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Using Wolfson DAC'S as a general purpose dac — Parallax Forums

Using Wolfson DAC'S as a general purpose dac

steprogsteprog Posts: 227
edited 2015-01-03 12:07 in Propeller 1
Hi,
I am in the middle of a design and wanted to get any opinions out there. I have a project where I am putting a circuit downhole that will have a 24bit a/d prop and fiber optic link to another board uphole. The uphole board will decode the A/D data and put it out analog. Basically, it is a 3 channel "digital wire" for an existing geophone.

I have been not keeping up with the devices for a while now but find getting a general purpose DAC with 24 bit accuracy is not that easy. It seems the application driving this technology is stereos and such. So I am thinking of using the dac's produced by Wolfson for my application.

I just wondered, if anyone knows any problems with doing that out there... a penny for your thoughts
Thanks,
Greg

Comments

  • MJBMJB Posts: 1,235
    edited 2015-01-01 11:04
    what sample rate do you require?
    24 bit is a 'lot'
    can you ensure there is no noise crawling in?
  • steprogsteprog Posts: 227
    edited 2015-01-01 13:40
    no, can't make promises about noise, expect it actually, part of it is marketing and resolution. My biggest concern is whether those parts can be used as general purpose or not
    Greg
  • abecedarianabecedarian Posts: 312
    edited 2015-01-01 14:43
    Could you do two 12 bit DAC's into a summing amplifier?
    Something like: 
    DAC1_OUTPUT --> 10K ---+
                        >---- OP_AMP(+)-----|\
DAC2_OUTPUT --> 20K ---+                    | \
                                            |  >---+----> OUTPUT
                                            | /    |
                        +---- OP_AMP(-)-----|/     |
                        |                          |
                        +--------------------------+
  • steprogsteprog Posts: 227
    edited 2015-01-01 15:53
    interesting ideal, not sure the math works.. Does that mean you don't like the Wolfsons stereo dac's?
  • jmgjmg Posts: 15,173
    edited 2015-01-01 19:24
    steprog wrote: »
    ... It seems the application driving this technology is stereos and such. So I am thinking of using the dac's produced by Wolfson for my application.

    I just wondered, if anyone knows any problems with doing that out there... a penny for your thoughts
    Try it ? - and check for Zero Drift and zero access pins.

    I think the issues with Audio DACs is they are designed for AC coupled signals, so there is no design focus on DC Zero, or DC Zero drift, and some have no true 'zero' pin.
    - but they may be 'good enough' for many Industrial apps.

    If you can get a Differential Output version for a tolerable price, that would have better Zero performance.
    There are some Audio DACs now that include Charge Pumps, to give a -ve Rail and so give a true Zero mid-point.
    They would also be good candidates for checking for Industrial use.

    The WM8524 looks close ?
  • steprogsteprog Posts: 227
    edited 2015-01-01 20:59
    Thanks, since signal I am looking at is the output of a geophone, which is a AC signal by definition, it sounds like...maybe. Zero drift may not be that big a problem.. I am new to this application so.. a bit nervous to trail too far off the beaten track.
  • MJBMJB Posts: 1,235
    edited 2015-01-02 02:44
    what is the frequency range you need for the geophone?
    Probably the lower audio range - so you can check the DAC datasheets for the low freq behaviour.
  • jmgjmg Posts: 15,173
    edited 2015-01-02 08:05
    steprog wrote: »
    Thanks, since signal I am looking at is the output of a geophone, which is a AC signal by definition, it sounds like...maybe. Zero drift may not be that big a problem.. I am new to this application so.. a bit nervous to trail too far off the beaten track.
    Sounds well worth a try.
    The part number above seemed to be among the cheapest, and it included the charge pump.
    You could test for DC zero match, and check sloope-tracking between channels too, as very good tracking could allow you to generate differential test signals with common mode 'noise' added.
    If gain tracking is not so great, then an external summer would allow common mode on one channel, and test signal on the other
  • abecedarianabecedarian Posts: 312
    edited 2015-01-03 12:07
    steprog wrote: »
    interesting ideal, not sure the math works.. Does that mean you don't like the Wolfsons stereo dac's?
    No experience with audio DAC's, so no comment regarding them.

    And yes, the math is off. I guess New Year's Eve hadn't worn off yet. >blush<

    Another option might be to have two DAC's outputs fed into either end of a digital potentiometer, and the 'wiper' from the digi-pot provides the output voltage, and an ADC looking at the wiper output could give you feedback to adjust the DAC's and digi-pot match the output you expect to send.
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