Using ADCMP553 for signal recovery

Hi all,

I am trying to recover a signal from a photodiode (1 to 100Mhz)

Because the signal is very noisy, voltage range vary and my amplifier output is differential, I decided to use ADCMP553 comparator.

My problem is that comparator outputs just follow the inputs.

I expected a logic PECL output.

Schematic and scope capture include.

Thanks for your help.

  • 0
    •  Analog Employees 
    on Jul 1, 2013 2:52 PM

    Moving this question on ADCMP553 to the Amplifier Community.

  • 0
    •  Analog Employees 
    on Jul 28, 2013 8:02 PM

    Hi, PhilippeB.

    Apology for the very late reply. Few comments on your schematic:

    1. Are you matching your ADCMP553 input into a 75ohm termination? PECL is usually used to drive 50ohm transmission lines. The 75ohm termination is used for video applications. Change R25 and R26 with 82ohms, and change R19 and R20 with 130ohms.

    2. What are the values of R27, R28, R29 and R30? These should also comply with a 50ohm termination. Use 82ohms to R29 and R30, and 130ohms to R27 and R28.

    The recommended resistor values are used for proper termination. Try setting them first then let me know of your result.

    Regards,

    Anna

  • Hi, AnnaFeB

    Thanks for your reply.

    I follow your recommandations but unfortunatly with same results.

    I passed some other tests with comparator "alone" , 2 potentiometers (200 ohms, each center at 100 ohms) for inputs, and correct (130 and 82 ohms) termination.

    I still have a window where outputs change linearly :

    I mean that when I change value of one potentiometer to have an input voltage from 2.48V to 2.53V, my outputs also change from approx. 3.22 to 4.04V (and 4.04 to 3.22).

    So I am unable to work with signal smaller than 50mVpp.

    I will order new samples (we only had 2 items), and will also look at ADCMP58x family (or other).

    Our major constraint is a response time lower than 1ns (not much choice of component)

    Thanks for your help.

    Regards

    Philippe

  • 0
    •  Analog Employees 
    on Aug 13, 2013 4:54 AM

    Hi, PhilippeB.

    Few comments:

    1. For a small signal of 50mVp-p, the differential line should be very well balanced in order to limit noise thus having a good common-mode noise rejection. Also, your differential lines should be as close as possible to the ADCMP553 pins.

    2. If the output impedance of your amplifier (before the ADCMP553) is 50ohms, then you terminate the inputs of ADCMP553 as what I have suggested (use 130 and 80ohms). If it's not 50ohms, then can you provide me its output impedance so that we can know if a termination is still needed or to know what values to use.

    3. As seen on your scope shot, you set the output of ADCMP553 in AC mode. Can you set it to DC mode because usually PECL output is in DC.

    4. Using a potentiometer may result in instability especially if it's a mechanical pot. Instead you may try using a digital pot or a trimmed pot in order to have a fixed resistance.

    Regards,

    Anna

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:54 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin