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DAC37J82: DAC37J82

tamir meoded
Prodigy 70 points
Part Number: DAC37J82
Other Parts Discussed in Thread: LMH5401, DAC38J84

Hi,

Our design use DAC37J82 with Rload=50ohm  on each IOUTAP/N analog output.

Data input code is  two complement (range:  (-32k) to 32k)

IOUTP/N  output current is 20mA   (coarse_dac[3:0] =0xA,  RBIAS = 1920ohm, VEXTIO=0.9V)

Measuring IOUTP/N on scope (1Mohm input resistance) shows that

Vdiff = 1V  (as expected), however, both signals have min DCoffset of 130mV :

IOUTP/N signals voltages are oscillating between 0.13V-1.13V.

DCoffset is higher if coarse_dac[3:0] is higher then 0xA.

We tried to "play" with DC offset  with config80[15], config8[12:0] registers....DC offset change was not enogh to make IOUTP/N signals voltages to be between 0V-1V.

Pls advise.

Thanks

  • 0
    TI__Genius 11930 points

    Tamir,

    The QMC offset correction only allows you to change within a 12bit range in the DAC codes and this will be enough to get your offset down to 0.5V.

    To understand why you are centered around 0.63V and not 0.5V I would need to see the DAC output circuit.

    Can you provide the schematic for the DAC output?

    Regards,

    David

  • 0 in reply to David Chaparro
    Prodigy 70 points

    Hi David,

    Thanks for the your reply.

    Pls see attached pdf that describe our setup and scope voltage level measurements :

    - IOUTP/N  WITH output current is 21mA  

    - IOUTP/N  WITH output current is 10mA  

    - IOUTP/N  WITH output current is 15mA  

    All measurements are with   config0[15] = 0  (qmc_offsetab_ena= 0)

    As can be seen, all measurements shows DC offset of ~130mV  

    -As for setting registers related to DC offset . We tried the following values:

    config0[15] = 1 (qmc_offsetab_ena= 1)

    config8 qmc_offseta(12:0)=0x1000 (decimal [-4096] )

    config8 qmc_offseta(12:0)=0x0fff (decimal 4095 )

    Results shows a MAX change of ~50mV. Not enough to bring IOUTP/N signal to be between 0V- 1V with common mode 0f 0.5V.

    Regards,DAC_DC_offset_meausrements_forTI.pdf

    Tamir

     

  • 0 in reply to tamir meoded
    TI__Genius 11930 points

    Tamir,

    I am taking a look at what you provided and will get back to you with more information on why this is occurring.

    Regard,

    David

  • 0 in reply to David Chaparro
    TI__Genius 11930 points

    Tamir,

    I have modified a board to verify the outputs you are seeing and they are correct. The DAC behaves this way when you do not apply your own common mode to the outputs. 

    If you want the common mode of the outputs to be 0.5V you will have apply the common mode using another component. The best way would be to use a fully differential amplifier to adjust the common mode of the differential outputs.

    Regards,

    David

  • 0 in reply to David Chaparro
    Prodigy 70 points

    Hi David,

    Thanks for your effort and help.

    I dont fully understand the solution of using fully diff. Amp..

    I will appreciate it if you reply with a general drawing of the suggested circuit  or a link to web site with such solution.

    TNX

    Tamir

  • 0 in reply to tamir meoded
    TI__Genius 11930 points

    Hi Tamir,

    The reason I suggested using a fully differential amplifier is because they have a VOCM pin that allows you set the common mode of the output signal.

    For a suggested circuit I would recommend you look at the High-Bandwidth Arbitrary Waveform Generator Reference Design. This reference design uses a DAC38J84 and one of its outputs goes to a FDA, LMH5401.

    www.ti.com/.../TIDA-00684

    Regards,

    David