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LMH6624 spurious anomalies

Paul Jones2
Prodigy 10 points
Other Parts Discussed in Thread: LMH6624

I am using the LMH6624 in a high gain, non-inverting configuration with a gain of 51 (Rf = 499 Ohms, inverting input to ground resistor = 10 Ohms). Layout guidelines have been followed with ample ground plane in proximity to amplifier and cut out only under sensitive components and nodes. I am observing a very low level (< 10 millivolts)  rf component between approximately 200 MHz and 300 MHz at the inputs and outputs, basically ALL OVER the circuit board. It seems impervious to changes in gain determining components. I am using a series 50 Ohm resistor at the non-inverting input. Aside from this low level spurious behavior, the desired signal pulse is being amplified correctly and I am filtering out the high frequency with a simple low pass filter.

QUESTION: What is the source of this spurious behavior? Is the the amplifier utilizing an internal chopper-stabilizing system to nullify offset or 1/f noise and am I observing residue of the chopper frequency ? The data sheet makes no mention of such a system but the amplifier is touted for ultra low noise and offset.

Thanks in advance for any help.

  • 0
    TI__Mastermind 22825 points

    Hell Paul,

    No, the LMH6624 does not have any internal chopper stabilization which could explain the source of your 200MHz- 300MHz spurs.

    I recommend making sure the spurs are there with the circuit turned off completely, to confirm that they originate in your board (and not picked up, etc.). Also, it would be good to make sure that your scope probe is not introducing the spurs by virtue of its capacitance. So, some isolation resistance to sensitive loads when probing would be good. A spectrum analyzer might help you pinpoint the exact frequency and offer some clues as well. Also, the power supply itself could be another suspect.

    Please let us know if you uncover more and need some help.

    Regards,

    Hooman