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Old 14th August 2012, 07:42 PM   #181
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Something practical. LowDist I/V MC nearly ready.
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Old 14th August 2012, 08:49 PM   #182
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Quote:
Originally Posted by Calvin View Post
As Zinsula correctly remarked in #159, THD improves with rising Ibias/Idac relationship. But there´s a sweet-point of optimum bias depending on the chosen transistors. The PCM1795/1796 or 1798 DACs have lower currents (-3.5mA+-2mA) than the 1792/1794A and allow for lower power requirements. Sourcing of matched transistors may be easier if SMD casings can be used, due to lower currents and power requirements. You just need to increase the value of the Riv for same level output.

The probabely best match to the PCMs is the TI/BB SRC4392 asynchronous sample rate converter and DIX.

jauu
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May i suggest other DAC.
AD1853 3mA pp 30pf
and it's brother
AD1955 8.6mA pp 100pf
i was told both are way better sounding than PCM1794.Especially AD1853.
I'm looking forward to build this I/V with something similar to this
USB DAC 2 - Hi-Resolution System
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Old 14th August 2012, 09:05 PM   #183
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I like the AD1955 on paper, but the one thing that irks me somewhat is how the digital filters precision turns to crud when used with 192kHz. Now TI don't show their digital filters response for different sampling frequencies, so one only assumes that they are all the same, I wonder if this is a valid assumption. The trouble with the PCM1792/4 chips is that they are of old design and the engineers currently working at TI aren't totally familiar with how it works inside. At least that's the impression I got when talking to them.
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Old 14th August 2012, 09:05 PM   #184
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Originally Posted by MiiB View Post
Do you have to run the DAC diferential. Cant you just use the one output.... I mean for the summation after you need a circuit to Sum the outputs and make them unbalanced..with added circuit complexity. I know you gain some noise benefits and 2. harmonic cancellation, but at what cost...The balance could well be the simplicity of one circuit..
As dirkwright wrote, you can use a single phase of the differential outputs. There are both positive and negative consequences of doing this.

Some positive consequences: When used with a current-conveyer I/V (such as is the circuit under discsussion in this thread), or with a current feedback transimpedance I/V stage, reducing the magnitude of the signal current will usually reduce the distortion produced by the I/V stage. In addition, the amount of circuitry is reduced.

Some negative consequence: Slightly increased even order distortion and reduced SNR due to elimination of push-pull operation. In addition, the output impedance of a zero feedback I/V circuit, using a high impedance node for the output, will double versus the differential circuit. For example, with smms73's circuit, the I/V resistor effectively sets the output impedance. If both output phases were utilized, the I/V resistor could be reduced by half (and with it, the output impedance) and produce the same output voltage amplitude.
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Last edited by Ken Newton; 14th August 2012 at 09:16 PM.
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Old 14th August 2012, 10:25 PM   #185
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thanks Ken. what is the simulator that you use?
I use the freeware version of Spectrum Software's Micro-Cap.
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Old 14th August 2012, 10:53 PM   #186
Calvin is offline Calvin  Germany
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Hi,

other current output DACs may be used alternatively. It seems to me that the actual DAC-cores of modern sigmadelta DACs are not the sonically decisive part but the periphery around that DAC-core. I found the mentioned ADIs sonically on par with the TIs, though an exact evaluation is very difficult, since small differences in clocks, PCB-layout, digital filter, etc. are of larger sonic influence than the DAC-core itself. The analog stages of the eval-boards of the ADI-DACs for example, equipped with OP275 OPAmps, simply sound terrible.
A interesting feature of the TIs and the AD1955 is the possibility to bypass the digital filter and to drive the DAC-core from a external digital filter at rates of up to 384kHz (see Anagram or PMD).
As to my knowledge the PCM1792/1794A feature a more powerful arithmetic machine for the digital filter than the (therefore cheaper) PCM1796 which may lead to small sonic differences.

jauu
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Old 14th August 2012, 10:57 PM   #187
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I did not know that digital is messed up that massive ! No information available to the designer-user, a sad affair.
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Old 15th August 2012, 09:09 AM   #188
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by Terry Demol View Post
[...]BTW, your choice of 327 / 337 in most locations is a pretty good one for 2
reasons - who can guess what they are?
I like them because
- cheap ;-)
- low noise, low Rbb (thanks to Joachim who pointed that out some time ago)
- high beta (the ...-40 types), up to almost 100mA flat
- can carry quite high currents (although you of course are limited with a TO92 which has a thermal resistance junction - ambient of around 250K/W)
- up to 200-300 mA Ic almost flat Ic vs Uce curve (low early voltage)
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Old 15th August 2012, 11:14 AM   #189
RCruz is online now RCruz  Switzerland
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Old 15th August 2012, 12:18 PM   #190
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Originally Posted by Ken Newton View Post
dirk,

Correct, each ouput pin is never more than 3.9mA from the quiescent point. So, the pk-pk current output from EACH PIN can never be more than 7.8mA. I think confusion may stem from the fact that, indeed, no single pin can output 15.6mA. However, the pk-pk differential net CHANGE in current between pins is equal to 15.6mA. These currents are anti-phase, so you won't ever measure an in-phase 15.6mA from the output pins. If you look at the 'X' chart on post #155, you can see that the current difference between IoutP and IoutN swings from 7.8mA peak at one polarity, through BPZ where the 'X' arms cross, to 7.8mA peak at the opposite polarity. That's a peak-peak difference of 15.6mA.

Think of that 15.6mA net change in terms of the voltage a differential I/V stage would produce. If the I/V resistor were 1k, then IoutP by itself can produce a signal voltage of 1k x 7.8mApp(+/- 3.9mA) = 7.8Vpp. However, the outputs are differential, so we have IoutN to account for. IoutN by itself can also produce a signal voltage of 1k x 7.8mApp = 7.8Vpp. If fed anti-phase inputs from IoutP and IoutN both, a differential I/V will then produce a signal voltage of 2 x 7.8Vpp = 15.6Vpp. Which means that 15.6Vpp / 1K = 15.6mApp current is conducted through the I/V resistor.
So, are you saying that we should use 15.6mA for the current generator in our simulations when it is connected in differential mode as shown for example in the attached figure?
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