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Wishlist: Op Amp Characterization Curves in Datasheets
Wishlist: Op Amp Characterization Curves in Datasheets
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Old 19th December 2014, 11:02 PM   #1
johnc124 is online now johnc124  United States
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Default Wishlist: Op Amp Characterization Curves in Datasheets

Since it's the holiday season I thought a wishlist thread would be appropriate! So what additional characterization curves do you wish to see added to op amp datasheets in the future?

I'll start the list:
THD for high source impedances to characterize input impedance linearity
FFTs of the output distortion to show which harmonics are dominant
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Old 20th December 2014, 12:19 AM   #2
Mark Johnson is offline Mark Johnson  United States
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Wishlist: Op Amp Characterization Curves in Datasheets
The important part of this idea is the quantitative answer to the question "How much will it increase our sales, and how quickly?"
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Old 20th December 2014, 12:20 AM   #3
jcx is offline jcx  United States
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you could just pick from Groner's op amp distortion file
SG-Acoustics Samuel Groner IC OpAmps
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Old 25th December 2014, 10:21 PM   #4
AndrewT is offline AndrewT  Scotland
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if the opamp datasheets are too honest the result can be reduced sales.

A manufacturer like Analog show a lot more than many others because they are confident in the quality of their product compared to equivalents from their competitors.
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regards Andrew T.
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Old 26th December 2014, 04:16 AM   #5
dchisholm is offline dchisholm  United States
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Quote:
Originally Posted by johnc124 View Post
. . . I'll start the list:
THD for high source impedances to characterize input impedance linearity
Quote:
Originally Posted by jcx View Post
you could just pick from Groner's op amp distortion file
SG-Acoustics Samuel Groner IC OpAmps
That's by far the best source of measured data. The last update is now several years old. I'd revise the wishlist entry to something like, "Somebody who has the motivation, access to necessary test equipment, and time, to continue Samuel Groner's work.".

Quote:
FFTs of the output distortion to show which harmonics are dominant
Occasionally (but not often) you see separate curves for second and third harmonics. See, for example, Figure 28 (page 13) in the THS6012 Data Sheet at http://www.ti.com/lit/ds/symlink/ths6012.pdf .

Dale
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Old 31st December 2014, 02:54 PM   #6
johnc124 is online now johnc124  United States
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Originally Posted by AndrewT View Post
if the opamp datasheets are too honest the result can be reduced sales.

A manufacturer like Analog show a lot more than many others because they are confident in the quality of their product compared to equivalents from their competitors.
This is a good point. But I want to remind everyone: I work for Texas Instruments, on the Precision Amplifiers team. So I'm well aware of the downsides of over-characterization. There's also the downside of too much data diluting the important metrics. An example of this is older op amp datasheets which have a spec table for every possible power supply voltage combination. It gets old quick...

My intent for this thread was simply to see how we (TI) can do better in the future. There is quite a bit of analog engineering talent on here, and I felt it was prudent to reach out to the diyaudio community and get some feedback on what you would like to see in the future. I agree that Samuel Groner's work is really impressive, but is it 100% complete? Is there no other testing you would like to see done? Regardless of the perceived effect it might have on op amp sales.

On the topic of curves for individual harmonics, the separate curves for the the 2nd and 3rd harmonics are common in high-speed amplifiers, but I actually find them misleading. The reason is that it excludes higher order harmonics. Often I see the 5th harmonic become dominant in an op amp output stage when it is required to deliver significant current.
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Old 31st December 2014, 03:27 PM   #7
dmills is offline dmills  United Kingdom
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One thing I would like to see, and it really is a one liner is detail on which supply pin the Vas integrator capacitance is referenced to, this is really helpful when thinking about supply decoupling as the presence of that capacitor makes this pin an additional input, the information is sometimes out there, but it can be hard to track down.

Distortion Vs load at somewhere other then 1KHz would also be appreciated, knowing the sand can drive 600R to +20dBu is nice, knowing it can still do that at 20Khz is nicer.

