Hybrid DAC with calibration

[Bernhard]

My new project:

A 16 bit DAC that uses one DAC chip per bit = 16 DACs per channel.
Chips are parallel data input / current output DACs BB PCM54.

They work as precision switchable current sources.
All 16 currents are summed up on the virtual ground of the I/V stage.

Every bit's output current is very fine adjustable, within the magnitude of 2 LSBs, having a low-ohm pot + resistor network between Iout and virtual ground.

Calibration will be done watching harmonics fade away on the analyzer.

If it works ok, there are options to calibrate without pots and resistors.

[Rescue Toaster]

Shouldn't the error of even the MSB be less than one LSB? At least on a decent chip?

And isn't it likely you'll introduce glitch noise?

[BrianL]

Do you think that a bunch of pots and resistors that you trim (with what instrumentation?) will be better than the precision trimming of an individual DAC? I think not.

Surely it would be easier to buy and 18 or 20 or 22 or 24 bit DAC that has linearity to better than 16 bits.

Read data sheets carefully

[Bernhard]

Originally posted by Rescue Toaster
Shouldn't the error of even the MSB be less than one LSB? At least on a decent chip?

Tolerance should be much tighter, about 1/50 LSB but it isn't.
That is why a few chips have adjustment for MSB + next 3 bits.

[Bernhard]

Originally posted by BrianL
Do you think that a bunch of pots and resistors that you trim (with what instrumentation?) will be better than the precision trimming of an individual DAC? I think not.

Surely it would be easier to buy and 18 or 20 or 22 or 24 bit DAC that has linearity to better than 16 bits.

Read data sheets carefully

The precision trimming of an individual DAC is not that precise, thats why there are selection grades.
It is a simple game, all you must do is make disappear those vertical lines on the screen.

Nobody should believe that any 20bit chip is better compared to a 16 bit one.

The most expensive 18bit BB instrumentation DAC has 16bit accuracy from precise factory trim and needs user calibration with 4 pots to perhaps reach 18 bit.

[Bernhard]

I have built one channel and it works so far.
So far means the sound is distorted, harmonics -36dB @ FS which is about 2% ??? distortion.
What I had overlooked is that audio DACs have poor gain error of 2%
Now I will put in bigger pots and see how far the DAC can be calibrated.
Later the bit errors can be compensated to one or two LSBs by choosing different codes and the fine adjustment again with the pots.

[janneman]

Originally posted by Bernhard
I have built one channel and it works so far.
So far means the sound is distorted, harmonics -36dB @ FS which is about 2% ??? distortion.
What I had overlooked is that audio DACs have poor gain error of 2%
Now I will put in bigger pots and see how far the DAC can be calibrated.
Later the bit errors can be compensated to one or two LSBs by choosing different codes and the fine adjustment again with the pots.

Bernhard,

Two comments:

- the poor gain error should not cause this 2% distortion. This is a linear distortion meaning that your output level in RMS terms would be off from the design value, but should be undistorted.

- even if the gain error would cause distortion, the fact that you have 16 DACs in parallel would average it out and the result should be much better.

I think there is another mechanism causing the high distortion. How sure are you that the input signal is distortion free?

Jan Didden

[janneman]

Too late for edit: You may check the input sigbnal and general setup by temporarily disconnecting 15 of the 16 DACs. How does that look?

Jan Didden

[Bernhard]

Hi Jan

each DAC is only switching from BPZ to the corresponding fixed bit current.
One DAC per bit.
I use the DACs as precision current sources with integrated switch.

[janneman]

Originally posted by Bernhard
Hi Jan

each DAC is only switching from BPZ to the corresponding fixed bit current.
One DAC per bit.
I use the DACs as precision current sources with integrated switch.

I still don't get the whole picture. What's BPZ?

Jan Didden