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LMH6629: Alternative way to compensate.

alperen akk
Intellectual 310 points
Part Number: LMH6629
Other Parts Discussed in Thread: TINA-TI

Hello,

I need to use LMH6629 with low gains. The datasheet suggests to use input lag compensation technique however, for my application noise is the most important spec, and therefore I can't use this method. I can however sacrifice bandwidth. I only need 12 MHz of Bandwidth and I need to operate LMH6629 with 2 gain or as buffer, if possible. Is there a way to operate this opamp with such low gains sacrificing bandwidth?

  • 0
    Guru 140200 points
    Hi Alperen,

    what about using the minimum gain and decreasing the output signal afterwards by the help of a voltage divider? At the same time you could decrease the bandwidth down to 12MHz (with a suited cap) and by this furtherly decrease the noise.

    Can you tell us more about your application?

    Kai
  • 0 in reply to kai klaas69
    Intellectual 310 points

    Thanks for your suggestion Kai,

    I am trying to build a 3-opamp AC coupled, single supply, differential amplifier out of 3 lmh6629's. The last stage is kind of a problem here because its gain is very low. I can actually split the gain between two stages, but I end with a minimum gain of 16 in this case, I need about 4 to 5 gain for this differential stage. I like your suggestion and I can achieve the gain I want using that method, but I use very low value resistors to achieve low noise, and increasing gain would increase my power consumption quite a lot. That why I don't want to implement this method. I might end up using a different opamp just for the last stage. 

    I just want to know if it's possible to use feedback capacitor in parallel with the feedback resistor to both stabilize and decrease the BW of the opamp? Any thoughts on that?

  • 0 in reply to alperen akk
    Guru 140200 points
    Hi Alperen,

    I cannot give you a "go", because I cannot simulate the phase margin. Datasheet does not present enough information. E.g. open loop output impedance is missing.

    But you could simulate the circuit with TINA-TI and hope, that the model will show representative resuslts. I made some simulations, and if the model is valid, it could work. I have simulated a difference amplifier with 200R resistors and 68p caps. But take care, the circuit does not allow any load capacitance at output of LM6629. Then immediately instability begins. So, you would need to isolate any output capacitance by some series resistance (50R or so).

    Have you verified that the external compensation really increases the noise? Have you already simulated your circuit with TINA-TI?

    Kai
  • 0 in reply to kai klaas69
    Intellectual 310 points
    Hello once again!

    Yes I tried to simulated the circuit in TINA software, but I haven't verified that external compensation increases the noise in TINA. What I understood from the datasheet is that the external compensation network increases the noise gain above the minumum allowable gain while keeping the signal gain constant across the BW. I don't know the whole theory behind it, but some other documents also have the same comment about this compensation scheme. The increase in the noise gain might happen beyond my frequency of interest and if that is the case it doesn't really matter if the noise gain increases because it's going to be filtered anyway.

    Thanks for your response again. I need to study this compensation scheme and see what's going on.