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Old 31st August 2012, 07:56 PM   #91
smms73 is offline smms73  Portugal
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Dirk is best to use buffers with low speed OP amp's. If you use buffers with high speed OP amps you can degrade the phase margin enough to make the circuit unstable. But as 5 element says is no need to use fast amplifiers in the iv section. The NE5534 is enough.
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Old 31st August 2012, 07:59 PM   #92
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This seems to work good. I'm seeing something like 1.5uVrms at the - input pins for 7.8mA p-p current output from the generator. So, they are really low impedance. Distortion and noise are both very good. Obviously, the lower we can make the feedback resistor in the IV stage, the lower the input impedance and noise will be in the circuit. Buffers!!! we need MORE buffers!!!! hahahaha.
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Old 31st August 2012, 08:00 PM   #93
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Originally Posted by smms73 View Post
Dirk is best to use buffers with low speed OP amp's. If you use buffers with high speed OP amps you can degrade the phase margin enough to make the circuit unstable. But as 5 element says is no need to use fast amplifiers in the iv section. The NE5534 is enough.
I think the LME49990 is a better choice because of lower noise and distortion, at least I think that's right.
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Old 31st August 2012, 08:05 PM   #94
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The NE5534 has a surprising good transfer linearity in shunt mode so distortion may not be lower when you use LME49990. It has other advantages though like very good PSRR, lower voltage noise, higher speed and a better output circuit.
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Old 31st August 2012, 08:10 PM   #95
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Originally Posted by Joachim Gerhard View Post
The NE5534 has a surprising good transfer linearity in shunt mode so distortion may not be lower when you use LME49990. It has other advantages though like very good PSRR, lower voltage noise, higher speed and a better output circuit.
OK, maybe in real life it's better, but not in my simulator. The NE5534 is slightly worse, though very close, to the performance of the LME49990.

I like the idea of using buffers because we can then lower the feedback resistor even more. One thing I just realized is that the FB resistor is already so low that we are basically driving a headphone. So, we need a buffer that can drive a headphone like the LME49600 in order to have a FB resistor of 150 ohms or so. Unfortunately there are no spice models for that particular chip.

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Old 31st August 2012, 08:28 PM   #96
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Dirk don't forget to use a buffer in the output opamp , its important.
Try to simulate with the discrete buffers I use in post #3.
But use the php ones at input and npn ones at the output opamp like the circuit.

Buffers also decrease output impedance and distortion of the opamp as they increase open loop gain. So you get a better opamp for cents.
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Old 31st August 2012, 08:34 PM   #97
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I think the schematic in post three is essentially what Dirk has done in post 92 - follow the opamp with a buffer stage of some sort that will isolate the opamps output from the feedback resistor and the capacitive load of the cap in parallel. This allows you to lower the value of the feedback resistor and keep the performance high and perhaps lower the overall system noise.

I do not see why this wouldn't work unless the very fast current feedback opamp has a problem with the added complexity around it and it might oscillate. That's never stopped anyone in the past mind you and building it might be the only way of testing it properly.

As I mentioned before though, looking at the distortion numbers of a simulator is all well and good, but in this actual situation I'd be more interested in seeing people building and measuring. This is mostly because, as I said before, you need to get things just right, otherwise the performance of the DAC itself will be compromised and it's that that could very well be the bottleneck and not the surrounding circuitry.

You may very well find that the added complexity yields nothing in terms of the actual measurements. Kind of like swapping in an ultra low noise opamp only for the Johnson noise to keep dominating for no real net improvement. I do like the idea of providing a buffer that can allow the opamp to function purely in class A, of course the buffer needs to be biased into class A too otherwise what's the point
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Old 31st August 2012, 08:49 PM   #98
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Originally Posted by smms73 View Post
Dirk don't forget to use a buffer in the output opamp , its important.
Try to simulate with the discrete buffers I use in post #3.
But use the php ones at input and npn ones at the output opamp like the circuit.

Buffers also decrease output impedance and distortion of the opamp as they increase open loop gain. So you get a better opamp for cents.
Can your buffer drive 150 ohms cleanly? That's what I need to know.

Thanks for the reminder about the output opamp. Will do chief!
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Old 31st August 2012, 08:52 PM   #99
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Originally Posted by 5th element View Post
I think the schematic in post three is essentially what Dirk has done in post 92 - follow the opamp with a buffer stage of some sort that will isolate the opamps output from the feedback resistor and the capacitive load of the cap in parallel. This allows you to lower the value of the feedback resistor and keep the performance high and perhaps lower the overall system noise.

I do not see why this wouldn't work unless the very fast current feedback opamp has a problem with the added complexity around it and it might oscillate. That's never stopped anyone in the past mind you and building it might be the only way of testing it properly.

As I mentioned before though, looking at the distortion numbers of a simulator is all well and good, but in this actual situation I'd be more interested in seeing people building and measuring. This is mostly because, as I said before, you need to get things just right, otherwise the performance of the DAC itself will be compromised and it's that that could very well be the bottleneck and not the surrounding circuitry.

You may very well find that the added complexity yields nothing in terms of the actual measurements. Kind of like swapping in an ultra low noise opamp only for the Johnson noise to keep dominating for no real net improvement. I do like the idea of providing a buffer that can allow the opamp to function purely in class A, of course the buffer needs to be biased into class A too otherwise what's the point
I've switched to VFB opamps on the advice of Joachim.

I'm slowly gathering the gear I need to do really good measurements. It takes time and it's expensive.

Thanks for your input!
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Old 31st August 2012, 08:56 PM   #100
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Originally Posted by 5th element View Post
I think the schematic in post three is essentially what Dirk has done in post 92 - follow the opamp with a buffer stage of some sort that will isolate the opamps output from the feedback resistor and the capacitive load of the cap in parallel. This allows you to lower the value of the feedback resistor and keep the performance high and perhaps lower the overall system noise.

I do not see why this wouldn't work unless the very fast current feedback opamp has a problem with the added complexity around it and it might oscillate. That's never stopped anyone in the past mind you and building it might be the only way of testing it properly.

As I mentioned before though, looking at the distortion numbers of a simulator is all well and good, but in this actual situation I'd be more interested in seeing people building and measuring. This is mostly because, as I said before, you need to get things just right, otherwise the performance of the DAC itself will be compromised and it's that that could very well be the bottleneck and not the surrounding circuitry.

You may very well find that the added complexity yields nothing in terms of the actual measurements. Kind of like swapping in an ultra low noise opamp only for the Johnson noise to keep dominating for no real net improvement. I do like the idea of providing a buffer that can allow the opamp to function purely in class A, of course the buffer needs to be biased into class A too otherwise what's the point
Thanks very much for your time.
Believe me the buffers are working in class A .

As I say before is not very wise to mess the close loop of a fast amplifier.
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