High frequency IMD would be interesting, as would open loop output impedance, mainly for looking at things like reconstruction filters where there may be significant energy present well up into the MHz region, where some parts run out of GBP.
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Old 31st December 2014, 03:36 PM   #8
johnc124 is online now johnc124  United States
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Originally Posted by dmills View Post
One thing I would like to see, and it really is a one liner is detail on which supply pin the Vas integrator capacitance is referenced to, this is really helpful when thinking about supply decoupling as the presence of that capacitor makes this pin an additional input, the information is sometimes out there, but it can be hard to track down.

Distortion Vs load at somewhere other then 1KHz would also be appreciated, knowing the sand can drive 600R to +20dBu is nice, knowing it can still do that at 20Khz is nicer.

High frequency IMD would be interesting, as would open loop output impedance, mainly for looking at things like reconstruction filters where there may be significant energy present well up into the MHz region, where some parts run out of GBP.
These are fantastic inputs! To you first point, the big indicator is going to be which power supply rail has worse PSRR. But the internal topology may not allow for a simple 1 line answer in the datasheet.

Great point on the open loop output impedance. All of our newer op amps include this curve (open loop, NOT closed loop). The effect the open loop output impedance can have on high frequency attenuation (especially in Sallen-Key filters) can be significant.
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Old 31st December 2014, 04:04 PM   #9
jcx is offline jcx  United States
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datasheet measured PSRR, CMRR and open loop gain should follow the relation in “A General Relationship Between Amplifier Parameters, And Its Application to PSRR Improvement” E Sackinger, J Groette, W Guggenbuhl, IEEE Trans CAS vol 38, #10 10/83 pp 1171-1181

this may require adopting a test fixture for the CMRR, PSRR # that actually measures them independent of the op amp's gain curve - Pease wrote some on this


I would like to see updated minimums for popular oldies like LM3886 which must have moved to new processes with presumably much tighter parameter spreads since the original datasheets


manufacturer supplied free Spice models also suck - nobody seems to even use their better Spice Modeling app note macromodel techniques consistently
and input parasitic Z, CM parts should go to the respective power supply pins instead of the fictitious Spice node 0 - which has no place in a op amp model

some seem to have given up altogether at output Z modeling above the unity gain intercept
to be useful for stability analysis Spice models should be representative up to 10x the gain intercept

I have complained, got a "response" that trimmed the Spice model unity gain intercept phase to <1 degree of the datasheet value while being off >20 degrees just an octave away - below the intercept frequency!

Quote:
Originally Posted by jcx View Post
actually open loop gain can change noticeably with loading vs totally unloaded - factors of 2 aren't impossible


macromodeling has always been hit or miss - often the "new guy" may be assigned to familiarize him with the products - even if he's never built a model before, hasn't even read the app notes on modeling

http://www.analog.com/static/importe...tes/AN-138.pdf

http://www.ti.com/lit/an/sbfa009/sbfa009.pdf

http://www.ti.com/lit/an/snoa265b/snoa265b.pdf

http://www.ti.com/lit/an/snoa247a/snoa247a.pdf

just checking the pin assignment on the op amp package shows any model with spice node "0" in it is suspect - particularly with any sub component of the model directly connecting from the op amp pins to Spice gnd

even the notes above often get this wrong with CM Z, leakage parts going to gnd

what is frustrating is the bad use of node "0"/gnd in modeling has been clear from before monolithic op amps and Spice - as early Philbrick/Analog Devices papers show input parasitics/compensation referenced to the power supply pins of the op amp modules
schematics of the Spice macromodels also make for easier analysis of their usefulness by experienced analog circuit designers - just the Spice text listing is very opaque and suggests someone is just blindly computer generating it

Last edited by jcx; 31st December 2014 at 04:29 PM.
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Old 31st December 2014, 05:09 PM   #10
dmills is offline dmills  United Kingdom
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Quote:
Originally Posted by jcx View Post
input parasitic Z, CM parts should go to the respective power supply pins instead of the fictitious Spice node 0 - which has no place in a op amp model
Amen to that.

Opamp spice models seem to (more often then not) assume an opamp is a three terminal device, not a (at least) 5 terminal one, it is a pain.

Regards, Dan.
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