Start with AD8137

Hello,

I'm starting a board for an audio application with an AD8137 on each input analog input of an AD1938.

To start, in this case, IN_GND = VREF1 = GND. The problem is that i have VO- = almost +VS and VO+=almost -VS whatever i have on IN+/IN-. I've tried to pull up with a resistor |PD but nothing change. I've also tried to isolate this schematic from the AD1938.

VS+ = 5V, Vs- = -5V.

I checked my schematic and PCB, nothing seems to be wrong.

thank you for your time.

  • 0
    •  Analog Employees 
    on Feb 9, 2015 8:42 PM

    Hi,output will

    I want to make sure about a few things:

    Are you AC coupling the output of this circuit to the ADC input on the AD1938? Your buffer is referenced to GND, but the AD1938 uses its own CM (1.5 V DC) as its reference. If you DC couple these circuits, the AD8137 will draw current from the AD1938 inputs and affect the ADC performance. You will need to put a capacitor between these circuits, 10 uF would be a good value.

    When you say that the VO+ and VO- signals are close to -VS and +VS, do you mean that you see DC voltage at these nodes? Have you tried measuring the nodes with no AD8137 in place, and then installing a new op amp?

    Regards,

    Coleman

  • Indeed, i've got 10uF on each VO before the AD1938.


    Also, I see DC voltage on VO+ VO- and i see it on each AD8137 of the board. If, hope not, the AD8137 are out, is there something wrong with this schematic ?

  • 0
    •  Analog Employees 
    on Feb 9, 2015 9:03 PM

    Hi,

    The connection between VREF1 and IN_GND is not clear from your schematic. How are they connected? For best signal integrity, noise performance and EMI compliance, I recommend a single solid power plane in the design, one GND reference only. Have you tried shorting the VREF1 node directly to the power supply ground?

    The schematic symbol looks correct, please check that it is mapped to the pcb footprint pins correctly.

    Regards,

    Coleman

  • Hello,

    I replace the driver with a new one, try to shorting VOCM (VREF1) to GND, try to shorting IN_GND to GND, try to use VOCM (VREF1) with the common mode voltage of the AD1938, same result every time, the outputs saturate and the input signal is degraded. I don't know where to look right now.

    I may have made a mistake somewhere, if anybody have an idea, thank you.