The pcm1794a datasheet I/V converter and how to improve it. - Page 17 - diyAudio
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Old 4th September 2012, 02:07 PM   #161
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This is what I suggest. Block the DC at the input and balance the input and feedback resistors so that the DC offset at the output will be 16uV, which is insignificant. Noise is still good with S/N near 140dB. AC response is flat from 1Hz to infinity apparently. It's just a flat line. THD is extremely low. After looking at the schematic for the NE5534, I realized that I'd played around with discrete opamps before, and so I realized that it's incorrect to just tie one of the input bases to ground. There needs to be a resistor there that is equal to the feedback resistor. Also, I have realized that a blocking capacitor is necessary for good DC balance in the circuit. I have further realized that when the DC balanced is achieved, then the opamp performs better. So, this is what I came up with now.

Actually, this may not work with the DAC because it appears to want a DC path to ground. I'm not sure of that now. Why would the DAC need a DC path to ground? I know I read about it in some application notes, but I don't see why this would not work. We're only interested in the AC signal after all.

As for my discrete IV converter, I have not done anything more with it. I'm trying to find the problem with a power supply a built, and I'm building Pete Millett's sound card interface and hoping to buy the PCM4222 ADC soon. I need good test gear before getting serious about building stuff.
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Last edited by dirkwright; 4th September 2012 at 02:27 PM.
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Old 4th September 2012, 02:38 PM   #162
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If a DC path is needed we have discussed the option to insert an opposing current directly into the output of the DAC with a J-Fet CCS.
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Old 4th September 2012, 03:04 PM   #163
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Quote:
Originally Posted by Joachim Gerhard View Post
If a DC path is needed we have discussed the option to insert an opposing current directly into the output of the DAC with a J-Fet CCS.
Yeah, I don't understand right now why a DC is needed at all. They say it in the various application notes, but what would happen if there wasn't one? We're interested in an AC current after all, not a DC one.
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Old 4th September 2012, 03:12 PM   #164
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I can not tell. The only experience i have in years with DACs myself is the new ESS with voltage output. I designed a buffer for them but that is all. There is a guy on this forum, his avatar is Calvin. He seems to know all about that.
I am planning though to build a DAC so i am contributing here a bit and watch what you and our friend from Portugal is coming up with.
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Old 4th September 2012, 03:30 PM   #165
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The TI circuit doesn't show a clear path to a real ground for the Iout- pin. The Iout+ pin does have a clear path to real ground. So, it's confused in my opinion.

I'm working on a zero field IV converter.
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Old 4th September 2012, 04:25 PM   #166
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This is where I'm at with it. Performance is pretty good.
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Old 4th September 2012, 04:38 PM   #167
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Could the input offset be low enough to get rid of the electrolytic ?
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Old 4th September 2012, 04:39 PM   #168
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Maybe you put it in to get rid of a peak.
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Old 4th September 2012, 04:44 PM   #169
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Quote:
Originally Posted by Joachim Gerhard View Post
Maybe you put it in to get rid of a peak.
The capacitor is necessary. Without it the DC offset on the output is 11 volts!

In the original schematic, there was just a capacitor between the transformer + and ground. I found I needed a huge capacitor to get good response with low feedback resistors and there was still a nasty peak. I reduced the capacitor and added the resistor to dampen the response, which is now nice and smooth down to about 1Hz. The bad news is that the 75 ohm resistor adds to the input impedance as seen by the current generator. I think it's about 50 ohms or so. Still, this DAC seems to be OK with higher than zero load impedances.
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Old 4th September 2012, 06:07 PM   #170
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The circuit is just a variation of the single opamp balanced to unbalanced converter, where the first resistor on the - pin is replaced by the transformer secondary winding. I don't know why the capacitor is required. So, R3 has to be set to the DC resistance of the transformer secondary and R1 and R2 have to be equal.
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