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Old 12th August 2012, 04:50 AM   #131
regal is offline regal  United States
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Quote:
Originally Posted by smms73 View Post
Hi Zinsula,
i was told that the output impedance of the pcm1794 is 2M with 22pf, try to find in internet for the real value, but no luck .
this circuit was design with that in mind, with a source of 1k the distortion is a lot worse (0.008%). but i have made some changes in the input and got 0.000064% thd with 1k input impedance see circuit below

how do you come to 1k , i think is very low for a current source, if someone knows the real value please post.
Dug thru my notes and the PCM1794 has an output impedance of 670. which is in line with other high current output S-D DAC's.

The 2M number is way way off any current DAC ever made.
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Old 12th August 2012, 08:15 AM   #132
MiiB is online now MiiB  Denmark
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So the best way to spice the PCM1794 would be a current source with app 700 ohm to GND..?? If this is the case, then input impedance really starts to matter.. The voltage source with serial impedance seem just too ideal to match real-world conditions

Last edited by MiiB; 12th August 2012 at 08:27 AM.
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Old 12th August 2012, 09:26 AM   #133
regal is offline regal  United States
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Originally Posted by MiiB View Post
So the best way to spice the PCM1794 would be a current source with app 700 ohm to GND..?? If this is the case, then input impedance really starts to matter.. The voltage source with serial impedance seem just too ideal to match real-world conditions
Textbook current source output impedance is with the output impedance in parallel (to ground on the simulator) but a DAC chip current source doesn't "originate from ground").

Best I've found that models reality is voltage source with series reg. In this case the voltage source would be 3.4pk *780 = 2.625V + 780 series resistance giving a 3.4maPk current sine wave.

Its impossible to model what really comes out of a current output DAC as they aren't square wave but dang close.

So you see that current output DAC chips really aren't current sources unless considered in relation to a opamp I/V' input impedance (virtual ground). Its all relative when defining a current source vs a voltage source, and further shows that modern DAC's are designed specifically for opamp I/V. Most current out DAC's you buy these days are just v-out designs with the internal opamp moved external to allow better specs.

This i/v is special enough that you should consider buying a multibit DAC, at least for comparison.

Last edited by regal; 12th August 2012 at 09:31 AM.
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Old 12th August 2012, 11:58 AM   #134
MiiB is online now MiiB  Denmark
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Would the offset then be modeled at a separate current source in parallel or simply as a voltage offset on the generator..??
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Old 12th August 2012, 12:04 PM   #135
xslavic is offline xslavic  Moldova
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youre right,designed for opmaps,but few opamps have the high freq response of a discrete stage ,the thing is to remove the resistor between the dac out and input of the opamp
-- so dont listen to that c..p that some LT or Lm opamp is better or more musical,important is the SNR and CMR and of course channell separation,you dont even imagine what opamps you can get for 20 usd,thats 10 high quality dip8
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Last edited by xslavic; 12th August 2012 at 12:26 PM.
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Old 12th August 2012, 02:34 PM   #136
regal is offline regal  United States
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Originally Posted by MiiB View Post
Would the offset then be modeled at a separate current source in parallel or simply as a voltage offset on the generator..??
Add a current source to model after the resistor for the offset. Be careful not all DAC's define thier offset the same. Some the offset current is + others - direction, I think the TI's are an oddball, I know the TDA1541 swinggs from 0 to -4mA (-2mA offset) but I think the TI PCM17xx are a positive offset , with S-D dac's it usually works out that they like to see 1/2 their single supply voltage on their output. So a DAC that runs off a +5V rail wants 2.5V DC at the i-out, and a 3.3V rail DAC wants 1.65V. Since PCM17xx run off +5V, you will know you have the right offset current when simulation shows 2.5V at the input to the I/V.
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Old 12th August 2012, 02:51 PM   #137
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Originally Posted by regal View Post
Dug thru my notes and the PCM1794 has an output impedance of 670. which is in line with other high current output S-D DAC's.

The 2M number is way way off any current DAC ever made.
Regal,

What is the source of that 670R figure? I'm not disputing it, it's just that the output impedance of the PCM179X series has been shrouded in mystery because T.I. hasn't seen fit to document it in their datasheets. By the way, the number I've seen reported (unofficially and indirectly) is 100k, supposedly from T.I. engineers in Germany.
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Last edited by Ken Newton; 12th August 2012 at 03:07 PM.
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Old 12th August 2012, 03:00 PM   #138
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Originally Posted by smms73 View Post
here is the circuit with the current mirror, but i don't like it very much.
This circuit gave me an inspiration. Thank you!

I am trying out a "darlington current mirror common base complementary" amplifier... it's has lower THD than my other design, but higher noise. I'm not sure whether or not I'm going to go with it yet.
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Old 12th August 2012, 03:40 PM   #139
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Originally Posted by smms73 View Post
As the doubt about the need of matched transistors appears very often, i post here the same circuit with bc327 for pnp and bc547c for all npn, they are very different and the thd is 0.000008% (7,8ma/100k), its no need for matched transistors. hope this end the subject, and we can move on.
I tried modeling just the front end (common base portion) of your circuit and I'm getting way too much gain. It's something like 100dB. Of course, I removed the emitter degeneration resistors, and the base stopper resistors as well. Hum ho.
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Old 12th August 2012, 04:43 PM   #140
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I tried modeling just the front end (common base portion) of your circuit and I'm getting way too much gain. It's something like 100dB. Of course, I removed the emitter degeneration resistors, and the base stopper resistors as well. Hum ho.
Ideally, voltage gain would be much higher, increasing as the virtual ground impedance of the grounded-base stage input is reduced. But what matters here is current flow. The circuit conveys DAC output current to the 400R I/V resistor, while isolating the DAC output from the voltage swing developed across the I/V resistor.
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Last edited by Ken Newton; 12th August 2012 at 04:53 PM.
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