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LMH6628QML-SP: Load capacitance (is there any restriction for load capacitance)

Alessandro Schirmer
Intellectual 290 points
Part Number: LMH6628QML-SP
Other Parts Discussed in Thread: LMH6628,

Tool/software:

Hello TI-Team,

we are using the LMH6628QML-SP in a space application. The LMH6628 generates/buffers an offset voltage and a CML voltage for a single to differential amplifier.

We have to implement a RC-Filter behind the output of the LMH6628 with R=10R and 10uF. Regarding Figure 23 in the datasheet SNOSAQ1B the capacitive load CL ends at 1nF in series with RS of 10 Ohms.

If it matters, the offset voltage is filtered with 10R/47nF (14 channels parallel) and CML voltage is filtered with 33R/100nF (14 channels parallel). The (second) filtering is directly at the inputs of the single to differential amplifier.

Q1: Are there any concerns/restrictions regarding the capacitive load (10uF)?

Q2: The input of the LMH6628 has no frequency as it is DC. What is in case of power up regarding the stability due to the high capacitive load.

Best Regards, Alessandro

  • 0
    TI__Mastermind 28825 points

    Hello Alessandro,

      I apologize for the delay. I will be able to get to these questions by today.

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    Intellectual 290 points

    Hello Sima,

    I would still appreciate your answer.

    Thanks, Best regards, Alessandro

  • 0 in reply to Sima Jalaleddine
    Intellectual 290 points

    Hello Sima,

    I would still appreciate your answer.

    Thanks, Best regards, Alessandro

  • 0
    Intellectual 290 points

    Dear TI-Team,

    is there anybody who could help me?

    Best regards, Alessandro

  • +1 in reply to Alessandro Schirmer
    TI__Mastermind 28825 points

    Hello Alessandro,

      I apologize for the really long delay on this issue. We appreciate the patience getting these solutions.

      Q1. You are on the right track here. You need an isolation resistor to isolate the capacitance from the amplifier and prevent instability. I ran a stability analysis on this to double check. I first double checked that at datasheet configuration, the open-loop AOL and Closed Loop Output Resistance. These two specs are the most important in making sure the model can correctly predict stability.  and 10 ohm resistor will give you around 76 degree phase margin compared to no isolation resistor which gives 17 degrees of phase margin. For optimal stability, system should have >45 degrees of phase margin. The closer the number is to 0, the more unstable since phase margin in sim starts at 180, and a full inversion (-180) will make a system fully unstable. Loop gain is what is shown in the first picture below, which is AOL - noise gain in log form. And the phase is found by looking at the 0dB crossing of the loop gain which is where AOL and Noise Gain intersect. 

        Below is the closed-loop frequency response of no isolation vs with isolation. With isolation, this creates an RC type filter which limits bandwidth to 1.58kHz. Since your input is DC, this will not be an issue. Also, having isolation will create a voltage divider. But, again, since this is DC, capacitor will not have this impedance affect unless you are paralleling a lower resistor value after this amplifier. 

        Q2: The input of the LMH6628 has no frequency as it is DC. What is in case of power up regarding the stability due to the high capacitive load.

         Is the input applied before the device is fully powered up? If no input, then there is no issue. If there is an input prior to fully power on of device, then the issue is not due to the high capacitive load, but with the abs max rating of the device. The input of the device may not exceed +/-0.5V above/below VS+/-. 

      Let me know if you have further questions on the above. I will make sure to be on e2e, and keep your questions in top priority.

    Stability LMH6628.TSC

    Closed Loop Response LMH6628.TSC

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    Intellectual 290 points

    Hi Sima,

    thank you very much for your detailed answer. This really helps to understand it.

    Concerning Q2: The power pins are supplied before the input pins. So we are fine with it.

    Best regards, Alessandro