D25 Irms=3D7.25 Rser=3D0.004 Lser=3D0 mfg=3D"KEMET" pn=3D"C1206C105K3PAC" type=3D"X5R" SYMBOL cap -320 -320 R0 SYMATTR InstName C5 SYMATTR Value 1=B5 SYMATTR SpiceLine V=3D25 Irms=3D7.25 Rser=3D0.004 Lser=3D0 mfg=3D"KEMET" pn=3D"C1206C105K3PAC" type=3D"X5R"

That is, every "=" saved as "=3D" while "µ" (mu) saved as "=B5", all of which required external editing (replace all) before LTspice running under wine on linux would properly load the file. I don't know if the same problem will occur under MS-windows. If it does, maybe it depends on how you copied the text into your newsreader. You could fix the "µ" by manually editing to "u"; see chart on page 63 of scad3.pdf.

You can manually fix the line breaks, if you see them when you post, by adding a "+" at the beginning of broken off parts. For example, SYMATTR SpiceLine V=25 Irms=7.25 Rser=0.004 Lser=0 mfg="KEMET"

• pn="C1206C105K3PAC" type="X5R"

[...]

I tried that. LTspice will not accept hard carriage returns in the middle of the attribute. It generates an error while loading. If you press "Ignore", it omits the rest of the line.

This gives very strange component attributes:)

I see only two solutions. Avoid long lines, or archive to a zip file and post it somewhere. abse is convenient, but not permanent. So if someone wants to review your work a decade from now, they will be disappointed.

Another alternative might be to encode the file in Base64 and append it to the newsgroup posting. That means everyone would have to download a Base64 decoder. It would be instantly fatal.

So what we need is a way to store binary files that will persist and are easy to decode, or find a way to allow long lines in LTspice. Any ideas?

Thanks,

Mike

and

scii

Thanks for all the advice Mike. I'm not even sure that is the same Alum-electro that I'm using. I didn't 'spice' this circuit till a few days ago. About a year or so ago, I proto-ed it on copper clad and played with values till the noise was low enough. (A bit of a pain since to test it I had to close it up inside a metal box.) Then it oscillated at certain bias currents, and I f'ed around till that went away. (I'm mostly just a circuit 'hack' though I hope I'm starting to understand things.)

George H. (Sorry for the Walt confusion on this thread, 'my' Walt knows nothing about circuits, but makes some 'killer' mustard.)

" 3D"KEMET" 3D"KEMET"

all

Thanks Joe, (I'm still an LTspice newlywed, so forgive my blunders.) I'll get rid of all the 'named' caps and try again.

Does this work?

Version 4 SHEET 1 1140 1108 WIRE -688 -448 -768 -448 WIRE -560 -448 -608 -448 WIRE -160 -448 -560 -448 WIRE -128 -448 -160 -448 WIRE -128 -400 -128 -448 WIRE -560 -352 -560 -368 WIRE -496 -352 -560 -352 WIRE -384 -352 -416 -352 WIRE -272 -352 -384 -352 WIRE -768 -320 -768 -448 WIRE -560 -320 -560 -352 WIRE -384 -320 -384 -352 WIRE -128 -288 -128 -304 WIRE -48 -288 -128 -288 WIRE 16 -288 -48 -288 WIRE -128 -256 -128 -288 WIRE -48 -256 -48 -288 WIRE -768 -224 -768 -240 WIRE -560 -224 -560 -256 WIRE -528 -224 -560 -224 WIRE -384 -224 -384 -256 WIRE -384 -224 -528 -224 WIRE -528 -208 -528 -224 WIRE -128 -160 -128 -176 WIRE -48 -160 -48 -192 FLAG -528 -208 0 FLAG -768 -224 0 FLAG -128 -160 0 FLAG -160 -448 Vin FLAG 16 -288 Vout FLAG -48 -160 0 SYMBOL npn -192 -400 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL voltage -768 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -592 -448 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value2 AC 1 SYMATTR Value "" SYMBOL res -576 -464 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res -400 -368 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 42 49 VTop 0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res -176 -368 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 10 SYMBOL res -144 -272 R0 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL cap -576 -320 R0 SYMATTR InstName C1 SYMATTR Value 100=B5 SYMATTR SpiceLine Rser=3D0.13 SYMBOL cap -400 -320 R0 SYMATTR InstName C2 SYMATTR Value 100=B5 SYMATTR SpiceLine Rser=3D0.13 SYMBOL cap -64 -256 R0 SYMATTR InstName C3 SYMATTR Value 100=B5 SYMATTR SpiceLine Rser=3D0.13 TEXT -216 -536 Left 0 ;Cap multiplier (Hobbs fillter) TEXT -224 -504 Left 0 !.ac oct 100 1 1e7

Mike wrote:

Test. Is ignoring plz.

Mike

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[...]

Hi George,

I kinda jumped the gun. But I love mustard, especially on Polish sausages. So either Walt is fine:)

I just posted a test attachment using Fairchild's model for the MPSA14. I assume this is converted to Base64 during transmission, then back to plain ascii when received. My newsreader automatically saves attachments to a download directory. I just checked, and sure enough the files were there and worked fine.

If you have a minute, see if your newsreader will recover these files properly and see if they will run in LTspice.

I'll give google groups time to archive the files, then see if they show up there.

If so, and everyone can read these files in the multitude of newsreaders out there, maybe this solves the problem.

And maybe it will also save zip files. That would be handy.

Thanks,

Mike

...

...

...

Mike noted that "LTspice will not accept hard carriage returns in the middle of the attribute" so disregard that.

...

...

It works much better -- loads into LTspice under wine ok. Still gets errors when Run, however, due to the µ (mu) transferring as =B5. However, one can fix that in LTspice, as opposed to using an external editor again.