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Old 7th February 2012, 06:50 AM   #11
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I could measure around 2.5 mV AC, with 50 / 50 duty cycle squarewave signal at 1kHz / 0dB, definitely not more than 3 mV.... square wave looked very clean with minimal ringing, but it took some tuning to achieve this. Measurement was done referenced to analogue ground reference point that was DC coupled to frame - at I/O connectors (RCA's). The peak stored value was around 2.5 mV - that was with the squarewave signal – not the sinewave. The same value was recorded when I used grounded AD811 pin (noninv pin) as reference for the measurement.

Boky

Last edited by Extreme_Boky; 7th February 2012 at 06:56 AM.
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Old 8th February 2012, 07:34 AM   #12
regal is offline regal  United States
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Quote:
Originally Posted by Extreme_Boky View Post
I could measure around 2.5 mV AC, with 50 / 50 duty cycle squarewave signal at 1kHz / 0dB, definitely not more than 3 mV.... square wave looked very clean with minimal ringing, but it took some tuning to achieve this. Measurement was done referenced to analogue ground reference point that was DC coupled to frame - at I/O connectors (RCA's). The peak stored value was around 2.5 mV - that was with the squarewave signal – not the sinewave. The same value was recorded when I used grounded AD811 pin (noninv pin) as reference for the measurement.

Boky
Thats about perfect, but if there is a resistor in series to the opamp on the i-out pin we would have some input impedance and compliance? I assume you are measuring at the dac pin. Just seems a resistor on the i-out pin in series with the I/V would be in opposition to what we want to accomplish, maybe the resistor is in paralell with the opamp like below? That way its impact is negliagle to input impedance and compliance?

Click the image to open in full size.

I have considered an I/V like this but with a much lower impedance prefilter (20 ohm resistor max, much higher cut-off). This way at lower frequencies thru the audio band, the input impedance is dominated by the opamp (near zero) and as the frequency moves toward the clock speed (from the steps) there is a limit to how high the input impedance can go due to the prefilter resistor in paralell with the opamp. Also the prefilter capacitors which turn to high impedance at higher frequencies are still in paralell with the resistor so no harm done. Plus the prefilter helps limit the bandwidth of the steps.
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Old 8th February 2012, 08:12 AM   #13
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You could do the search on: "JT DAC No 3" for the circuit I used. It is the same one Walt Jung explained in "High_Performance_Audio_Stages_Using_TransZ_Amps.p df". I used different values for Rf and Rs to optimise bandwidth/ringing, and gain.

Total gain of the stage and the transimpedance of the CFB OP used will influence the dynamic compliance "swing". For this reason, the LME49713 might be a better choice. I tried it - it was not as revealing as AD811 was....

I would say that AD811 is the closest thing to a single-gain-stage, optimised for I/V conversion. It is the fastest, with the lowest static impedance of any IC known to me….

Boky
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Old 8th February 2012, 11:38 AM   #14
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Quote:
Originally Posted by regal View Post
I have considered an I/V like this but with a much lower impedance prefilter (20 ohm resistor max, much higher cut-off).
Why use the resistor? It just limits the usefull voltage on output.
IMO tha capacitor itself does the same thing - that's the usual input capacitor in older datasheets (and older schematics). In newer schematics, it was done away with - probably is not important that the output impedance to be low at that frequency anyway (SR of steps themself), relevant is only the settling time of the DAC (that's around some 200ns)?
Notice in the schematics below the "evolution" to empty capacitor pads.
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Last edited by SoNic_real_one; 8th February 2012 at 11:48 AM.
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Old 8th February 2012, 01:08 PM   #15
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Question 1k resistor?

I wonder where the 1k resistor at the inverting input of the AD811 is for in the JT-DAC 3.
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Old 8th February 2012, 05:37 PM   #16
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Because AD811 it's a current feedback OpAmp, not a voltage one. Look in datasheet for more...
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Old 8th February 2012, 08:10 PM   #17
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Quote:
Originally Posted by SoNic_real_one View Post
Because AD811 it's a current feedback OpAmp, not a voltage one. Look in datasheet for more...
Sorry, your reply makes no sense.
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Old 8th February 2012, 11:10 PM   #18
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Originally Posted by QSerraTico_Tico View Post
I wonder where the 1k resistor at the inverting input of the AD811 is for in the JT-DAC 3.
Maybe because you question does not make sense either?
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Old 9th February 2012, 02:14 AM   #19
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R10 and R11 are 1k resistors. the feedback resistors R12 and R13 run from OP output pin to DAC Iout, so the virtual ground point shifted.... and R10 and R11 do not influance the dynamic input impedance.

Boky
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Old 9th February 2012, 03:29 AM   #20
regal is offline regal  United States
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Quote:
Originally Posted by SoNic_real_one View Post
Why use the resistor? It just limits the usefull voltage on output.
IMO tha capacitor itself does the same thing - that's the usual input capacitor in older datasheets (and older schematics). In newer schematics, it was done away with - probably is not important that the output impedance to be low at that frequency anyway (SR of steps themself), relevant is only the settling time of the DAC (that's around some 200ns)?
Notice in the schematics below the "evolution" to empty capacitor pads.
Awesome post. Sounds like boards for the JT DAC are no longer available, but I want to try these cfb opamps, on paper the LME49713 looks ideal if it can be tamed.
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