• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 48 subscribers
  • Views 614 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LHM6629 InAmp oscillating.

Joel Subash
Prodigy 90 points
Other Parts Discussed in Thread: LMH6629

InAamp_3LMH6629_In_Ls.TSC

Hey guys,

Attached is the TSC of a instrumentation amplifier (built with three LMH6629) which I am trying to get to work. I am trying to amplify a 10MHz signal received by a 2uH center-tapped aircoil. Its running off a 9V battery and a reference voltage of 2.5 V (Vref) is generated by a general purpose op amp. Also an LDO provides a regulated 5V supply to the opamps (LMH6629, 900MHz GBW). Now I am seeing spurious voltages reaching the 5V supply when nothing is connected to the input and also when the coil is connected to the input, basically oscillating. Am i missing something here, what extra care should i take to ensure that the circuit does not oscillate. Any ideas thoughts will be very helpful. Thanks.

  • 0
    TI__Mastermind 22825 points

    Hi Joel,

    Here are my comments:

    1. Minimum Stable Gain: What LMH6629 package are you using? If SOT23-5, then you must raise the gain in the front end (U1, and U2) and also for U3 as the SOT23-5 package is for minimum stable gain of 10V/V or higher. The WSON-8 package can work down to 4V/V gain if COMP pin is tied LO (or left open). Otherwise, you may have to do some more exotic external compensation as described in the Compensation section of the datasheet.

    2. Decoupling Caps: I'm sure you are sprinkling local supply decoupling caps (~0.01uF ceramic to ground along with a heavier cap next to the regulator) next to every LMH6629 supply pin, but I'm noting it here just to make sure you are.

    3. Layout: Make sure your layout is tight and devoid of excessive parasitic. You could use the LMH6629 EVM (possibly) reworked to match your schematic.

    4. Simplifying the circuit first: I recommend you test your circuit piece by piece first (i.e. use lab signal generator to drive U1 input (50ohm termination next to + input and AC coupled) with U2 input tied to your virtual ground (with decoupling cap). I'd even disconnect U3 for now. Continue testing to make sure of the operation of each block because when the device you are working with is this fast, you must get everything right! I would even tied both U1, and U2 inputs to virtual ground (U4 output) first (again with good decoupling close by) and see if you can attain stability that way?

    5. Input Decoupling: Make sure you minimize your transformer leads the feed U1, and U2 pins (or add series R, shunt C next to LMH6629 input).

    6. Light loading: Your stage loads are light and sometimes a heavier load helps. Test to see if loading any (all) stages improves if you add a 100ohm (AC coupled) to ground. This could be a simple short lead 100ohm + 1uF cap that you touch to each output.

    Let me know if you do any of these or other experiments and what the effects are for further debugging.

    Regards,

    Hooman