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LMH6554: Noise Problem due to Vcm

Gozkul
Prodigy 105 points
Part Number: LMH6554
Other Parts Discussed in Thread: ADS4145, , LMH6654

Tool/software:

Hello TI Family,

We are using LMH6554 Fully Differential Amplifier for driving ADS4145 ADC. Here our block diagram our structure:

 

Our LMH6554 works 0V to 5V and our Vcm voltage 2.5V. We are supplying V+_1 and V+_2 pin with 5V LDO and V-_1 and V-_2 pins are connected to GND. We are supplying the Vcm pin with 1:2 voltage divider by using same LDO that feeding V+_1 and V+_2 pins. In this situation, we see random noise floating around the spectrum on the input and output pins of the LMH6554. Also, when we investigate our ADC Spectrum in FPGA, we see same noise in our operating band even LMH6554 and ADC are not connected each other. This means that generated noise effects our ADC without electrically conducted. Further, When we apply the supply voltages to the LMH6554 but do not apply the Vcm voltage, we observe that the noise disappears! Additionally, when we connect the Vcm voltage at the input of the LMH6554 to the Vcm voltage of the ADC(0.95V) instead of supplying it externally(2.5V), we also observe that the noise disappears. Next, we try voltage reference IC (MAX6126) for 2.5V Vcm supply, we see the same noise problem. Lastly, We’ve tried low noise LDOs and it seemed that the noise problem continues.

From this perspective, we examined the LMH6554 Data sheet and saw that it was written the following statement for Vcm.  “Also shown in both figures is a capacitor from the VCM and VEN pins to ground. These inputs are high impedance and can provide a coupling path into the amplifier for external noise sources, possibly resulting in loss of dynamic range, degraded CMRR, degraded balance and higher distortion.” According to all these findings, we are considering Vcm creates this noise. Considering these we have few questions about the problem?

 

  • What can be the reason that kind of noise because of the Vcm?
  • Could there be a correlation between the amplitude of the Vcm voltage applied at the input of the LMH6554 and the generation of such noise?
  • What could be the difference between applying the Vcm voltage externally to the input of the LMH6554 and using the ADC's own Vcm voltage?

Thanks in advance.

  • 0
    TI__Genius 14011 points

    Hello Gozkul,

    Please allow me some time today & next week to fully answer your questions.  Proper use of a capacitor on the VCM pin will help reduce noise coupling into the IC.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 105 points

    Hello Alec,

    Thanks for your return. Noise that we mentioned which can be seen in our operating band that travels as you can see the uploaded video. 

    Click here to play this video

    We have tried different values of bypass capacitors and also RC Filter before the LMH6554 supply. There was nothing changed. We are waiting for your detailed response.

    Thanks in advance. 

    gozkul

  • 0 in reply to Gozkul
    TI__Genius 14011 points

    Hello Gozkul,

    Thank you for responding about trying bypass capacitors and RC filters before the LMH6554 device supplies.  Could you share these bypass capacitor and RC circuit values?

    I also cannot load or watch the video you have sent.  What video file format are you attempting to send?

    Could you described how the noise travels?  

    Can you also share your operating bandwidth?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 105 points

    Hello Alec,

    We've tired bypass capacitors above and below of 220nF such as between 390 pF and 1 uF. We have also tried RC filter before supply and VCM voltages of FDA which was 22R-4.7uF but we could not see any differences. 

    We are working at 70 MHz with 7 MHz bandwidth.

    I have re-uploaded the video that we saw the noise problem in MP4 format. Please let us know if it works or not.

    We are waiting for your response.

    Best Regards,

    gozkul

  • 0 in reply to Gozkul
    TI__Genius 14011 points

    Hello Gozkul,

    I can now view the video.  Please allow me time to review and suggest a fix.  If you have tested a range of bypass capacitors, there may be another solution we can try.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 14011 points

    Hello Gozkul,

    You could try forcing the common-mode voltage of the ADC to be 2.5V from the same supply as the VCM pin of the LMH6654.

    The problem may be the common-mode voltage of the ADC is different from the 2.5V supplied to the LMH6554, which results in the observed behavior.

    Using the VCM pin of the ADC to drive the VCM pin of the amplifier is a recommended practice.

    Best,
    Alec