dac I/V convertion with very low distortion - Page 47 - diyAudio
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Old 1st September 2012, 01:57 PM   #461
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I do not see the problem. Input and J1 are on the same potential.
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Old 1st September 2012, 01:59 PM   #462
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Here is one possible solution. The problem I am addressing is a nasty spike in the AC frequency response around 1Hz. The damping network solves the problem but increases the input impedance to 78 ohms. Noise is still low but THD is slightly worse.

I tried deleting the extra pair of transistors but distortion was worse and input impedance went up also.
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Old 1st September 2012, 02:04 PM   #463
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Quote:
Originally Posted by Joachim Gerhard View Post
I do not see the problem. Input and J1 are on the same potential.
Oops, never mind. it's fine. There are 100uA DC or so flowing through the transformer. Not a big deal as long as the transistors are matched. This connection from post 457 solves the nasty 1Hz spike as well. I'm just not awake yet.

There is an imbalance in the AC signal though due to the different input impedance on each side of the current mirror.

Input impedance for circuit in post 457 is 5.9 ohms.

I'm not sure I need the RLC filters since I doubt the transformer will convey any really high frequencies. I have incomplete information on the transformer characteristics so I have to guess at things like capacitance.

Oh, and good morning.

Last edited by dirkwright; 1st September 2012 at 02:31 PM.
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Old 17th October 2012, 07:47 PM   #464
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Just to say that this project is not forgotten. I am working on it. But things are moving a little slow, because i dont have a lot of time lately. but i have run some tests on the pcm1794 and this type of I/V converter seems a very good option.

I have already made a final version, that i will not post for now, and i am designing the pcb.

the thread about the pcm1794 tests:

Testing the pcm1794
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Last edited by smms73; 17th October 2012 at 07:59 PM.
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Old 25th October 2012, 04:12 PM   #465
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Yesterday i have finished the prototype of the I/V converter . And made some tests.
I must say, that i am very happy with the results.
As predicted the distortion is very low , and i can not measure it with the equipment that i have (emu1212m) better than 0.0003%. Anyway i post here a picture of 2,8 Volts rms output in 1kohm. 8ma p-p.
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File Type: jpg prototype.jpg (467.3 KB, 233 views)
File Type: png 4v_8ma.PNG (41.3 KB, 206 views)
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Old 25th October 2012, 04:15 PM   #466
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Wow, that looks pretty damn good !
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Old 25th October 2012, 04:22 PM   #467
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This next test consists of a very fast pulse of 750ns duration, and serves to test the stability of the circuit.
As you can see by the photo the circuit is very stable and do not have signs of ringing or overshoot, the first photo is the input signal, the second is the output without the output capacitor, and the third is output with capacitor.
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File Type: jpg spike_test.jpg (611.5 KB, 193 views)
File Type: jpg out_nocap.jpg (604.1 KB, 136 views)
File Type: jpg out_cap.jpg (600.8 KB, 38 views)
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Old 25th October 2012, 04:27 PM   #468
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just for curiosity, the same test in a opa2134 connected as I/V converter.
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File Type: jpg opa2134.jpg (596.8 KB, 52 views)
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Old 25th October 2012, 04:38 PM   #469
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The time domain behavior of your circuit is excellent.
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Old 25th October 2012, 04:46 PM   #470
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by Joachim Gerhard View Post
Wow, that looks pretty damn good !
Hallo Joachim.

Yes, I think I'm almost done , the circuit just need small changes to be finished.
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