ADA4938-2 problem

Hi,

I have a confusing problem with ADA4938-2. I connected ADA with unity gain and dual supply to AD9253. Vocm is provided by ADC. Schematic and parts values seem to be correct but output of ADA is very noisy. Any suggestion??

  • Hi Den,

    Could you share your schematic for us to check? Thanks.

    Anyway, I would assume the following conditions: AD9253 AVDD = 1.8V and ADA4938-2 is in single supply (let's say 5V). Given the condition that the Vocm is coming from AD9253's VCM which is AVDD/2 = 0.9 V or from the VREF which is 1.0 V, in these cases those values are out of range for ADA4938's Vocm.

    The ADA4938 operates at Vocm of 1.3V away from the supply rails. So at single supply (VCC=5V), this range will be from 1.3 V to 3.7 V. You may use 2.5V supply to power ADA4938 if you wish to drive the AD9253, but the Input and Output Ranges would change.

    Regards.

    Jino

  • Dear Jino,

    Thanks for your quick reply. I connected Vcm of AD9253 to Vocm of ADA4938. So common mode voltage is set on 0.9 volts. The main problem is strong spurs in op amp output. Here is some information.

    1.Schematic:

    2. FFT of ADC samples (op amp Connected to ADC ):

    3. FFT of ADC samples (op amp disconnected from ADC)

    Best Regards,

    Den.

  • Hi Dougl,

    ADA4938-2 supplies are +5V and -5V

    Thanks

  • Hi Denkins,

    Thank you for your interest in ADA4938 and AD9253. I'm sorry but I cannot see the schematic clearly. Would you please confirm what the ADA4938 supply voltages are?

    I believe Jino's point is that, depending on the amplifier supply voltages, the ADA4938 might not be able to support an output VCM = 0.9V even if 0.9V is applied to the VOCM pin.

    Thank you.

    Doug

  • Dear Doug,

    Both FFTs are noise floor of ADC. First with op amp and second without it. I really confused about this. I've designed  many software radio cards including ADC, DAC, FPGA and ... and this circuit is first of them that failed in analog section.