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Old 6th August 2012, 06:46 PM   #1
smms73 is offline smms73  Portugal
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Default dac I/V convertion with very low distortion

This circuit dont use feedback, have 3,5 ohm of input impedance, and very low distortion , it gave only 0.000005% of distortion in the LTspice simulator at 2,5Vrms.


Yesterday, come to the conclusion that i need a new dac, so i will use the pcm1794 and dir9001, just because already have them.
I dont like the I/V of the pcm1794 datasheet, so i remember to use a circuit that i have design for a non feedback amp.

in attachment there is the asc file for the simulation. dont forget to go to control panel / compression , and turn off all compression for better visualization of fft.

if you have any doubt please fell free to ask
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File Type: jpg dac1.jpg (235.6 KB, 2741 views)
File Type: png bod1.png (50.4 KB, 2530 views)
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File Type: asc iv1.asc (4.7 KB, 81 views)
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Old 6th August 2012, 07:01 PM   #2
DF96 is offline DF96  England
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Just a few comments. Real life might have more distortion because you won't get such near-perfect cancellation in the push-pull grounded base BJTs - you might need offset trimming. You seem to have bias applied from voltage sources via emitter followers: the followers will degrade the perfect voltage sources (in simulation - real life will be different), and of course followers work by using feedback. You might not have signal feedback, but your bias uses feedback.
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Old 6th August 2012, 07:33 PM   #3
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by DF96 View Post
Just a few comments. Real life might have more distortion because you won't get such near-perfect cancellation in the push-pull grounded base BJTs - you might need offset trimming. You seem to have bias applied from voltage sources via emitter followers: the followers will degrade the perfect voltage sources (in simulation - real life will be different), and of course followers work by using feedback. You might not have signal feedback, but your bias uses feedback.
DF96 this is only a simulation circuit, not the final project.
of course the circuit need offset trimming, a dc servo will be use.

the voltage sources will be replaced by zeners or voltage references.

when i say no feedback, i mean no closed loop (global feedback).

i will post a more simplified circuit, for better understanding.

Last edited by smms73; 6th August 2012 at 07:39 PM.
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Old 6th August 2012, 07:53 PM   #4
smms73 is offline smms73  Portugal
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this is the more simplified circuit. and have 0.008% distortion.

the only sources of distortion are the currents from the bases of the transistores( that's the reason for using the extra four transistors ),
and the fact that the 2 current sources in the first circuit are not perfect.
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Old 6th August 2012, 10:46 PM   #5
zinsula is offline zinsula  Switzerland
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You are using a perfect current source, infinite z out. DAC's in real life are worse.
Use a voltage source instead, and put a 1k resistor between it and your input. This is closer to reality. Use 1Volt per ma the DAC puts out.
Better would be to build and measure though...not much sense to believe these Simulation numbers, you might be disappointed.
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Old 7th August 2012, 01:49 PM   #6
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This is very similar to Dr. Leach's moving coil head amp:
Moving Coil Cartridge Head Amps

So, you will achieve lower noise and lower input impedance by removing the emitter resistors R1, R2. If you use current mirrors as suggested by Dr. Leach, then a further improvement should occur. You can lower the input impedance even more by doubling up the input BJT's. I have a similar circuit that has about 1 ohm input impedance.

The DACs that you like have DC current on their output pins which will upset the balance of your circuit. The PCM1794a for example puts out -6.2mA of DC current. Something has to be done about that as well.
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Old 7th August 2012, 02:18 PM   #7
Jay is offline Jay  Indonesia
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Nico Ras has just posted a link to the following article with some relevancy with DAC/opamp design issue: "Dangers of Rounded Numbers, Typical Specifications, and Simulations"

The Dangers of Rounded Numbers, Typical Specifications, and Simulations - Tutorial - Maxim
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Old 7th August 2012, 02:31 PM   #8
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Quote:
Originally Posted by smms73 View Post
This circuit dont use feedback, have 3,5 ohm of input impedance, and very low distortion , it gave only 0.000005% of distortion in the LTspice simulator at 2,5Vrms.

....

if you have any doubt please fell free to ask
My doubt is - DAC chips don't put out sinewaves, yet you're simulating with a sinewave source. So I can't but help thinking all those zeroes in your distortion figure are rather meaningless.

A modern CMOS DAC such as the PCM179x puts out some very fast edges with rise times perhaps <1nS. Have you considered using transient simulation ? - you can input a .wav file containing a sinewave which will be somewhat closer to reality because it'll have steps. It will still not really be close enough though as it won't have all the out of band hash that's intrinsic to S-D converters.
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Old 7th August 2012, 02:32 PM   #9
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Quote:
Originally Posted by Jay View Post
Nico Ras has just posted a link to the following article with some relevancy with DAC/opamp design issue: "Dangers of Rounded Numbers, Typical Specifications, and Simulations"

The Dangers of Rounded Numbers, Typical Specifications, and Simulations - Tutorial - Maxim
Well, according to a couple of engineers I know from Analog Devices, the version of SPICE they use is far advanced from anything you can buy on the street. People who design chips must get it right the first time because tooling is so expensive. That's my understanding anyway.

As for us amateurs, we can tweek a circuit all we want because we have the time to do so. Obviously, circuits like this I/V converter requires matched transistors. Many audiophile circuits required matched components, which is not in the realm of practical for mass produced stuff.

Of course, the article is correct in that relying too much on a SPICE model, or expecting exactly the same results from a real version of the model, is not realistic. I know that. I consider the tests (FFT, noise, etc) in SPICE to be a mere guide for the performance of the real device.
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Old 7th August 2012, 02:35 PM   #10
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Quote:
Originally Posted by abraxalito View Post
My doubt is - DAC chips don't put out sinewaves, yet you're simulating with a sinewave source. So I can't but help thinking all those zeroes in your distortion figure are rather meaningless.

A modern CMOS DAC such as the PCM179x puts out some very fast edges with rise times perhaps <1nS. Have you considered using transient simulation ? - you can input a .wav file containing a sinewave which will be somewhat closer to reality because it'll have steps. It will still not really be close enough though as it won't have all the out of band hash that's intrinsic to S-D converters.
Yeah, you're right, plus the fact that SPICE models for these DAC chips are not available as far as I can tell. I have not advanced far enough yet to be able to create a sine WAVE file with the stair step overlay that would be the actual output from a DAC.
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