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Old 18th July 2013, 11:17 PM   #11
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by JanErik View Post
According to the datasheet, PCM1794 can feed 7.8 mA p-p, would it not bottom out with that 22R if max voltage is 5 V? It seems PupDAC has 22R though, but the other has 220R.
Using 220R is a bad idea , lots of distortion, but using the 22R will have very low distortion, I think the PupDac can be a good sounding DAC . I never heard none of those two DAC, but have made some tests with the PCM1794 last Year , and I like the sound of a simple 47 ohms I/V converter, nowadays I use a discrete I/V converter. But a resistor as I/V converter can be a very good solution , and for some people is the only one.

See this for more information about the chip:

Testing the pcm1794
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Old 19th July 2013, 01:24 AM   #12
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It might be better to use a THAT 1200 in place of the opamp in the pupDAC I/V converter.
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Old 19th July 2013, 06:35 AM   #13
Calvin is online now Calvin  Germany
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Hi,

the problem with passive IV is, that due to the small required resistor value, the resultant voltage swing needs voltage amplification of app. +20dB.
It is imho far better to use a current conveyor, or more colloquial "current buffer" instead. It could be as simple as one single transistor, connected in common base (or common gate), driving into a resistor of sufficiently high value to achieve the desired voltage.
qusp has proven that with a single MOSFET (wasting loads of power though), Nelson Pass with a complementary JFET-pair and Jocko with bipolar transistors. My own single-ended circuits achieve better than -90dB THD at 0dBfs with common +-15V supplies.
The circuit in #3 is a more elaborated complementary Version.

The conveyor is used to present the DAC output a low enough impedance so it can work into optimum load conditions. The conveyor simply passes the input current either 1.1 to its output, or mirrors the input current 1:1. A resistor is connected at the conveyor´s output, converting the current into a voltage.
Since the CB or CG circuit is extremely fast and without feedback, it can follow the current steps of the DAC with ease. Only a small cap in parallel to the IV-resistor is needed to bandwidth limit for Freqs >500kHz. With high clocked DACs there´s no need at all for additional analog post filtering. The digital filters inside the DACs achieve more than enough stopband attenuation nowadays.
The signal voltage may then be buffered to drive the connected load device.
The conveyor has the big advantage, that the value of the IV-resistor may be as large as one can think of. In theory up to infinite, but there are of course practical issues. The achievable signal voltage values may be so large though, that one could drive loudspeakers if only a sufficiently powerful buffer is used.

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Old 19th July 2013, 07:08 AM   #14
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Calvin, yesterday I read your threat about the buffers, and I liked your ideas, I would like very much to know more about your I/V topology, if possible of course. I think you have already seen my ideas for a I/V converter.
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Old 19th July 2013, 10:20 AM   #15
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Quote:
Originally Posted by jcx View Post
driving Cload doesn't have much to do with feedback caps if you're not using noise gain input C to gnd too, the negative input "virtual gnd" really isn't real gnd

in fact the OPA6132 datasheet has an example with 1 nF feedback C

not knowing the DAC Iout Z can't really say if it would work but the reasoning should be kept straight
"Driving cload" was an oversimplification, you're right. Still, feedback caps seem best avoided with opa1632 in I/V. (cfr Buffalo DAC (ESS Sabre 9008) )
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Old 19th July 2013, 02:31 PM   #16
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I suppose you could use a single transistor in common base configuration for a current conveyer with the PCM1794a without a DC blocking capacitor if you set the bias so that the idle current was the DAC's negative DC bias. Interesting idea.
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Old 20th July 2013, 08:27 AM   #17
Calvin is online now Calvin  Germany
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Hi,

