Why 4*LO has problems in AD8333

Really thank you for your help.

I am struggling with the application of AD8333.

I am confused that when I input 4*LO sine wave to Din of DS90C401, its output (Dout-, Dout+) was not a sine wave. Even though I adjust the amplitude of the input. It still could not work.

I don't how to solve this issue. 

The purple trace with a sine wave is the input to Din of DS90C401,but the other two traces were Dout(+, -). I don't know why they were not sinewave.

Thanks for your kind help

  • 0
    •  Analog Employees 
    on Dec 12, 2019 12:55 AM 11 months ago

    Hi,

    Can you provide a schematic of your circuit? Do you have the transimpedance amplifiers on the outputs of the AD8333? See Figure 64 in the AD8333 datasheet as a reference.

  • I am very grateful for your kind reply. I actually followed the reference design of the AD8333 datasheet. I firstly simulated this circuit in Multism, but it seemed to have problems that are shown below.

    As the figure above, I input a reference signal with 4MHz(sinewave) to 4LO directly, and I set  Reset to a high level. RFIP and RFIN were a pair of differential signals which were converted by the previous VGA circuit. The output was as follows. In fact, It did not accomplish I/Q demodulation when I set Reset to High.

    However, when I put Reset to a Low level. It seemed to work with 2Lo signal plus DC component, but its signal was definitely weird, like below.

  • In addition, I also made an AD8333 circuit board without the previous VGA differential conversion module because I did that on another board in advance. According to the official recommendation, I used LVDS DS90C401 before my input reference signal entered 4LO. I looked up a lot of articles that told me the LVDS was used to process a digital signal instead of an analog signal. So I was confused that if 4LO's input should be an analog or digital signal. If it is a digital signal like a square wave, how they perform the mixing operation?  The following figure shows the schematic.

    When I changed a sine wave as the reference and I canceled the connection of DS90C401. The output was still wrong. I/Q could not track the tested signal's amplitude or phase. 

    I am really upset about its application because I have spent plenty of time to study it. But the outcome was always filled with errors.

    Best wishes

  • 0
    •  Analog Employees 
    on Dec 16, 2019 10:54 PM 11 months ago in reply to Carecr

    Hi,

    What is the DC common-mode of the LO in the above schematic?

    In your first screenshot, it looks like the Dout(+,-) are going below ground. Where are these points being measured? The LO common-mode voltage range for the AD8333 is 0.2V to 3.8V.