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Building the ultimate NOS DAC using TDA1541A
Building the ultimate NOS DAC using TDA1541A
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Old 21st April 2019, 07:16 AM   #7261
koldby is offline koldby  Denmark
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Originally Posted by Alexandre View Post
I think I can make a small contribution at this point. Back when we were discussing whether or not sign magnitude would work, it became clear to me (and others) that one of the segments need addition of 1 LSB. (But this would be a big step in complexity in the decoder.)

Another way is to accept only 16 bits of data and make the LSB always 1 on the relevant segment (John´s suggestion). This would result in a 16 bit sign magnitude dac (no longer 17 bits).

Building the ultimate NOS DAC using TDA1541A

-Alex
Yes the word length needs to be 17 bits, but I do not see why the decoder needs to be changed? A 24 bit long word would just be truncated to 17 bits in the dac as a 24 bit word would be truncated to 16 bits in a normal TDA1541A
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Old 21st April 2019, 07:43 AM   #7262
koldby is offline koldby  Denmark
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And , for what it is worth, I personally do not care if it has 16 or 17 bits resolution. To me that is not the main benefit of this decoder.
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Old 21st April 2019, 04:04 PM   #7263
koldby is offline koldby  Denmark
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Originally Posted by Alexandre View Post
I think I can make a small contribution at this point. Back when we were discussing whether or not sign magnitude would work, it became clear to me (and others) that one of the segments need addition of 1 LSB. (But this would be a big step in complexity in the decoder.)

Another way is to accept only 16 bits of data and make the LSB always 1 on the relevant segment (John´s suggestion). This would result in a 16 bit sign magnitude dac (no longer 17 bits).

Building the ultimate NOS DAC using TDA1541A

-Alex
Looking back on the discussion whether this signed magnitude circuit was infackt a 17 bit dac, it seams to me that you actually was advocating for that it indeed was???

Now after I actually have made the circuit and seen that the max swing out is 8mA and knowing that the smallest current change (1 bit)for a TDA1541A is around 61 nA the math is also predicting 17 bits resolution:

8 x 10^-3 / 61 x 10^-9 = 131.147

And

2^17 = 131.072
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Old 21st April 2019, 05:30 PM   #7264
Alexandre is offline Alexandre
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This is what I think: It is possible to make it a 17 bit sign magnitude dac. BUT, to do it right, it needs an adder. One of the segments need addition of 1 LSB. Otherwise there will be a missing code at the zero crossing.

Without the adder, the zero crossing will be slightly anomalous: the digital input will increase by one count and the DAC output will not change.

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Old 21st April 2019, 05:41 PM   #7265
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by Alexandre View Post
This is what I think: It is possible to make it a 17 bit sign magnitude dac. BUT, to do it right, it needs an adder. One of the segments need addition of 1 LSB. Otherwise there will be a missing code at the zero crossing.

Without the adder, the zero crossing will be slightly anomalous: the digital input will increase by one count and the DAC output will not change.

-Alex
Sorry, you’ve got it wrong.
Even when 1 bit was missing, which is not the case, it doesn’t turn 17 bits into 16 bits but into 2^17-1
Hans
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Old 21st April 2019, 05:57 PM   #7266
Alexandre is offline Alexandre
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it doesn’t turn 17 bits into 16 bits but into 2^17-1
I didn´t say that.

I said: to do it right, you need the adder.

Since it doesn´t have the adder, you guys might want to investigate the output of the decoder to make sure that one of the segments always has zero in the LSB, and the other always has one. (This is the only way to do it without the adder, and it will be 16 bits, obviously.)

-Alex
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Old 22nd April 2019, 03:33 PM   #7267
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Mathematics for the new millenium. It means what you want it to mean when you want it to mean it.
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Old 22nd April 2019, 06:21 PM   #7268
koldby is offline koldby  Denmark
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Mathematics for the new millenium. It means what you want it to mean when you want it to mean it.
Very constructive. Thanks
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Old 22nd April 2019, 06:54 PM   #7269
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Old 23rd April 2019, 09:27 AM   #7270
koldby is offline koldby  Denmark
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I will try to explain , how I see all this controversy over the 17th bit, even though I am not at all a theoretical expert:

Let´s assume a perfect TDA1541A with exactly -2 mA current out when shorted to ground for both segments.

If the positive segment is biased to 2 mA it will put out 4 mA with 16 1's input and 0 mA with 16 0's.
If the negative segment is biased to -2 mA it will put out 0 mA with 16 1's input and -4mA with 16 0's at the input.

In this situation we have the problem around switching from positive segment to negative segment, because they are both at 0 mA.
Said in another way, when the original MSB is changing from 0 to 1 nothing happens at the output.

This would cause distortion as far as I can see, but if the bias is adjusted to lowest distortion, I think the bias to the two segments have a difference of 61 nA.
Lets assume the negative element has a bias of -2,000061 mA this would add the missing LSB to the negative segment.

There will then be a small DC error and a FS error, and perhaps that is what I see in the measurements I did on this circuit?

I am just trying , humbly, to understand and explain what I see in the actual circuit.

I hope not this sounds too silly, please be kind to an old man doing his best to understand Johns brilliant circuit and making it work.

Cheers
Koldby
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