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Setting the full scale current output of AD5426

Dear friends, 

We are trying to design a 125kHz bipolar square waveform generator to communicate with a old digital computer. Its a form of BPSK type digital signaling.

We shall connect the AD5426 thru a SPI bus to the uC, and will be using ADR01 for the 10V reference. For dc isolation, we used a transformer coupling as specified in AN-912. Using the spreadsheet provided, we got the following values,

  • Transformer ratio - 1CT:1
  • DAC output resistor / termination - 75ohms, 75 ohms (we assumed no reconstruction filter will be required for our application)
  • Output load - 75ohms
  • Full scale DAC current output - 20mA
  • Voltage across the load - 0.354 Vrms (1.000 Vpk-pk)
  • Output to be amplified on the isolated side

I am a bit confused how the full scale current output is being calculated for a balanced current output DAC. How do I achieve the 20mA output ??

What will be the maximum current output the DAC AD5426 can deliver ?? How is it being calculated.

For your reference, I have attached a snapshot of the oscilloscope readings of the captured data from the system, you can see the 180 degree phase shift.

Please feel free to share any answer, opinion or criticism. I am here to learn.

With my best regards,

Tapajit Mondal

  • Hi TapajitM,

    The AD5426 is able to output a current of 1mA for 10V reference voltage, following the formula below.

    Iout = Vref/R ,where the input resistance is R(typ)=10KΩ.

     Multiplying DACs would not be an option applications requiring such a high output current.

     

    ADI offers Current Source/Sink DACs with higher output currents up to 300mA Isourcing and -60mA Isinking.

    Another alternative to Multiplying DACs for waveform generation are DDS (Direct Digital Synthesizer), providing digitally programmable waveform sequences. Because the device is pre-programmable, it eliminates continuous write cycles from a DSP/μC in generating a particular waveform. These parts offer different programmable output modes including square waves.

    Regards,

    Estibaliz

  • Many thanks for the clarification.

    I would like to ask, if I were to follow the application note AN-912, which DAC should I use ?? Can you please suggest one ?? They have showed two examples with 20mA DAC output current in the same application note.

    Implementing a DDS is beyond the scope of my time-frame, unfortunately.

  • Hi TapajitM,

    The AD5770R offers sink/source output currents in the range of -60mA up to 300mA. This is a multichannel Precision DAC that can provide both the +20mA and -20mA to drive the transformer.

    Regards,

    Estibaliz

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