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Old 12th January 2013, 01:57 PM   #31
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Originally Posted by Shinja View Post
on AD797's datasheet, there is a description such decoupling scheme.
Yes, but notice that the very small value carbon resistor is used to minimize the lead inductance of a large tantalum capacitor, not the smaller 0.1uF bypass capacitor. This technique appears to only be useful when using relatively large bypass capacitors and only if you use a carbon resistor, or some other type that has very low inductance at radio frequencies.
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Old 12th January 2013, 02:05 PM   #32
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If the inductance contributed by the resistor concerns you then select a smaller package size - 0603 perhaps or 0402 if your eyesight is good enough. Its the total loop inductance that's important, not just that of the resistor - so if the resistor adds to the loop area it will increase the inductance. OTOH if the resistor's just replacing a wire that would be there anyway, it won't add inductance. This is assuming normal thick film SMT resistors of course, not wirewound ones.
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Old 12th January 2013, 02:16 PM   #33
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Originally Posted by abraxalito View Post
If the inductance contributed by the resistor concerns you then select a smaller package size - 0603 perhaps or 0402 if your eyesight is good enough. Its the total loop inductance that's important, not just that of the resistor - so if the resistor adds to the loop area it will increase the inductance. OTOH if the resistor's just replacing a wire that would be there anyway, it won't add inductance. This is assuming normal thick film SMT resistors of course, not wirewound ones.
I think you misunderstand. This does not concern me at all. I won't be using this technique because I don't think there is enough benefit to bother. TI does not suggest the technique for their extremely high bandwidth BUF634 for example. Maybe they don't know as much as AD, I don't know.

I assumed that the datasheet portion under discussion applied to through hole devices (since the AD797 is available that way) which have more lead inductance than SMD's. The datasheet specifies a carbon resistor.
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Old 12th January 2013, 02:28 PM   #34
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Originally Posted by dirkwright View Post
I think you misunderstand. This does not concern me at all. I won't be using this technique because I don't think there is enough benefit to bother.
That's totally fine by me - your reasoning though wasn't particularly sound because in many cases the loop area isn't increased by adding the resistor, so the 'extra inductance' argument doesn't hold water. That's what concerned me in giving my response. If the part being decoupled is in a DIL8 package for example, both the cap and resistor comfortably fit beneath the chip without adding inductance. That's how I decouple TDA1545 DACs when in DIL packages.

The use of snubbers is less important for decoupling purely analog circuits that aren't handling fast rise time signals and hence aren't taking big, fast gulps of current like CMOS digital chips do. I do recommend them for such applications, on pure analog handling audio band signals its a toss-up whether there's any benefit.
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Old 12th January 2013, 02:38 PM   #35
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Originally Posted by abraxalito View Post
That's totally fine by me - your reasoning though wasn't particularly sound because in many cases the loop area isn't increased by adding the resistor, so the 'extra inductance' argument doesn't hold water. That's what concerned me in giving my response. If the part being decoupled is in a DIL8 package for example, both the cap and resistor comfortably fit beneath the chip without adding inductance. That's how I decouple TDA1545 DACs when in DIL packages.

The use of snubbers is less important for decoupling purely analog circuits that aren't handling fast rise time signals and hence aren't taking big, fast gulps of current like CMOS digital chips do. I do recommend them for such applications, on pure analog handling audio band signals its a toss-up whether there's any benefit.
For THD's, the leads of the added resistor have inductance at RF, so I can't see how they would be of much benefit in that particular case. Of course, SMD's are a different story. I prefer THD's because my hands are not nearly as steady as they used to be.

Obviously, digital circuits have different power supply requirements than analog ones, and it's debatable whether or not an opamp in an IV converter for a DAC sees enough digital surges to warrant this kind of snubber circuit. I would like to see some hard data on that before bothering with it. Do you have any?
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Old 12th January 2013, 02:47 PM   #36
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Originally Posted by dirkwright View Post
For THD's, the leads of the added resistor have inductance at RF, so I can't see how they would be of much benefit in that particular case. Of course, SMD's are a different story. I prefer THD's because my hands are not nearly as steady as they used to be.
Fair enough, I only use SMT passive parts these days (other than where I need major power dissipation in resistors or large-valued inductors) and was only speaking of SMT. If the resistor is through-hole then yes its going to add inductance so I can see why you'd not be interested.

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Obviously, digital circuits have different power supply requirements than analog ones, and it's debatable whether or not an opamp in an IV converter for a DAC sees enough digital surges to warrant this kind of snubber circuit. I would like to see some hard data on that before bothering with it. Do you have any?
What kind of 'hard data' did you have in mind? I'm not interested in making measurements or scope shots of any differences from adding series resistors if that's your wish. After all it would be totally irrelevant data to you as you're not into SMT.
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Old 12th January 2013, 02:56 PM   #37
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Originally Posted by abraxalito View Post
What kind of 'hard data' did you have in mind? I'm not interested in making measurements or scope shots of any differences from adding series resistors if that's your wish. After all it would be totally irrelevant data to you as you're not into SMT.
I may be forced to use SMD's in the near future, so I have some interest in this idea. The LME49990 is particularly difficult to implement, according to their data sheet (and comments by others), for example, and this technique may be useful for that part. I'd be interested if you know of someone who has made this comparison if you don't have it. I assume that Analog Devices didn't invent this idea out of thin air but instead came up with it to address a specific problem they were having with the AD797. I understand the theory of course, but would like to see data on how much of a difference it makes. I can try to do a Google search myself but I am not hopeful about finding something useful.
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Old 12th January 2013, 03:02 PM   #38
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Power supply decoupling would be the least of your worries if you're considering either of those opamps in close proximity to anything digital as I understand they don't co-exist very happily with RF. Walt Jung (who works for ADI) even designed out the AD797 in his regulator as it misbehaved when powering digital stuff.

<edit> Here's a paper which anyone considering an opamp with input LTP for I/V would do well to read and digest : http://www.uemc.polito.it/papers/opampsusc_01.pdf
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Old 12th January 2013, 04:02 PM   #39
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Originally Posted by abraxalito View Post
Power supply decoupling would be the least of your worries if you're considering either of those opamps in close proximity to anything digital as I understand they don't co-exist very happily with RF. Walt Jung (who works for ADI) even designed out the AD797 in his regulator as it misbehaved when powering digital stuff.

<edit> Here's a paper which anyone considering an opamp with input LTP for I/V would do well to read and digest : http://www.uemc.polito.it/papers/opampsusc_01.pdf
Thanks for the paper.

No, I'm not considering either opamp for an IV converter for a DAC. The IV converter I have is based on the LME49710.
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Old 12th January 2013, 04:08 PM   #40
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Out of that ex-National range the only part I'd recommend for I/V duty would be the LME49713 because its CFB therefore no LTP to suffer the assault of all that RF.
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