Sergio, I like to keep things simple. As such I prefer the single-ended common base schematic with current sourcing DACs like the PCM179x.
I1 may be a simple single N-JFET current source (still lowest in noise compared to more complex and more stable ccs)
D1 may be a second bipolar connected as Diode. In fact Q1/D1 is best chosen a Dual for thermal tracking of the signal current injection point.
With Riv connected to teh supply line, the quality of the supply needs to be high, since the positive PSRR is 0. The Jocko is better in this regard, but the active current source introduces some noise and I never managed to get much better than -60dB THD at 0dBfs, some 20-30dB worse than the passive solution.
Both require a DC-blocking cap or a level translator towards the output.
The Bakuun and folded cascode may omitwith DC-caps at the cost of more complexity and noise and probabely no better THD than the Jocko.
Q1,2,3 and Q5 may be replaced by JFETs for better THD, due to Gate-leakage beeing lower than Base-current, but input impedance may be higher than with bipolars.
Surfing the CFP-train at the time, I will look into te possibility of CFP-ing the conveyor-transistor again. Tried that a couple of years ago, but somehow lost interest. See also AES reprint 4326(B-1), 101st Convention, Los Angeles, Nov 8-11, 1996 "Low Input Impedance Current-to-Voltage Conversion Circuit for Current-Output Digital-to-Analog Converters", Michael Smedegaard

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Old 20th July 2013, 12:38 PM   #18
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A few years back, I tried this one, based on a suggestion of Cauhtemoc from here.

It worked well but there was no magic improvement over the opa1632, quite the contrary.
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Old 20th July 2013, 02:28 PM   #19
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Quote:
Originally Posted by Calvin View Post
Hi,

the problem with passive IV is, that due to the small required resistor value, the resultant voltage swing needs voltage amplification of app. +20dB.
It is imho far better to use a current conveyor, or more colloquial "current buffer" instead...
Hi, Calvin,

Current conveyor circuits have drawbacks as well. For example, unless such a circuit utilizes some local or global feedback loop to attain low distortion I/V conversion, the conveyor bias current needs to be quite high relative to the peak signal current, maybe by a factor of ten. This also places significant and broadband dynamic current demands upon the local regulators supplying the current conveyor. In addition, some of the best current output DAC chips (PCM179x and AD1955) need to source or sink their own D.C. bias currents in to a current conveyor, which could severely upset the circuit operating symmetry if not offset by some matched complementary secondary current source or sink.

Those implementation concerns keep pushing me away from current conveyors (despite my appreciation for their conceptual logic) and back to an low value passive resistor I/V, followed by simple passive low-pass filtering, followed by active open-loop voltage amplification.
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Last edited by Ken Newton; 20th July 2013 at 02:30 PM.
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Old 20th July 2013, 02:43 PM   #20
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Quote:
Originally Posted by Ken Newton View Post
Hi, Calvin,

Current conveyor circuits have drawbacks as well. For example, unless such a circuit utilizes some local or global feedback loop to attain low distortion I/V conversion, the conveyor bias current needs to be quite high relative to the peak signal current, maybe by a factor of ten. This also places significant and broadband dynamic current demands upon the local regulators supplying the current conveyor. In addition, some of the best current output DAC chips (PCM179x and AD1955) need to source or sink their own D.C. bias currents in to a current conveyor, which could severely upset the circuit operating symmetry if not offset by some matched complementary secondary current source or sink.

Those implementation concerns keep pushing me away from current conveyors (despite my appreciation for their conceptual logic) and back to an low value passive resistor I/V, followed by simple passive low-pass filtering, followed by active open-loop voltage amplification.
Yeah, I played around with symmetrical complementary current conveyors in simulation ad nauseum, and not only did they have poor PSRR, as Ken said the bias current of the DAC requires some kind of additional compensation, otherwise the circuit is completely knocked off balance. Another drawback is the requirement for matched parts in any discrete complementary arrangement.

I tried zero field input transformer I/V circuits as well, but the transformer always adds substantial distortion.

I've settled on the normal opamp based I/V converter myself, though I am intrigued by the use of a THAT 1200 as an instrumentation amplifier in this kind of circuit.

To each their own, as they say.
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