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Old 31st August 2012, 06:35 PM   #81
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by Ken Newton View Post
Dirk, I must admit that I stopped experimenting with IC op-amp based I/V circuits quite some time ago. I achieved by far the best sound quality via a simple passive (resistor) I/V which is subsequently A.C. coupled directly (without any intervening active circuit) to my discrete FET based linestage for voltage amplification. Although, the linestage itself is functionally a discrete op-amp. The transparency of this configuration far surpassed the current-feedback IC op-amp based I/V converter I was using until then. My FET linestage has plenty of gain for this application (the fullscale signal from the DAC is about 0.4VRMS), so an active I/V stage located on board the DAC would be one active stage too many. So, I couldn't honestly offer you an opinion on the latest current-feedback IC op-amp based I/V implementations.
Hi Ken .
Can you give us details about the dac chip you use, and the value of the iv resistor?
I am now trying new possibilities in my discrete balanced iv. And I am thinking in using a 5ohms resistor iv. Do you think is to much ?
If I use the pcm1794 in mono I can use a 2,5ohm resistor.
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Old 31st August 2012, 06:39 PM   #82
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I think an interesting option would be to use a low value passive resistor IV like you mention, and follow it with a normal gain stage. I think the noise will be higher, but maybe not enough to be a bother.
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Old 31st August 2012, 07:00 PM   #83
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There is already a lot of designs that use passive iv resistors. And I want to try it too. WY do you think the noise will be higher, 5 ohms seems a very low value.
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Old 31st August 2012, 07:11 PM   #84
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I don't think the PCM1792/4s like driving a load impedance of anything other then something very low at all. I did experiment with using little resistors, placed just before the I/V opamps, on the output of the DAC and even with small values the performance was degraded.

I think there is probably a very good reason why most current output DACs either use a virtual ground, or tie the non inverting input to some common voltage, either supplies by the DAC or an external source.

Really though, there is very little wrong with the implementation in the TI datasheet. It might be boring, but if you choose opamps that are not fazed by a capacitive load and have enough output current to drive a 600 ohm load comfortably, then you're not going to go wrong. Due to the cap in the feedback path limiting the bandwidth that the opamp is asked to work with, you don't need one with a particularly high slew rate either.

Of course the gentle low pass filtering is necessary to remove the noise from the DAC too and should lower the overall EMI, so I don't see why people are interested in trying to get rid of it and then need fast opamps as a result to cope with the high frequencies that are then let through.

Now you can use opamps that aren't happy driving capacitive loads providing that you isolate the cap with a small series resistor (22-100R). In the given application the opamps are also working at unity gain, so I don't think you even need opamps that are that quiet either.

Having worked extensively with the PCM1792 I know that the PCB is extremely important in actually allowing it to provide the performance that it is capable of. Although looking extensively at the IV stage is a decent idea, I'd be interested at first, in just getting it to performance how it should given the TI application, that was, at the end of the day, used for the measurements given in the datasheet.
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Old 31st August 2012, 07:13 PM   #85
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Quote:
Originally Posted by smms73 View Post
There is already a lot of designs that use passive iv resistors. And I want to try it too. WY do you think the noise will be higher, 5 ohms seems a very low value.
Actually, when I look at this more closely, I think I'm mistaken. If you adjust the IV resistor and the feedback resistor to obtain the same output of 1.41Vrms, the noise is much worse with a small size IV resistor and a large feedback resistor compared to a large IV resistor and a small feedback resistor. The one shown is pretty bad. If the IV resistor is changed to 1k and the feedback resistor is also changed to 1k, the output voltage is the same but the noise is far lower. Of course, we can't use a 1k iv resistor, so this is just an abstract test. The current generator was set to 1mA sine wave.
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Last edited by dirkwright; 31st August 2012 at 07:23 PM.
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Old 31st August 2012, 07:18 PM   #86
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Quote:
Originally Posted by 5th element View Post
I don't think the PCM1792/4s like driving a load impedance of anything other then something very low at all. I did experiment with using little resistors, placed just before the I/V opamps, on the output of the DAC and even with small values the performance was degraded.
Yeah, I saw that in some of the application notes for other current output DACs. They had a resistor going from the - pin to either ground or a voltage reference. I tried that in Tina and the noise went sky high. Very bad idea.

I think you're right about the other points you make as well.
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Old 31st August 2012, 07:33 PM   #87
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Quote:
Originally Posted by 5th element View Post
I don't think the PCM1792/4s like driving a load impedance of anything other then something very low at all. I did experiment with using little resistors, placed just before the I/V opamps, on the output of the DAC and even with small values the performance was degraded.

I think there is probably a very good reason why most current output DACs either use a virtual ground, or tie the non inverting input to some common voltage, either supplies by the DAC or an external source.

Really though, there is very little wrong with the implementation in the TI datasheet. It might be boring, but if you choose opamps that are not fazed by a capacitive load and have enough output current to drive a 600 ohm load comfortably, then you're not going to go wrong. Due to the cap in the feedback path limiting the bandwidth that the opamp is asked to work with, you don't need one with a particularly high slew rate either.

Of course the gentle low pass filtering is necessary to remove the noise from the DAC too and should lower the overall EMI, so I don't see why people are interested in trying to get rid of it and then need fast opamps as a result to cope with the high frequencies that are then let through.

Now you can use opamps that aren't happy driving capacitive loads providing that you isolate the cap with a small series resistor (22-100R). In the given application the opamps are also working at unity gain, so I don't think you even need opamps that are that quiet either.

Having worked extensively with the PCM1792 I know that the PCB is extremely important in actually allowing it to provide the performance that it is capable of. Although looking extensively at the IV stage is a decent idea, I'd be interested at first, in just getting it to performance how it should given the TI application, that was, at the end of the day, used for the measurements given in the datasheet.
Thanks very much for your answer. I really appreciate.
Can you please look at the circuit in post #3 , and tell me what you think, I really like to use buffers ween there is a lot of current swing.
they don't degrade the open loop phase margin. The OP amps can be different the ones shown.
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Last edited by smms73; 31st August 2012 at 07:39 PM. Reason: error
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Old 31st August 2012, 07:38 PM   #88
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I was wondering if adding a buffer in the feedback loop for the CFB opamp LME49713 would make it possible to use a much lower feedback resistor.
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Old 31st August 2012, 07:43 PM   #89
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I think not but you can try an input zobel. Cap over feedback resistor does not work with CFB opamps.
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Old 31st August 2012, 07:44 PM   #90
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Originally Posted by Joachim Gerhard View Post
I think not but you can try an input zobel. Cap over feedback resistor does not work with CFB opamps.
Yeah, I was afraid of that. I'm aware of the capacitor in the feedback network issue. Thanks.
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