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Old 27th October 2012, 06:57 PM   #11
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Hmm, that's interesting. Generally the application notes in datasheets from good companies like Analog are quite solid but this case is a bit concerning. Earlier on the data sheet says to use figure 39 for the same purpose and the later text refers to Cf in figure 53 being connected to ground, which it isn't. I've accidentally implemented a similar circuit with a comparable op amp (LME49990) and it oscillated until I removed the cap between the op amp inputs. Even if it doesn't oscillate on the A797 I still would still be wary of low phase margin.

By the way, it's not usually desirable operate DACs below the minimum load resistance called out in the datasheet. Exactly how cranky it makes them varies from part to part but in the best case you'll still see an increase in THD compared to an in spec load.

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Originally Posted by GLENZWORLD View Post
Is feedback resistance and capacitance part of output load?
It does load the output but usually the feedback loop(s) are analayzed separately from other loading considerations.

Last edited by twest820; 27th October 2012 at 07:09 PM.
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Old 28th October 2012, 08:23 AM   #12
AndrewT is offline AndrewT  Scotland
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A few manufacturers warn against hanging any capacitance off the -IN pin. Even to the extent of being wary of parasitic capacitance from the PCB traces around the _IN pin trace.

I see this on many chipamps and recommend they remove the capacitor to -IN pin connection.
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Old 28th October 2012, 09:20 AM   #13
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Old 29th October 2012, 09:42 PM   #14
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Quote:
Originally Posted by twest820 View Post
Hmm, that's interesting. Generally the application notes in datasheets from good companies like Analog are quite solid but this case is a bit concerning. Earlier on the data sheet says to use figure 39 for the same purpose and the later text refers to Cf in figure 53 being connected to ground, which it isn't. I've accidentally implemented a similar circuit with a comparable op amp (LME49990) and it oscillated until I removed the cap between the op amp inputs. Even if it doesn't oscillate on the A797 I still would still be wary of low phase margin.

By the way, it's not usually desirable operate DACs below the minimum load resistance called out in the datasheet. Exactly how cranky it makes them varies from part to part but in the best case you'll still see an increase in THD compared to an in spec load.

It does load the output but usually the feedback loop(s) are analayzed separately from other loading considerations.
thanks again for the input and I did notice tda1549 min output r load is 3k but with combined resistance of r1 and rf as well as 797 input impedance being very high I thought it might still work

I also noticed in text cf shunting noise to ground and did not understand but I am a noob so there is lots I don't understand and phase margin is one.

If I keep the phase flat out to 20 to 23 khz with gain -3db about 160khz I think will work on active filter but inverting configuration what should I try to achieve for phase margin?

and what of group delay? if it shows peaking is that bad?

incidentally I did try a cap allready in that spot across inputs and got very audible distortion. that was with rs 260ohm and .022uf giving pole of 28 khz

when I removed it cleared up so I think you are right and I may need closer to fig 39 which is also closer to original curcuit.

I am still wondering if someone was able to bypass dac op amps (internal) on tda1549 and use the 797 i/v externally then strait to rca maybe passive filtèr between

I will try to use labview see what it says

thanks again
Glen
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Old 29th October 2012, 09:45 PM   #15
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Quote:
Originally Posted by AndrewT View Post
A few manufacturers warn against hanging any capacitance off the -IN pin. Even to the extent of being wary of parasitic capacitance from the PCB traces around the _IN pin trace.

I see this on many chipamps and recommend they remove the capacitor to -IN pin connection.
thanks I am just learning and found the same by accident

see above post
Glen
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Old 11th November 2012, 08:25 PM   #16
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Originally Posted by GLENZWORLD View Post
what of group delay? if it shows peaking is that bad?
If you refer to the definition of a Bessel alignment you'll see any non-Bessel alignment exhibits peaking. How much peaking is acceptable depends on the design requirements but, if phase deviation is kept below subjectively audible limits, any associated peaking is most likely also inaudible. I'm not, however, aware of any rigorous studies of this in a DAC context. (The closest results I know of are Siegfried Linkwitz's investigation of group delay audibility in LR2 and LR4 crossovers but there are so many other factors involved there---most notably driver cone breakup and operation at frequencies where the ear has better phase descrimination---I wouldn't consider the findings particularly relevant here.)
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Old 13th November 2012, 12:40 PM   #17
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even though AD call AD797 unity gain stable and it can in fact be used that way, it takes quite some skill to get it behaving properly and its definitely not a dropin replacement, which is why the AD797's popularity with opamp rollers horrifies me! some of them even running them as buffers, one guy I saw paralleled 4 of them per channel, airwired without any change in compensation, in fact without any comp at all...

its a nice chip, I like it a lot, but with many new much better behaved chips available now it really doesnt make much sense to use it for low, or unity gain applications driving cables, headphones etc.

I would also go for something like the lme49990
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Old 13th November 2012, 05:04 PM   #18
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Quote:
Originally Posted by twest820 View Post
If you refer to the definition of a Bessel alignment you'll see any non-Bessel alignment exhibits peaking. How much peaking is acceptable depends on the design requirements but, if phase deviation is kept below subjectively audible limits, any associated peaking is most likely also inaudible. I'm not, however, aware of any rigorous studies of this in a DAC context. (The closest results I know of are Siegfried Linkwitz's investigation of group delay audibility in LR2 and LR4 crossovers but there are so many other factors involved there---most notably driver cone breakup and operation at frequencies where the ear has better phase descrimination---I wouldn't consider the findings particularly relevant here.)
thanks again for your reply

I have been looking at this and thinking I cound change op to something else or try it in that curcuit or try bypassing internal opamps on tda1549 and use ad797 strait from dac out then to rca (maybe passive filter between)

supposedly Rudolf Broertjes did this (best known for his dac i/v) but he has not replied to me so I may just try it.

here is basic schematic


Glen
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Old 13th November 2012, 05:12 PM   #19
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Originally Posted by qusp View Post
even though AD call AD797 unity gain stable and it can in fact be used that way, it takes quite some skill to get it behaving properly and its definitely not a dropin replacement, which is why the AD797's popularity with opamp rollers horrifies me! some of them even running them as buffers, one guy I saw paralleled 4 of them per channel, airwired without any change in compensation, in fact without any comp at all...

its a nice chip, I like it a lot, but with many new much better behaved chips available now it really doesnt make much sense to use it for low, or unity gain applications driving cables, headphones etc.

I would also go for something like the lme49990
yes all us noobs did not realize when you change the op amp some need the filter and other values to change

I am going to buy a used oscilloscope soon and then I will know for sure what the circuit is doing

see above for new plan of attack which I think ad797 is better suited but I need to check gains with and without internal ops on tda1549

I don`t think they are unity and even in the first curcuit it is not unity 1.46 . I know not far from unity

thanks for the reply
Glen
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Old 13th November 2012, 05:55 PM   #20
qusp is offline qusp  Australia
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thats close enough to just call unity.... so I did... really without careful layout and without some additions, I wouldnt use it lower than 4x. AD797 not properly implemented will be handily beaten by lme49990, which is a fairly bulletproof chip too, cheap as well.
AD797 even has completely different pins that need to be used to gain best performance from it (pin 8) that are probably just tied to ground or unconnected on your layout. once you combine figure 39. figure 42. and figure 44 (along with the recommended psu bypassing) without a suitable PCB..... I dont think your noise performance will be lower than lme49990, which requires none of that (except the bypassing)

great chip, but its a princess (sorry Scott =) you know its true)

Last edited by qusp; 13th November 2012 at 06:00 PM.
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