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Marginal Stability in ADA4938-2

Category: Hardware
Product Number: ADA4938-2ACPZ

Hi,

I'm using the ADA4938-2 to drive an AD9653 ADC. I am observing marginal stability in the ADA4938-2 circuit when I include C121 and C127 in the design. However, if I remove these components, the circuit is stable. I cannot reproduce this issue in simulation. Could you explain to me why this is happening? I would like to include active differential lowpass filtering in this design to limit the noise bandwidth from this amplifier. Is there a way to modify this circuit to limit the bandwidth while keeping the circuit stable?

Please review the following images:

The schematic of the ADA4938-2 driver:

Schematic

The noise measured with C121 and C127 populated (time domain [top], frequency domain [bottom], there are two channels with identical circuits [red and blue]):

Noise with capacitors installed

The noise measured with C121 and C127 removed (time domain [top], frequency domain [bottom], there are two channels with identical circuits [red and blue]):

Noise with caps removed

Thanks,

Peter

  • Hi Peter,

    I didn't use this IC, but what I observe is:
    - you are using 100R for Rf and Rg, whereas in data sheet the smallest value is 200R
    - it looks like the ADA4938-2 is less stable than ADA4938-1 for low load resistance - compare Fig.21 / Fig.24
    - this IC is not intended to use a capacitor parallel to the RGs

    Did you obey all the layout rules on page 23? Mainly that the ground plane is removed under the RFs and RGs.
    What would improve stability:
    - maybe if you could increase gain (to 2)
    - try small series resistors for C121 and C127
    - try to have lowest ESL-capacitors for C121 and C127, parallel more pieces. Be extremely careful of layout with shortest possible lead length to C121 and C127
    - try using 200R for RFs and RGs
    - you could try to build two-pole low pass filters for both IN- and IN+. Somewhere I did see such a circuit

    The stabiltity issue results from that with C121 and C127 you lower the gain for highest frequencies well below 1 and you increase the feedback for high frequencies.