Paralleling up DAC chips

[jewilson]

Thorsen,

Each PCM1702 has a full scale current output of 1.2ma and LSB current of 2.28882054nA. The DAC output is current source and adding more dacs increases the current noise by the number of DAC added in parallel. So were talking about summing junction and the current noise add's, it's not multipled.

The normal 1702 has an output impedance of 1000 ohm when two are parallel the impedance drops to 500ohm, three 250. Since the noise current adds, and output impedance drops by half the noise does not improve it should remain the same constant.
However, it now is possible for the spectrum of the noise to change, that may or many not be of any importance. The improvement you get here is at the output of VI stage. Many opamps have much lower 1/f noise current and voltage when the input impedance is low, example is a 5534 or a OP37. So that is some improvement there.

[ThorstenL]

Konnichiwa,

Originally posted by jewilson
Thorsen,

It's Thorsten, if you must use my name.

Originally posted by jewilson
Each PCM1702 has a full scale current output of 1.2ma and LSB current of 2.28882054nA.

Yes, but what is the noise on the output? At -110db (limit), or at around 4nA (Rms - around 5.6nA Peak). Thus the (thermal) current noise from the PCM1702 is larger, considerably so, than the LSB.

Originally posted by jewilson
The DAC output is current source and adding more dacs increases the current noise by the number of DAC added in parallel. So were talking about summing junction and the current noise add's, it's not multipled.

The noise does add, but as should well know, if we add random noise sources (or non-linearities at that) two equal level noise sources raise the noise (or nonlinearity) statistically only by 3db, not by 6db, while the signal level goes up 6db.

This is the underlying principle in combining multiple DAC's without added digital filtering to stagger their output and in paralleling devices for low noise.

Originally posted by jewilson
Since the noise current adds,

Which they do not, they add according to the "sumsquare" function, not linear....

Originally posted by jewilson
and output impedance drops by half the noise does not improve it should remain the same constant.

Not if the basic observedlaws of electronics are still valid.

Originally posted by jewilson
The improvement you get here is at the output of VI stage. Many opamps have much lower 1/f noise current and voltage when the input impedance is low, example is a 5534 or a OP37. So that is some improvement there.

That is a FURTHER improvement, beyond that from the smple paralleling devices.

Sayonara

[ThorstenL]

Konnichiwa,

Originally posted by jewilson
Thorsen, I looked over the BB EVM-1702 and the measurements described don’t match the methods used in the data sheets for the PCM 1702. In fact the specifications for the data sheet or better in many cases. Maybe you know is the EIAJ filter the same as the A weighted filter?

I looked it over. Best compare the DEM-1702 and EVM-1702. The DEM-1702 with 1pcs PCM1702 is rated at 108db dynamic range (typhical), the EVM-1702 with 4pcs PCM-1702 is rated at 118db dynamic range (typhical), both measured under identical conditions (and illustrating that "datasheet" figures are rarely realised with a real device incorporating the Chip.

We see an overall improvement of 8db for paralleling 4 devices over 1 Device, of which 6db are ascribable to the paralleling of the devices and the other 2db likely to lower analog stage (input current noise induced?) noise due to lower source impedances and different Op-Amp's. Anyway, my guesses on this....

Sayonara

DEM-1702 - http://focus.ti.com/lit/ug/sbau016/sbau016.pdf

EVM-1702 - http://focus.ti.com/lit/ug/sbau029/sbau029.pdf

[jewilson]

Evaluation board with 4 each 1702 DAC’s

PRECISION DAC BOARD WITH 20-BIT
RESOLUTION
STANDARD DIGITAL AUDIO INTERFACE
(S/PDIF, EIAJ-1201) OPTICAL OR COAX
INPUT
SAMPLING RATE: 32kHz to 48kHz
COMPLETE ISOLATION FOR DIGITAL/ANALOG
HDCD DECODE FUNCTION
8X OVERSAMPLING DIGITAL FILTER
4 PCM1702 DACs PER CHANNEL
connected in parallel)
HIGH PERFORMANCE
THD+N: –90dB (16-bit)
–100dB (20-bit)
Dynamic Range: 98dB (16-bit, EIAJ)
***112dB (20-bit, EIAJ)***
S/N Ratio: 118dB (EIAJ)

-----------------------------------------------------------

Evaluation board with 1 each 1702 DAC

COMPLETE 20-BIT STEREO
D/A CONVERSION SYSTEM
NEW SIGN-MAGNITUDE DAC: PCM1702P
8x DIGITAL FILTER: SM5842AP
HIGH PERFORMANCE
THD+N at (F/S): 0.0015%
***Dynamic Range: 108dB (EIAJ)***
S/N Ratio: 120dB (EIAJ)
Non Zero Cross Distortion
SERIAL DIGITAL INTERFACE
ANALOG OUTPUT: 3V

