DAC MSB adjustment

[Oli]

Just an update on my progress with the static method....

My Raspberry PI Python script works perfectly. Without amplification and using a digital multimeter I was able to verify that :

MSB = 0.00V,
+ve full scale = 3.00V
-ve full scale = -3.00V

Going a stage further and using a low noise opamp configured for 100 x gain (PI supply powered off) I have obtained some promising results.

Despite some noise... I'm working on this:

MSB + 1 = 140mV
MSB = 120mV
MSB -1 = 110mV

(these figures represent the amplified DAC voltages and amplified noise.)

Next I will add and tweak adjustment resistors to see if I can make the interval constant.

I'm not saying that this method is perfectly accurate, but it appears that I may be able to make reasonable improvement to the MSB adjustment.

I should say that one advantage of this method is that there is no switching noise from digital circuitry / by contrast the dynamic method possibly involves greater noise and countermeasures- I am simply using breadboard. At this stage not even decoupling capacitors!

More to come.

 

[tzobor]

Hi guys,
Sorry to barge in!
I have a Denon DCD-1460 with PCM61P. I have tried to adjust MSB using the test CD from here and a -60dB signal of my own. I tried using the sound card, a Creative Audigy 4 with oscilloscope software and THD measuring software, but when I turned the pots nothing changed. For any channel. I have the service manual, so I am sure I was turning the right pot.
I am no engineer, so I was doing it by ear, sure, but wasn't THD to change, or the oscilloscope graph show something?
Also, the THD values where vastly different between R and L channels, and both greater than those of a Technics SL-PG390 and a Marantz CD63 MkII.
Maybe you can give me a hint?
Thanks!

 

[Shinja]

That Denon is utilized digital offset system to avoid MSB transition in small signal.The offset is also compensated by external DAC. When signal get large, the digital offset and compensation is switched off. It is not sure how level is the threshold. Try adjustment with higher level signal eg. -20dBFS. The compensation DAC looks 74HC00 pin1 to pin6.Pin3 output is 5V/1360kohm=3.7uA and pin6 is twice current to summing junction which is close to 9th and 8th bit current.
Bad distortion level is almost no idea. The distortion must get worse when the offset system switch on or off but I do not know how level, when, or how often it is.

 

[tzobor]

Thank you!
I will look into it.
My tracks were at -60dB (as per the PCM61P datasheet) and 0dB.
Track 15 from the Linearity Test CD that I have downloaded from here and is used in several threads is also at -60dB, I think.