0.01mV precision DAC, differential output?

Discussion in 'General Electronics Chat' started by richiechen, Oct 25, 2012.

  1. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    (Updated 26/10 2012)


    Dear All

    I have solved the power problem and here comes my design purpose, schematic and some questions:

    I would like to control an optical modulator by a DAC. The DAC needs a precision of 0.1mV. The DAC will be connected with the modulator through a coaxial cable.

    1.One end of modulator is connected with ground. In order to avoid ground loop, I think differential output(from LP filter after DAC) should be connected with the modulator, is that correct?

    2. After DAC, there will be a low-pass filter, before connecting with the optical modulator. The output of the low pass filter will be differential pairs. Which kind of DAC should be used to connect with the differential output LP filter, a differential DAC or a voltage output DAC?

    3. I have read in a tutorial (http://www.hit.bme.hu/~papay/edu/Acrobat/GndADCs.pdf) that the connection between DAC with data bus should be buffered to reduce digital return current. Is it also true for a microcontroller based application? If so, what kind of buffers should be used?

    My current design: MicroController (No Buffer)---> Single end output voltage DAC--->single input differential output low pass filter---->Coaxial cable---->Optical modulator.

    It would be really appreciated if you could give me any suggestions/advice on the questions/design.
    BTW, I have watched 5 things you need to know about DACs. It is very useful.

    Thank you very very much.

    Regards
    Richie
     
    Last edited: Oct 26, 2012
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Careful consideration to what currents go where is better. Given a clean DAC signal, you should be able to make a "star" ground connection between a single ended DAC output and the modulator.
     
  3. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    Just updated the thread.

    Really appreciate if you could have a look.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    We have no idea what your ground network looks like. you need to provide a picture and/or a detailed description
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    You state you want to build an digital to analog converter (D2A).

    The doc you reference is for an analog to digital converter (A2D).

    One of these things is not like the other...

    A D2A does not need to drive a bus as it has no output logic lines.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    If you read the doc he referenced, you'll see that it addresses both ADCs and DACs.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Richiechen, is your optical modulator a commercial product? If so, can you link to the datasheet?
     
  8. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    Ron H

    Thanks for your reply.
    Here comes the datasheet of the optical modulator:
    http://www.jdsu.com/ProductLiterature/ddmz10g_ds_cc_ae.pdf

    In page 3, there is a schematic of the modulator. The DC bias pin is the voltage input pin which receives signal from DAC.
    The DC bias pin should be connected with coaxial cable and the outer layer of the DC bias pin is connected with modulator case(grounded). Therefore, one end of modulator, which is the signal receiver, is already connected with ground.

    BTW, I dont quite understand the inside structure of the modulator. Could you please give me a hint?


    Regards
    Richie
     
  9. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    Ernie

    You are right saying that ADC needs buffer because of the driving capability.

    However, as the document says, DAC needs a buffer to reduce the digital return current from ground. Without a buffer, the current has to be returned to MCU. With a buffer, the current could be returned to the buffer directly and the current will be smaller.

    My question is, in a microcontroller bases application, is the buffer necessary? And where to find the buffers?

    Thank you.

    Regards
    Richie
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Sorry, after scanning several pages of inapplicable information I stopped looking.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Richie, do you know that the modulator is optimized for RF application? It appears to have bandwidth down to DC, but It says nothing about linearity. Google "mach-zehnder modulator tutorial" and click on the second link on the page. It says that the transfer function is nonlinear, compressing the positive and negative peaks (see the section on Mach-Zehnder modulators).

    And where will you get an optical demodulator with .01mV precision (whatever that means)?
    What are you planning to do with the modulated signal?
     
  12. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    You are right saying that the modulator is non-linear. Actually I am trying to use the non-linearity to control the modulator. I will send a test signal to the modulator and analyze the distortion of the response.

    The modulator needs a DC bias voltage to operate correctly. And the bias voltage will need a step size of 0.1mV.

    If this is the case, do you think differential output is necessary?


    Regards
    Richie
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    OK, now I get it.
    The signals that need to be differential are the RF inputs. The grounding requirements for the bias depend in part on how far away the modulator is from the bias source.
    As I said in post #4,
     
  14. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    The distance depends on the customer's need. Can I just make the output differential?

    If so, a single end DAC is better + differential LP filter is better
    OR
    a differential DAC + differential LP filter is better?

    Thank you
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Are you thinking of connecting one side of the differential DC bias to channel 1, and the other to channel 2? Would you make the differential voltage symmetrical around ground, or what? I think you need to contact JDSU and get some detailed applications information. You don't know enough about what you are trying to do to make a commercial product.
     
  16. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    You are right!! I should contact JDSU!

    What I planned to do is to make the DAC output differential. It means DAC output has no end connected with ground. Both end will be connected with modulator bias 1.
    It means one end of DAC will be pulled to ground by the modulator. However, DAC itself has no end connected with ground. Therefore, no ground loops will be formed.

    Richie
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Are the modulation signals coming from the same board as the DAC? Their shields need to be grounded.
     
  18. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    The modulation signals are not from DAC. DAC only has DC bias signal.

    Why modulation signal's shield needs to be grounded?

    Richie
     
  19. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    If it's not grounded, there is no return current path for the signal, which is on the center conductor. They could be grounded through a capacitor, though.
     
  20. richiechen

    Thread Starter Member

    Jan 1, 2012
    93
    0
    The modulation signals are sent through coaxial cables. The shield of the coaxial cable connects the modulator ground and the V- of the modulation signal.

    Excuse, I still confuse why the signal has to be grounded.

    Thank you.

    Richie
     
Loading...