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Old 1st May 2015, 05:03 PM   #1
donovas is offline donovas  South Korea
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Default 'slow down' ad797

hello. i build composite headphone amp. ad797 inner loop. but it oscillate. maybe slew rate too much? how do i slow down slew rate? this inverting input- so cap from output to inverting input? 100pf for start? thank you.
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Old 1st May 2015, 05:05 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by donovas View Post
i build composite headphone amp. ad797 inner loop. but it oscillate.
First post a photo of your pcb and the schematic.
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Old 1st May 2015, 06:25 PM   #3
donovas is offline donovas  South Korea
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i p2p this. works with 811. but i socket them both so i can change different opamps. i want slow down 797 so that it work with other opamp.

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Old 1st May 2015, 06:26 PM   #4
donovas is offline donovas  South Korea
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also invert input version instead. it also work. only with 811

change 22pf higher?
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Old 1st May 2015, 06:47 PM   #5
rayma is online now rayma  United States
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Originally Posted by donovas View Post
also invert input version instead. it also work. only with 811 change 22pf higher?
First I'd try removing the 22pF cap, and instead adding a series RC network directly across the 797 input terminals.
Start with a 100 Ohm resistor in series with 2.2nF. If this helps reduce the oscillation, we can further adjust the values.
BTW, make sure that the input is always terminated, either with the signal generator, or a short to ground.
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Old 1st May 2015, 07:22 PM   #6
juma is offline juma  Germany
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Originally Posted by donovas View Post
change 22pf higher?
Change its place - put it between pin 6 and pin2 of AD797. See what you get and adjust its value if needed.
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Old 1st May 2015, 07:57 PM   #7
donovas is offline donovas  South Korea
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Originally Posted by rayma View Post
First I'd try removing the 22pF cap, and instead adding a series RC network directly across the 797 input terminals.
Start with a 100 Ohm resistor in series with 2.2nF. If this helps reduce the oscillation, we can further adjust the values.
BTW, make sure that the input is always terminated, either with the signal generator, or a short to ground.
across +/-? wow, value very high. i will try, thank you

i will try 22pf in someplace else, too.
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Old 1st May 2015, 07:59 PM   #8
rayma is online now rayma  United States
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Originally Posted by donovas View Post
across +/-? wow, value very high. i will try, thank you

i will try 22pf in someplace else, too.
Try without the 22pF first. The RC network adjusts the HF noise gain of the amplifier.
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Old 2nd May 2015, 12:20 AM   #9
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Also - sockets are a big problem with opamps like these!

Those decoupling caps need the shortest, most-direct return to power 0v possible; your 0.1uF caps need to have zero lead length. Sockets add inductance, and ruin things; without really 'tight', well-considered layout I am not surprised you have oscillation problems.

My suggestion - take the AD797 out and try something with considerably less GBW product: NE5534, OPA604 or similar - and get the circuit working as nice as you like. Then develop it for a faster opamp, and refine the physical layout using the lessons learned.
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Old 2nd May 2015, 12:27 AM   #10
rayma is online now rayma  United States
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Originally Posted by martin clark View Post
Those decoupling caps need the shortest, most-direct return to power 0v possible; your 0.1uF caps need to have zero lead length.
Sockets add inductance, and ruin things; without really 'tight', well-considered layout I am not surprised you have oscillation problems.
My suggestion - take the AD797 out and try something with considerably less GBW product: NE5534, OPA604 or similar - and get
the circuit working as nice as you like. Then develop it for a faster opamp, and refine the physical layout using the lessons learned.
Yes, ideally an all SMT layout with a solid ground plane would be best in this case.
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