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Old 10th October 2007, 04:53 PM   #1
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Question D1 Offset Questions

I'm working on integrating the Pass D1 I/V to my AD1955 DAC. I'm first experimenting with SPICE sims as an educational tool to learn more about how this circuit operates.


In the sim, I was able to adjust the sources of Q3 & Q5 to within +-44mv. The offset is -28mv.

How would someone adjust this 28mv offset to 0?

Thank you!

-David
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Old 10th October 2007, 05:00 PM   #2
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Here's the original D1 I/V for reference, just in case someone sees and error in my schematic capture that I missed.

-David
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Old 10th October 2007, 06:17 PM   #3
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I've changed the values of R26 and R33 from 3K3 to 4K7. The offset was reduced to -2.35mv.

Are these the correct resistors to adjust?

Like the first post, the measurements are from the Sources of Q3 and Q5. In simulation this was the closes to zero thta could be obtained.

Any thoughts or comments are appreciated.

Thank you!

-David
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Old 10th October 2007, 07:00 PM   #4
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OK one last sim...

R26 & R33 adjusted to 4K7, and P4&P5 are tuned perfectly as well. The offset is now perfectly adjusted, however there is one side effect.

In the attached plot, the amplitude is higher above zero than below. Any ideas on what could cause this behavior?

Thank you!

-David
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Old 10th October 2007, 11:10 PM   #5
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Made suttle changes to resistor values and changed the curent source to a single source.
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Old 10th October 2007, 11:15 PM   #6
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The offest issue is fixed, but the issue of the amplitude above offset for Q3&Q6 Source pins is unbalanced with a higher amplitude above the cross-over point.

Here's a larger plot that shows the I/V MOSFET Source pins and the output waveform of the buffer MOSFETs Q2 & Q5.

Is is expected behavior or is there something wrong?

Thank you,

-David
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Old 11th October 2007, 02:56 AM   #7
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I could adjust the dc offset to a few mV without any changes to the circuit...but however, i face the problem of drifting...they could measure 30mV at times.
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Old 15th October 2007, 10:56 PM   #8
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Can anyone confirm if this is the right output behavior for the D1?

With the attached SIM's see how the vertical alignment of the output voltage swings with the minimum and maximum settings of PL55.

-David
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