High-speed DAC for 8-bits

I am interested in a DAC for 8-bts that can work at high speed with output of about 2 to 3 Volts. I am using a FPGA to generate 8 bits, which I need to convert into an analog voltage to drive an optical modulator, which requires a voltage in a range from 0 to up to about 2 to 3.5 volts to modulate the light. My application requires fast conversion: I would like to convert this 8-bit code into the analog voltage within 10 ns, so I would like to know which DAC chip I should use and if possible I would like to have an evaluation board for this chip.

Many thanks.

  • 0
    •  Analog Employees 
    on Jun 2, 2014 9:31 PM

    10 ns seems slow for optical transmitters. Will you describe your need in more detail, please? Our high speed converter lines tend to have more bits for more traditional transmitter architectures, but there may be something that comes close to your needs.

  • Hi Danf,

    Thank you for your response. I need this converter for a project in an academic-research lab, where we need to drive an electro-optical modulator to produce light modulation ad different levels within 10 ns or less to potentially reach modulation rates up to100-200 MHz. This is certainly very slow for real optical transmitters, but at this stage in our project, a “slow” modulation rate (~100 MHz) is enough for us. To perform full optical modulation, between states on and off of light, I need to drive this modulator with a voltage around 3 V.


    We don’t need a lot of resolution, so 8 bits will be good enough for us, and I will be generating the digital inputs for the converter using a FPGA. This FPGA will deliver digital signals with rising and falling edges < 1 ns, and uses a crystal oscillator as a master clock, which can be configured to work close to a rate of 200 MHz with PLLs.

    Do you think we can use a high-speed converter with more bits, but only using 8 bits of them as a 8-bit converter? Could you suggest one which we can get the evaluation board for it?

    Thank you

  • 0
    •  Analog Employees 
    on Jun 10, 2014 7:22 PM

    You might try the AD9748 8-bit single DAC, but it may not be quite fast enough for you at 210 MHz. The next step could be the AD9734 10-bit that you could just use the top 8 bits and stuff 0's for the LSBs. You can order the AD9736 14-bit EVB and stuff 0's for the bottom 6 bits to see the same effect. You don't have to clock those parts at their max DACCLK frequency, but they give you room for faster settling if you need it.

  • Danf,

    Thank you for the help. I have been looking at the AD9748, for which there is an evaluation board (EVB) available described in the datasheet. However, there is not enough information about the EVB in the datasheet describing how to change among different possible output configurations described in the AD9748 datasheet. I wonder if you can point me to the manual or a guide for this evaluation board.

    Specifically, I would like to use a configuration described as "differential coupling using an opamp", but by looking at the datasheet it seems that the EVB may not support this configuration which requires an OpAmp. 

    Thank you

  • Danf,

    We got the evaluation board for the chip AD9748, and I have a question about the input pins of the digital bits (DB0X-DB7X).

    In page 19 of the spec sheet of the AD9748 it indicates that the inputs in the "header" for DB0-7X are pins 27, 25, 23, 21, 19,17, 15 and 13, and DB0-7X are directed to the Digital bit inputs of the chip DB0-7. However, by looking at the evaluation board layout in page 21, these pins do not go to the right pin input of the chip, as described in page 7. Instead, it looks like to me that the "header" inputs corresponding to DB0-7X should be 15, 13, 11, 9, 7, 5, 3, 1, where the MSB (DB7) is pin 1.This would match the evaluation board layout and the in configuration of the chip in page 7.

    Could you please tell me if this is correct?

    Thank you