AD7934 full-differential mode

Dear All,

I'm using AD7934 with FPGA interface. I want to use AD7934 in differential mode.

I've used AD8138 for converting single-end to differential signal.

ADC is working in single-end mode (M0=0,M1=0) quite nicely. I tried all four input channels VIN0-3 in single mode all are working nice.

But When I configure ADC  for fully diferential and even pseudo differential mode but no success. Output keeps fluctuating.

Kindly help me

Thanx in advance

  • 0
    •  Analog Employees 
    on Mar 20, 2013 7:00 PM

    Hi,

    I moved this discussion to the precision ADC community, someone should be able to help you here.

    Best Regards,

    David

  • 0
    •  Analog Employees 
    on Mar 20, 2013 10:24 PM

    Hi,

    It's great that you get proper results in single-ended mode. I am assuming you configured the AD7934's input channels for fully differential and pseudo differential mode by setting MODE0 and MODE1 as detailed in Table 9 of the datasheet. Did you follow the typical connection as discussed in the datasheet? Can you share your circuit schematics as well as scope shots of your output data for a given input so I can take a look?

    Regards,

    Karen

  • Thanx for reply,

    Following way I've connected AD8138 (ADC driver) to AD7934.

    pin-4(AD8138) connected to pin-25 (AD7934)

    pin-5(AD8138) connected to pin-26 (AD7934)

    When I checked signal between VIN0-VIN1 of AD7934 it is coming as expected diffential

    for sinewave input. Then I applied 2V DC at pin-1 of AD8138 and 2.5V on pin-2(VOCM of AD8138). I observed around 0.5V voltage between VIN0-VIN1.

    I tried following Control register value : 309h,109h,129h and so many different combinations.

    But the result is fluctuating continuously even for DC input.

    please help

    Waiting for reply

  • I think I made a mistake in writing here, Control registers I tried are

    311,331,129,

    In pseudo differential mode, I was getting out put correct but only for either +ve only or -ve only even in 2's complement mode also.

    Connection for pseudo was as per datasheet, VIN1 connected to 2.5V and signal applied on VIN0

  • 0
    •  Analog Employees 
    on Mar 24, 2013 9:32 PM

    Hi, Ravi.

    Apologies for the delayed response. Yes, if you could please send in your schematics especially on the actual connections of AD8138 and AD7934, it would be very helpful. Please also include scope shots of your output.

    I have a few notes from what you have provided. What is your VDD? From your control register values, you are using 0-2Vref range of the ADC with the internal 2.5V reference. Note that in this case, VDD must be between 4.75V to 5.25V.

    Please also note that for pseudo differential operation Vin- is typically -0.3 to 0.7V for VDD = 3V and -0.3 to 1.8V only for VDD = 5V.

    Regards,

    Karen