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LMH6505: Noise during mid-range gain adjustment

Gary Kitten
Prodigy 130 points
Part Number: LMH6505
Other Parts Discussed in Thread: BUF634A

We're trying to add a LMH6505 to our photodiode detector to remotely control the gain, using a DAC as the control signal.  The peak voltage coming from the photodetector op amp is 1.35V, and we'd like to achieve a gain of 5x.  However, when I test the circuit, I see very noisy output in the middle of the gain range.  The extremes are OK, but the middle shows lots of noise in the Kilohertz range.  I added a capacitor to the Vg input and that helped some, but mid-range gain is still noisy.  Tried decreasing the 10u caps to 1u, but that was no help.

Is there anything I am overlooking or misinterpreting?  This is a 2 layer board, so no ground planes to be concerned about.

  • 0 in reply to Gary Kitten
    TI__Genius 16725 points
    Hi Gary,

    I cannot see your schematic. Can you re-post the schematic so I can see what you are doing?

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 130 points

    Karan, I'll try it again, as my first attempt was unsuccessful...  R7 is 2K and C8 is 10pF, in case it matters.

  • 0 in reply to Gary Kitten
    TI__Genius 16725 points
    Hello Gary,

    When you say the output is noisy, what is the magnitude of the noise? Can you provide maybe some scope plots? Secondly, have you measured at the output of the AD825 to make sure you have configured that correctly so we can narrow down the issue to half of the circuit?

    I am going to look over the circuit to see if I can notice any component values you might have used that look incorrect.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 130 points
    Karan,
    I have measured the output of the AD825 op amp, and it is very clean. This was an existing product that used manually selected resistors to vary the gain, and we're switching to the LMH6505 to allow remote, DAC-controlled gain adjustment.
    I'm out of the office due to illness today, so I can't take any scope shots, but I'll do that as soon as I get back in.
  • 0 in reply to Gary Kitten
    Prodigy 130 points

    Snapped a shot of the scope...Noise is about 30 KHz (excluding the high freq stuff).  The 831mV level in the photo is about mid-range for the total gain.  Any suggestions??

  • 0 in reply to Gary Kitten
    TI__Genius 16725 points
    Hi Gary,

    What you are seeing doesn't look like noise but oscillation. Your circuit seems to be unstable. You mentioned your output from the AD825 is clean but can you show me a scope shot of that output? Also something you can try to increase stability is to decrease the Rg resistor value to increase the gain of the device. Let me know if this helps.

    Here are some recommendations to avoid problems and to get the best performance:
    • Do not place a capacitor across RF. However, an appropriately chosen series RC combination can be used to
    shape the frequency response.
    • Keep traces connecting RF separated and as short as possible.
    • Place a small resistor (20-50Ω) between the output and CL.
    • Cut away the ground plane, if any, under RG.
    • Keep decoupling capacitors as close as possible to the LMH6505.
    • Connect pin 2 through a minimum resistance of 25Ω.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    TI__Genius 16725 points

    Hello Gary,

    On another note, not related to your issue. I think you can use the BUF634A instead of all those components for your output stage. The BUF634A
    can be used inside the feedback loop of op amps to increase output current and replace that output stage you have setup. It provides up to 250mA output current.

    Thanks!
    -Karan