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Old 15th December 2012, 05:32 PM   #2381
bcarso is offline bcarso  United States
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The need for loop gain in this particular case had more to do with a fairly low open-loop input impedance for the opamp, not because of the apparent closed loop gain. Scott W., do you recall a module from ancient times that was a sort-of-general-purpose "budget" part? I think the number was the AD118, but of course that number has probably been recycled a few times since --- I couldn't find anything on the web. Maybe it was advertised in Analog Dialogue.
Yes, it's advertised.
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Old 15th December 2012, 07:34 PM   #2382
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Yes, it's advertised.
BTW, that's not a good amp for low current measurements, but in this case the guy had lots of light. For serious astronomical photometry, we made an instrument based on another ADI product from that era, one of their varactor bridge ones. It replaced a Keithley store-bought electrometer.
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Old 15th December 2012, 10:15 PM   #2383
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Thanks for this Mr. Marsh. It's certainly what's required to replace J5. I'm reminded I have some SPICE models for Siliconix diodes too.
Du..uh! I be wrong.

Anyone have or can point to SPICE models for Siliconix constant current dioides?
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Old 16th December 2012, 01:17 AM   #2384
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Anyone have or can point to SPICE models for Siliconix constant current dioides?
Take a look at "diode.lib" from http://www.ece.utah.edu/~ece2280/diode.lib

Dale
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File Type: txt constant_current_diodes_Siliconix.txt (25.2 KB, 32 views)

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Old 16th December 2012, 02:14 AM   #2385
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Originally Posted by bcarso View Post
BTW, that's not a good amp for low current measurements, but in this case the guy had lots of light. For serious astronomical photometry, we made an instrument based on another ADI product from that era, one of their varactor bridge ones. It replaced a Keithley store-bought electrometer.
Still in production, we sold it to a third party who is now making it.
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Old 16th December 2012, 03:59 AM   #2386
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. . . a module from ancient times that was a sort-of-general-purpose "budget" part? I think the number was the AD118 . . .
From the Analog Devices "Product Guide" for 1975. Note that the AD118 overall performance is approximately equivalent to a '741. (And by 1975, the '741 was quite outdated even though many engineers didn't figure that out for another decade.)

At first I was puzzled by why they would do a side-by-side comparison of the modular opamp products on the same page as "microcircuit" (i.e., IC) products, since the IC's cost 1/4 to 1/10 as much as the modules. On subsequent pages I saw that the price differential became lower as performance increased, and in a few cases AD didn't even list an IC with comparable performance to some discrete modules.

Of course, that situation had pretty much changed by the early 80's. In the earlier stages of this thread we said there really wasn't much point to creating a discrete design with performance equal, or inferior, to generally-available IC's . . . and then it took a little work to find a set of realistic performance requirements that common IC's could not meet!

Dale
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File Type: pdf AD118_Opamp_1975_Product_Guide_Pg1.pdf (912.1 KB, 49 views)
File Type: pdf AD118_Opamp_1975_Product_Guide_Pg2.pdf (965.0 KB, 42 views)
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Old 16th December 2012, 12:26 PM   #2387
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Take a look at "diode.lib" from http://www.ece.utah.edu/~ece2280/diode.lib
Thanks for this & the attached file, Dale
____________

Scott, is there a SPICE model of AD797 which allows 'accurate' modelling of output Z & stability in a circuit like Fig 39 of the datasheet?
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Old 17th December 2012, 01:38 AM   #2388
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Thanks for this & the attached file, Dale
____________

Scott, is there a SPICE model of AD797 which allows 'accurate' modelling of output Z & stability in a circuit like Fig 39 of the datasheet?
No.
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Old 17th December 2012, 03:06 AM   #2389
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From the Analog Devices "Product Guide" for 1975. Note that the AD118 overall performance is approximately equivalent to a '741. (And by 1975, the '741 was quite outdated even though many engineers didn't figure that out for another decade.)

Dale
AD301AL my first project. I found a built-in offset and drift source that survived all previous layouts.

Folks forget about macro-models being anything more than gross approximations. We have again reached an impass on providing full encripted models via over-the-web simulation. The technology is there, but it will remain for major customers with NDA's only.
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Old 17th December 2012, 03:29 AM   #2390
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I've never understood the reasoning about not providing transistor level models - isn't anything you really care about is protected by patents, can't transistor level circuit topology can be reverse engineered by any interested competition from the die?

http://www.google.com/#hl=en&tbo=d&s...=1920&bih=1069

presumably the real secret sauce and barrier to competition is the $Billion fab lines, processing expertise

but even with macromodeling – it is very annoying when the manufacturer's haven't even taken care to exploit their own published macromodeling to a merely competent extent, not even getting them to a coarse eyeball match to datasheet curves in parameters where the macromodels could work

disclosure: I have an open complaint about the AD8067 model on Analog's support forum

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