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Old 7th October 2011, 11:48 PM   #1
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Default How to Calc resistor for I/V stage

I am trying to understand how to correctly figure the value of a resistor for a purely resistive I/V stage for a current output DAC like the TPA COD.

The BB PCM1794a Dac chip has a 7.8ma p-p output per channel @ 0dfs, 15.6ma of you use the chip in mono mode. so it is easy enough to calculate E=IxR to figure a resistor/voltage but at some point the Dac runs out of voltage!

So, .0078 x say 100 ohms = 0.78v p-p

vs.

say .0078 x 500 ohms = 3.9v p-p

Where is the limit? if the analog side of the DAC chip is running on 5V would the limit be just under 2.5V??

And do you want to size the resistor for max voltage swing? or is there some point you try and aim for?

The datasheet only gives an example I/V calculation for an active I/V stage. nothing about passive I/V stage calc.


Zc
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Last edited by Zero Cool; 7th October 2011 at 11:52 PM.
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Old 8th October 2011, 01:01 AM   #2
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The active stage puts a ZERO resistance (less than miliohms, depending of the OpAmp) on the output of the current source. That minimize the distortions and errors due to the finite internal resistance of the DAC current source.
A straight resistor will add distortions and noise. Electronics 101.
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Old 8th October 2011, 01:03 AM   #3
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I realize an active stage is better. BUT i have a reason why i want to use a passive resistive stage. So, back to my initial question.
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Old 8th October 2011, 01:28 AM   #4
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The limit is set by the DAC's output compliance voltage spec. That's not shown on the 1792's datasheet so you'd have to determine it empirically. TI assumes you'll follow their recommendations (which suck for sound incidentally) and use an NE5534

Active stage is only better in some aspects - myself I've played with active stages but normally end up preferring the sound of passive.

@SoNic - the milliohms does not extend across the full bandwidth because its generated by the opamp's open loop gain. As that falls at 6dB/octave so the input resistance rises by the same beyond the dominant pole frequency of the opamp. Depending on the opamp various amounts of IMD will be produced because DACs have no difficulty in exceeding the slew limits of opamps. You'll see in various application notes the opamp is fitted with a feedback cap to try to mitigate this.

Intermodulation - Wikipedia, the free encyclopedia
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Old 8th October 2011, 01:55 AM   #5
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I am following in the footsteps of a major audio mfg. that uses a passive I/V stage. I want to do a Passive I/V stage so i can play with output buffer designs. tubes, Jfets, maybe even Sit's! eh? later i can play with active stages Vs. the best sounding Passive resistive + buffer stage.
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Old 8th October 2011, 01:59 AM   #6
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Its probably quite difficult to find an opamp fast enough to give sound quality to beat a passive I/V. I reckon some of the National very fast parts might manage it. If you look at my blog you'll see I'm using AD603 as the buffer/amp stage for my passive I/V. This to my knowledge hasn't been used in a commercial product, but it is a very fast part with GBW in excess of 1GHz.
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Old 8th October 2011, 06:15 AM   #7
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Hi,

the voltage limit would be as Abraxalito says the so called voltage compliance (roughly +-1v, depending on the type of transistors used for the DACs current sources)....if the outputs were not protected by diodes.
The BB/TI DACs feature protection diodes in their outputs which limit the useful voltage range to <<300mV. As soon as the diodes start conduction the THD figures of the DAC rise. I wouldn´t recommend more than 22Ohms IV-resistor value for the 1792/1794.
This translates to a voltage level that asks for some amplification.
The imho best sounding way to do the IV-conversion is to use a currrent conveyor or current buffer stage like the Jocko and similar. These stages isolate the DACs output and the IV-resistor, presenting the DACs output the low impedance it likes to see and transferring the DACs current into the resistor. The signal voltage develops over the resistor and may be buffered with an output buffer stage.
In its simplest form the IV+Buffer may be build out of just 2 transistors, but a practical form would take up 5 transistors (see the Jocko). The bandwidth and speed of these very simple circuits is amazingly high since they don´t work with global feedback. And we need alot of speed because the DACs feature a fullscale settling time of only 200ns.
A nice read Zen -> Cen -> Sen, evolution of a minimalistic IV Converter

jauu
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Last edited by Calvin; 8th October 2011 at 06:25 AM.
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Old 8th October 2011, 12:16 PM   #8
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@Calvin, That is interesting you say that 22 ohms should be the limit when Twisted Pear recommends 470 ohms and a major audio manufacturer that uses these chips in their products use 150 ohms when the chips are used in stereo and about 100 ohms when the chips are used in mono mode. So this seems to be a very cloudy issue with no clear answers. I am currently running my dac chips in mono mode with about 100 ohms with no issues. even when a 1khz 0dbfs cd is played there is no visible clipping on the scope. and distortion is very low.


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Old 8th October 2011, 01:12 PM   #9
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It very much is a very cloudy issue - partly because TI doesn't tell us what the chip's limits are. Philips/NXP do say for their chips, but they set the AC compliance value extremely low - I think about +/-25mV (from memory). Plenty of NOS passive I/V designs with e.g. TDA1543 violate this spec by more than an order of magnitude without horrendous distortion. I use 47R on the output of my TDA1387s which more or less complies (ha) with the datasheet spec for those parts. I do though measure full scale distortion which is considerably poorer than the spec (around -66dB) - this might be more to do with the distortion in the buffer than from the DAC's compliance limits. I don't myself consider this amount of distortion to be a problem provided its low order and of course it decreases with reducing level.
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Old 9th October 2011, 02:13 AM   #10
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OpAmps are not THAT bad as you want to make us belive. A decent 55MHz open loop BW and 20V/uS is healthy enough for the 20kHz limit of audio signal. The rest will be filtered by filtes - yes they are needed to achieve low distortion.
Sure, if you belive in fairies with tubes, NOS, no filters and such, nothing can convince you that TI, WM and AD are not trying to "get you" with their "bad" design.
If tubes where a remote alternative to quality OpAmps, they would be present in datasheets. There is no conspiration, just poor sound quality from a I/V based on resitor and tube voltage follower.
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