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Old 8th August 2007, 02:18 PM   #1
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Default Voltage out DAC

Hello everyone,
I'm currently working on a differential DAC based around the AK4396, but I've run into a bit of a problem. Hopefully you will be able to help.

The problem is that the AK4396 have a differential voltage output centered around 2.5V. The normal practise is to feed this into a differential amplifier (most commonly an op amp), which gives a single ended output and recenters it around 0V.

Because this is for a differential DAC, the original plan was to only buffer the outputs and feed them straight to the amplifier. However doing so would effectively halve the voltage swing of the amplifier as it could never get below 0V, which is somewhat undesirable.

While I could theoretically use two op amps, with one of them being inverted, I would prefer to only use discrete components here, to have a better control the different stages. Really good op amps are also quite expensive. I've tried to use a differential amplifier made from discrete components (basically a modified long tailed pair), but without much sucess. Could anyone enlight me on the proper way to do this?

Another idea I had was to feed the AK4396 with -2.5V and +2.5V instead of 0V and 5V. This would mean that a 2.5V output from the DAC would equal 0V for the amplifier. I would prefer not to do this however, think of it more as a last resort.

Are there any other alternatives?

Any help would be most appreciated.
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Old 10th August 2007, 07:17 PM   #2
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Anyone?
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Old 10th August 2007, 07:41 PM   #3
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I don't get your question. Are you feeding a balanced amp? Trying to avoid caps? Just getting from diff to single-ended? Why would a differential amp not work?
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Old 10th August 2007, 07:42 PM   #4
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Just use an OPA1632 and be done with it. Read through the datasheet here:

http://direct.focus.ti.com/lit/ds/symlink/opa1632.pdf

Note:

The output common-mode voltage pin sets the DC
output voltage of the OPA1632. A voltage applied to the
VOCM pin from a low-impedance source can be used to
directly set the output common-mode voltage. For a
VOCM voltage at mid-supply, make no connection to the
VOCM pin.
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Old 11th August 2007, 07:57 AM   #5
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Quote:
Originally posted by analog_sa
I don't get your question. Are you feeding a balanced amp? Trying to avoid caps? Just getting from diff to single-ended? Why would a differential amp not work?
I will be feeding a balanced amp, that's not the problem. The problem is that AK4396 outputs 0V to 5V (i.e. it's centered around 2.5V). I need it to output -2.5V to +2,5V (i.e. centered around 0V). Otherwise I halve the output swing of the amplifier.

The common practice is to use a differential to single ended converter (such as an op amp). This automatically recenters the output at 0V.

I want a balanced output, and while I could use two differential to single ended converters for this (basically creating a differential to differential converter), I want to avoid op amps all together.

I've tried to make a differential amplifier from discrete components (basically a modified long tailed pair), but so far I've only had limited success at best.

If I can't get any of this to work, powering the AK4396 with -2.5V and +2.5V seem to be my only option.
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Old 11th August 2007, 08:38 AM   #6
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Two caps will eliminate the dc bias. And won't halve the swing

Or any differential input amp which will treat the dc as common mode.

Again, is your purpose to eliminate coupling caps?

Sadly, you can't eliminate opamps as they are internal to your dac.

A simple, ready solution is to use a Pass preamp - takes differential input and can output either balanced or se. And you can control volume at output if needed. In my setup a 1.7 Aleph works like a charm doing what you want. Of course it's full of coupling caps
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Old 11th August 2007, 01:36 PM   #7
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Quote:
Originally posted by analog_sa
Two caps will eliminate the dc bias. And won't halve the swing
The problem is not the DC bias. Since this is for a balanced DAC, it's the difference between the two outputs that matter. If both are at 2.5V, the difference is still 0V. The problem is that the outputs are not centered around 0V.

For example, say we have an amplifier with a maximum voltage swing of +/- 25V. If I where to recenter the output of the DAC at 0V (i.e. it would output -2.5V to +2,5V), I could use a gain of 10 before clipping occurs (DAC outputs -2.5V to +2.5V, multiplied by 10 is -25 to +25V).

If I where to feed the signal straight from the DAC into this amplifier, the maximum gain I could use is 5, or else it will clip (DAC outputs 0V to 5V, multiplied by 5 is 0V to 25V). The voltage swing have effectively been halved.

Adding a capacitor will not change this.

Quote:
Originally posted by analog_sa
A simple, ready solution is to use a Pass preamp - takes differential input and can output either balanced or se. And you can control volume at output if needed. In my setup a 1.7 Aleph works like a charm doing what you want. Of course it's full of coupling caps
I will integrate all of this into the DAC so I cannot use a ready made solution. I will take a look at the Aleph however, perhaps I can adapt it.
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Old 11th August 2007, 02:27 PM   #8
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For some reason you are assuming that the DC levels have some kind of effect upon the AC gain of your amp. This is simply not the case. Whether your input signal has a DC component of 2.5v is completely irrelevant. What matters is the AC voltage developed between the two outputs of the dac. In case of AC coupling this is simply trivial. If you prefer DC coupling then you might need to consider getting rid of the bias voltage.
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Old 11th August 2007, 07:29 PM   #9
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Quote:
Originally posted by analog_sa
For some reason you are assuming that the DC levels have some kind of effect upon the AC gain of your amp. This is simply not the case. Whether your input signal has a DC component of 2.5v is completely irrelevant. What matters is the AC voltage developed between the two outputs of the dac. In case of AC coupling this is simply trivial. If you prefer DC coupling then you might need to consider getting rid of the bias voltage.
What you are saying applies only to differential amplifiers. I don't want to limit myself to using only differential amplifiers.

If I connect the positive and negative output of the DAC to two single ended amplifiers the voltage swing will be halved. The output of the DAC will never get below 0V, and as such the output of a single ended amplifier will never get below 0V.

Continuing the previous example, the amplifier can potentially swing between -25 and +25V, but it will only swing between 0V and 25V, thus the voltage swing have effectively been halved.
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Old 11th August 2007, 08:09 PM   #10
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Quote:
Originally posted by Cauhtemoc


The problem is not the DC bias. Since this is for a balanced DAC, it's the difference between the two outputs that matter. If both are at 2.5V, the difference is still 0V. The problem is that the outputs are not centered around 0V.

For example, say we have an amplifier with a maximum voltage swing of +/- 25V. If I where to recenter the output of the DAC at 0V (i.e. it would output -2.5V to +2,5V), I could use a gain of 10 before clipping occurs (DAC outputs -2.5V to +2.5V, multiplied by 10 is -25 to +25V).

If I where to feed the signal straight from the DAC into this amplifier, the maximum gain I could use is 5, or else it will clip (DAC outputs 0V to 5V, multiplied by 5 is 0V to 25V). The voltage swing have effectively been halved.

Adding a capacitor will not change this.



I will integrate all of this into the DAC so I cannot use a ready made solution. I will take a look at the Aleph however, perhaps I can adapt it.
You can use an OPA1632 and set the output common to ground, that way you get balanced>se, re-centered at ground. It's all in the data sheet. You only get about 12V peak swing but you can add your +/-25V amp after that. Question: what do you need +/-25V swing for?

Edit: forget this post, someone else already mentioned this....

Other question: would you really go to +/- 2.5V supply just to avoid an opamp? You may get more problems and worse sound than what you got rid of.

BTW If you feed the DAC outputs capacitively to your amp, you have gotten rid of the 2.5VDC without compromising swing, that will then be +/-2.5V (or whatever the DAC puts out).

Jan Didden
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