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Rise time and slew rate
Rise time and slew rate
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Old 21st October 2013, 10:33 PM   #1
akis is offline akis  United Kingdom
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Default Rise time and slew rate

There is another thread dating back to 2005, so I will ask in this new thread.

I have tried to measure the slew rate on my op-amps. I used two tests. One was a square wave oscillator, from rail to rail (or as much as the op-amp can do). The other test is a straight forward 25x non-inverting amplifier.

With the square oscillator I got these results:

PART	Test1	Test2	Documen
TLE2072	31	22	45
LM4562	19	18	20
LT1361	159	140	800
TL082	12	11	n/a
MC33072	13	11	13
OPA2134	22	28	20
LM833	9	7	7
With the 25x amplifier I got this:

PART	Test1	Documen
TLE2072	12	45
LM4562	17	20
LT1361	39	800
TL082	4	0
MC33072	5	13
OPA2134	11	20
LM833	8	7
I am not sure if I am doing this right. Especially for the plain 25x amplifier, I am finding that the "600V/us slew rate" LT1361 does not live up to the promise.

The other amps, in the square wave oscillator mode are quite near the documented values.
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Old 22nd October 2013, 03:05 AM   #2
stratus46 is offline stratus46  United States
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Why do you care? I've never found a properly functioning opamp that _didn't_ meet the specs on the data sheet. What I can tell you is it's real easy to run OUT of slew rate when using opamps in active filters and high gain stages. You need to be more careful with high speed amps as parasitics can make life difficult but proper bypassing and keeping the impedances low helps a LOT.

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Old 22nd October 2013, 03:23 AM   #3
abraxalito is online now abraxalito  United Kingdom
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Originally Posted by akis View Post
I am not sure if I am doing this right. Especially for the plain 25x amplifier, I am finding that the "600V/us slew rate" LT1361 does not live up to the promise.
You'd do well to pay attention to the DS for this part, in particular page 10 'Circuit operation'. It refers back to this graph of slew rate vs input level.
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'The total potential here must be nothing less than astronomical.'
'Nothing less. The number 10 raised almost literally to the power of infinity.'
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Old 22nd October 2013, 07:05 AM   #4
akis is offline akis  United Kingdom
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I see from the specs now that the LT1361 specifies the slew rate, mostly, as a function of the input level, and unfortunately my signal generator has a max slew rate of 60V/us. In the square wave oscillator arrangement, the LT1361 did reach 159V/us however.

As you can see in the table below, the only op-amp that can follow the input curve faithfully is the LT1361. However other op-amps have a more linear curve, with no overshoots and no kinks on the way up.

buffer Vin=10Vpp +/-15V SR=60V/us AV=1			
TLE2072	35		45
LM4562	16		20
LT1361	57		800
TL082	12		0
MC33072	10		13
OPA2134	19		20
LM833	7		7
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Old 22nd October 2013, 05:59 PM   #5
Loudthud is offline Loudthud  United States
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Opamp specifications are quoted under very specific operating conditions. If you change the conditions, the specification will likely change. Texas Instruments quotes the slew rate for the TL082 with the amp connected to +/- 15V rails as a voltage follower and a load of 2K in parallel with 100pF. No mention of the signal amplitude but figure 19 of the old spec sheet I'm looking at shows the pulse response under the same conditions swinging +5 to -5V in about 0.75uS. That's 13.3V/uS. 13V/uS is the typical number, 8V/uS is quoted as minimum.
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Old 24th October 2013, 04:20 AM   #6
gootee is offline gootee  United States
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We can't quite be sure what it is that you are actually testing. It looks like you are possibly testing your circuits and their implementations, as much as the op amps.

A good high-speed circuit requires careful design, well beyond the schematic. In some cases, each different type of high-speed op amp could need both a tailored schematic design and a tailored physical implementation, to reach its maximum slew rate.

Usually, if your circuit or layout can't get the lower-speed op amps up to their rated slew rates, then it has no hope with the faster types.

But we don't know what you tried, which brings up a major point:

When presenting test results, it is extremely important to also present the details of the test setup and procedures, with enough detail that your results could be reproduced by anyone else who cared to try it.

JUST(!) for example: For all we know, you did this on (gasp!) a plug-in breadboard, with long wire connections, no ground plane, and large resistor values, etc, etc, which (for the high-speed amplifiers at least) would be wholly-inappropriate and would make your results quite misleading, and invalid relative to your stated objective. [I'm not saying any of that is the case. The point is that there is no way for people to know WHAT led to your results, or, especially for the less-experienced, whether or not blaming the devices themselves is even remotely reasonable or not.]

With the LT1361, even just using a socket, with an otherwise competently-designed PCB and circuit, could be a mistake, because of the additional parasitics.

You probably need to be using a PCB with a high-speed layout (or a bare copper pcboard ground plane and a "dead bug" style point-to-point construction), with short paths, small loop areas, low inductance ground and signal paths, low stray capacitance, low resistance values where needed, excellent decoupling/bypass capacitors and configurations, proper trace impedances and terminations, low-capacitance input layouts (e.g. removal of ground plane sections), proper high-speed measurement techniques and equipment, and more.

Here is some fairly-good but not overly-technical reading on the subject of high-speed amplifier design and layout (and measurement), and (the last one) amplifier slew-rate verification: (Some of it might be overkill, for your desired level of effort. Sorry.)









The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg

Last edited by gootee; 24th October 2013 at 04:34 AM.
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Old 24th October 2013, 12:06 PM   #7
marce is offline marce  United Kingdom
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A 4 layer board is really the minimum to get the best out of high speed (digital or analogue).
I would initially not recommend removing ground plane sections until some experience of these sorts of layouts is gained (removing it from under pads as mentioned in one of the above links can be done, but for audio I don't think you'd notice any difference, and if not done carefully can cause more problems by upping the decoupling caps impedance.
Just doing a 60 channel digitiser using 8 layers, and this gives you even better options for getting the optimum layout, though rather expensive for DIY, but for the ultimate control, signal integrity etc. cant be beaten (except by more layers and thus more ground planes).
For most stuff DIYers will face following best practices without going to overboard will give you optimum results, and a minimum 4 layer PCB is a good start, but is taking things up a notch.
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