ADA4941-1

Hi All,
I am looking for OPAMP which can convert single ended signal to differential signal to drive the ADC which has differential input. I have selected ADA4941-1 and tested for -2 and +2 gain configurations which are mentioned in Figure 58 and Figure 50 of datasheet. For -2 gain configuration, I am getting both the signals (positive and negative) but both are in 0 deg phase shift instead of 180 deg and for +2 gain, I am getting only positive signal (in negative line we are getting DC signal). For both the configurations (-2 and +2), I am connecting DIS pin to ground. Please suggest what might be the problem ASAP.
I am  tried to simulate same configurations (-2 and +2 gain configurations of ADA4941-1) in "ADIsimPE" but its showing error. The error is "*** ERROR *** The circuit is too large to simulate with this version.Too many analog nodes". Please suggest how to counter this error.
Thanks & Regards,
Lakshmaiah S
C.No: 9989787530/8978195982.


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[edited by: JValeriani at 7:44 PM (GMT 0) on 10 Oct 2019]
  • 0
    •  Analog Employees 
    •  Super User 
    on Oct 7, 2019 11:34 PM

    Hi Lakshmaiah,

    I cannot tell what might your problem be with not getting the right output from ADA4941-1. The DIS pin can be tied to the VEE pin to enable the part. Or, if you are running the device with a single positive supply, DIS should be tied to ground to enable the part.

    You can use LTspice to simulate your circuit. I've imported the Pspice model of the part into LTspice below and I get the proper differential output (Gain_dm= -2 V/V). May you can use the zip file below to find out what could be the issue?

    ADA4941-1 EZ Gain-2VperV 10_7_19.zip

    Regards,

    Hooman

  • Hi Hooman,
    
    Thanks for your response.
    
    I have tried gain of -2 configuration (Figure 58 of datasheet) in LTspice. In LTspice, for 0.5 VPP input, the outputs are 0.5 VPP on each line (1 VPP differential output). When I implement practically, same circuit is giving 1 VPP on each line (2 VPP differential output) in DSO but in SA it's showing -2dBm which is equal to 0.5 VPP in 50 Ω.  Please suggest what might be the problem. For your reference, Please find the attached test setup..
    
    Note: While testing, I am feeding signal through signal generator (0.5 VPP in 50 Ω) and measured the output in DSO (Kept in 50 Ω mode and used 50 Ω probes) and Signal Analyzer (50 Ω).

     DOCX
    Thanks & regards,
    Lakshmaiah S.
  • 0
    •  Analog Employees 
    •  Super User 
    on Oct 10, 2019 6:12 PM in reply to lakshmaiah

    Hi Lakshmaiah,

    Your RF Sig GEN (50ohm) needs to be terminated in 50ohm next to the ADA4941-2 to deliver 0.5Vpp to the amplifier. I'm not sure if you have that on your board as your drawing does not show it.

    I'm talking about making sure R5 is there to provide termination to the GEN output impedance R4:

    Regards,

    Hooman

  • Hi Hooman,

    Thanks for your response.

    I have tried with 50Ω (R5). Yes, I got the expected level in DSO. But I am unable to correlate the readings observed in DSO and SA. Please go through below table and suggest how to correlate DSO and SA results.

    Please note that DSO values are taken by keeping DSO in 50Ω mode.

    Description

    Input Voltage

    Measured Output Voltage

    CRO

    Signal Analyzer

    Without 50Ω

    0.5 VPP (-2dBm)

    2 VPP

    -2 dBm (0.5 VPP in 50 Ω)

    With 50Ω

    0.5 VPP (-2dBm)

    1 VPP

    -8 dBm (0.25 VPP in 50 Ω)


    Thanks & Regards,

    Lakshmaiah S

  • 0
    •  Analog Employees 
    •  Super User 
    on Oct 11, 2019 5:49 PM in reply to lakshmaiah

    Hi Lakshmaiah,

    Only the "With 50ohm" row should be considered as a 50ohm GEN requires proper input termination (50ohm) so that the power setting (-2dBm in this case) corresponds to the actual power delivered to the load.

    With this condition, having 0.5Vpp at the input, each ADA4941-1 output should swing 0.5Vpp (1Vpp differential). What you labeled at "CRO" may be the differential output measured which works out correctly?

    Your Spectrum Analyzer image states the following in the bottom right hand side:

    "AC Coupled: Accuracy Unspec'd < 10MHz".

    You're testing a 1MHz sine wave and thus your spectrum analyzer is not going to produce accurate results below 10MHz. That may be the issue you're having. Try DC coupling the SA or use one that can accuractely function at 1MHz. Or raise your frequency.

    Also, make sure you're not connecting the SA and the 50ohm terminated scope at the same time as this would represent a heavy (25ohm) load to ADA4941-1.

    Regards,

    Hooman