Kinda' noob question:
Is it correct to see the DAC output Z (670R) forming a voltage divider with the IV stage input Z?
If true then obviously an IV with a flat input Z from DC up to 10MHz of say 3R (I forgot to check but it is probably close to what Hawksford proposes - have to check...) will display less noise at its output than an IV with 10R of input Z.
May I conclude that when evaluating the two approaches: Jocko (OL) or Hawskford (CL) vs. opamp IV then measuring noise over a wide band should be one of the criteria?
I agree that slew rate limiting is also an issue, especially with poorly chosen OpAmps.
Tks!
Is it correct to see the DAC output Z (670R) forming a voltage divider with the IV stage input Z?
If true then obviously an IV with a flat input Z from DC up to 10MHz of say 3R (I forgot to check but it is probably close to what Hawksford proposes - have to check...) will display less noise at its output than an IV with 10R of input Z.
May I conclude that when evaluating the two approaches: Jocko (OL) or Hawskford (CL) vs. opamp IV then measuring noise over a wide band should be one of the criteria?
I agree that slew rate limiting is also an issue, especially with poorly chosen OpAmps.Tks!
I tried this around 1991 or so, the DAC is probably out of date now. The two capacitors and the BW of the op-amp form a critically damped two pole system, I don't know about the Dutch circuit but the principle is similar. The results at the time were remarkably good. The picture is from 797 datasheet
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Joseph K said:
George, you mean 48 kHz in fig.3-8a of Hawksford, but that is mutch worse than 3-10a which corresponds about with 8x oversampling, if I'm right.
Is this true? With the 1800 pF in parallel the gain has been lowered with 40 dB at 3.5 MHz. From 0 - 20 kHz the signal could be 8.8Vpp (with 0 dB on disk) indeed, but not at higher frequencies. Do I make a mistake?Joseph K said:
With OS is mutch better (see above), right? I still have somewere a wrong exchange of ideas. The 4 mA jumps are in both cases 200 ns, so....... what is the difference for the slew rate?................help!
Measured input impedance.....
With a PM3233 oscilloscope (10 MHz, 35 ns, 2 mV/cm) and a decent probe, I measured the output voltage over the pins 6 and 7 of the PCM63 (which is followed by my op amp IV with OPA314).
I could see LF-signals from a few hundred Hz up to 20 kHz. The higher the frequency the larger the signal,
BUT,
I could not find anything at 8 x fs which is about 3.5 MHz. NOTHING. Even without the 5 nF capacitor!
I think, I cannot trust my simulations. The IV is much better than expected from simulation.
I think I leave out the 5 nF either.
Later I will measure with W&G's PSM5 to see how small the recidues are at 3.5, 7, 10.5.... MHz.
With a PM3233 oscilloscope (10 MHz, 35 ns, 2 mV/cm) and a decent probe, I measured the output voltage over the pins 6 and 7 of the PCM63 (which is followed by my op amp IV with OPA314).
I could see LF-signals from a few hundred Hz up to 20 kHz. The higher the frequency the larger the signal,
BUT,
I could not find anything at 8 x fs which is about 3.5 MHz. NOTHING. Even without the 5 nF capacitor!
I think, I cannot trust my simulations. The IV is much better than expected from simulation.
I think I leave out the 5 nF either.
Later I will measure with W&G's PSM5 to see how small the recidues are at 3.5, 7, 10.5.... MHz.
PA0SU said:With OS is mutch better (see above), right? I still have somewere a wrong exchange of ideas. The 4 mA jumps are in both cases 200 ns, so....... what is the difference for the slew rate?................help!
I did find the answer myself!
From my radio-experience I know that in a heterodyne receiver a small distance between local oscillator (LO) and the frequencies to be received (RF) gives a lot of spurious. In general: the larger the distance, the less spurious. However there are regions you should better not use (see my web site).
In this case the RF is the audio-band ( 10-20,000 Hz) and the LO is the sampling rate..........
In our case the 'mixer' is the IV-conv. This should be very linear so that the spurious are as small as possible of course, but oversampling is a must from this point of view.
(I never understood the non oversampling lovers.......)
PCM1704 iso PCM63
With a PCM1704 one will find less intermod distortion than with a PCM63 because the 1704 outputs 1.2 mA and the 63: 2.0 mA!
Perhaps folks think the 1704 is a better DAC than the 63 because 'thay sould better'.
It could very well be that their IV-conv can't handle the 2.0 mA from the 63!!!
With a PCM1704 one will find less intermod distortion than with a PCM63 because the 1704 outputs 1.2 mA and the 63: 2.0 mA!
Perhaps folks think the 1704 is a better DAC than the 63 because 'thay sould better'.
It could very well be that their IV-conv can't handle the 2.0 mA from the 63!!!
Congratulations! Very nice work.
For OPA648 and similar op-amps there's an article on Walt Jung website : http://waltjung.org/PDFs/High_Performance_Audio_Stages_Using_TransZ_Amps.pdf The trick discussed there may help to reduce the distorsions.
For OPA648 and similar op-amps there's an article on Walt Jung website : http://waltjung.org/PDFs/High_Performance_Audio_Stages_Using_TransZ_Amps.pdf The trick discussed there may help to reduce the distorsions.
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