According to the data sheets the 4 DACs evaluation board at 20bit has a 112dB dynamic range and a S/N of 118dB. The evaluation board with 1 DAC has a dynamic range of 108dB, however the are claiming 120dB S/N. I do find this strange that the single DAC board has the maximum theatrical S/N ratio! Also, there is a slight improvement in THD .0015 for one DAC as compared .001 for 4 DACs, however this is still in the range of slightly less that seventeen bits. Is this due to the digital filter, analog filter which I dough or the isolator, anyway it only a ¾ bit better resolution with four dac.?

So according to these documents the single board DAC has 18 bit resolution and the 4 DAC board has just little less than 114.4dB needed to get to 19 bits resolution. Also, we have several differences in the design of the evaluation boards will influence the performance. The 4 DAC board has an Isolator the ISO 150 which decouple the ground current noise from digital side from the DAC’s. This could be worth as much as one full bit in noise and Dynamic range. As for as the PDM 100 vs. the SM5842A there may not be any improvement their.

[Bernhard]

Mostly 2 parallel chips measure better than one.

Linearity errors are random, so they have to partly cancel out , the more chips, the better.

PCMxx chip grades were not accurate in my testings.
I had very good non grade chips and very bad K grade chips.

I start to think about PCM 56 instead of TDA1541 because I found PCMs that are better than S1.

Also they require less space and need no caps.

I put own selection of PCM in a Technics player and it beats every TDA S1 I have.

[Jocko Homo]

OK....in theory, you get a 3 dB improvement in S/N ratio.

How much imformation do you guys think is actually down there? Is it really worth the extra $35 or so, assuming that you are using PCM1704s/

Just curious.........

Jocko

[ThorstenL]

Konnichiwa,

Originally posted by jewilson
So according to these documents the single board DAC has 18 bit resolution and the 4 DAC board has just little less than 114.4dB needed to get to 19 bits resolution.

Tarnation and blimy. I did not quite read it right. My fault. I guess I read the figures I expected.

Puzzeling, the results. the 4 X PCM1704 SHOULD have been better....

Sayonara

[ThorstenL]

Konnichiwa,

Originally posted by Jocko Homo
OK....in theory, you get a 3 dB improvement in S/N ratio.

How much imformation do you guys think is actually down there?

Very littlke from experience and from PERSONAL EXPERIENCE I prefer single DAC's over paralleled.... Which still does not make paralleling DAC's a dumb idea.... ;-)

Sayonara

[jewilson]

Bernhard

Linearity errors in a DAC are not caused by random errors. Example in ladder DACs they are caused by the errors in R/2R resistor networks internal switching and impedance nodes. These are can be different from DAC to DAC and they can be different on negative vs. positive out and require laser trimmed low TC precision resistors. DAC’s with current weighted switching for R/2R ladders have different types of errors. Also, logic buffering can add to the error of the DAC producing transient errors.
Plus the different errors from the Delta Sigma DAC’s.


I’m listing type of DAC errors

QUANTIZATION EFFECTS
Aperture Error
Absolute Accuracy (Total) Error
Integral Nonlinearity (INL) Error
Differential Nonlinearity (DNL) Error
Gain Error
Offset Error

You may find this helpful so you can get an understanding of the type of errors in DAC converters.

http://focus.ti.com/lit/an/slaa013/slaa013.pdf

http://www.electronicproducts.com/Sh...epana1.sep2003

http://www.analog.com/UploadedFiles/...8400AN-313.pdf

http://www.analog.com/UploadedFiles/...4731AN-345.pdf

http://focus.ti.com/lit/an/sbaa047/sbaa047.pdf

http://focus.ti.com/lit/an/sbaa002/sbaa002.pdf
----------------------------------------------------------------------

Thorsten

Very littlke from experience and from PERSONAL EXPERIENCE I prefer single DAC's over paralleled.... Which still does not make paralleling DAC's a dumb idea.... ;-)

Just for fun yea I would like to try the with my 1702 DAC it would be an interesting experiment and there are the possibilities it might sound better with the extra drive current. The problem I have it other than the statements I've made is the cost. These DAC are about $30 each, that's 8 of them is $240. Ouch, a good idea for TI I think. You can bet that application engineer got a pat on the back.

Well they don't sample this part any more too bad.

[fumihiko]

konnichiwa

parallering DAC chips
it!s PATENTED by MASAOMI SUZUKI (Accuphase Lab)
PAT-No ŽÀŠJ•½‚U?|‚S‚W‚Q‚S‚Q