Correct layout of ADA4937-2


The ADA4937-2 datasheet states that the ground and power planes should be cleared near the summing nodes and gives an example for the ADA4938-1:

This is a bit confusing to me because it only looks like the polygon pour on the top layer is removed. I have an 8 layer board with multiple ground- and power planes. Do I understand correctly that I should remove all the power/ground planes in that region?

Second, the ADA4938-2 is a bit more difficult: I need thick lines for the supply to connect them to the decap and in my case, I also have a feedback cap. So it gets more crowded and I cannot cut out so generously. I have created the following layout (for simplicity I've hidden overlay):

The dark black area is what I have cut out on all of my 8 layers.

Is this cutout enough or should I make it larger? Or smaller?

Is this an acceptable layout? Including the decap?



PS: This is my first "high performance" layout.

PPS: The unrouted components are pulldown resistors for "ENABLE" and a resistive divider for the output common mode.

  • Hi Eric,

    1. Yes, as much as possible there should be no ground and power planes in the pcb area where RF, RG and summing nodes are located. This means that no ground and power planes should be in that area from layers 2 to 8 of your pcb. This is to ensure that stray capacitance would not degrade the performance of the amplifier, especially at higher frequency signals.

    2. I'm confused with your ADA4938-2 layout. You have +5V, -5V supplies then ground at other 4 supply pins. May I know why? Anyway, regarding the ground and power planes the area would be sufficient.



  • Hi Jino,

    Great, thanks for the feedback!

    I am not sure where you see the -5V. I supply VS1+ and VS2+ with 5V and VS1- and VS2- with GND. Just to make sure, I attach an annotated picture (I also annotate Ri, Rf, Cf and decap)

    I also just found another issue with this IC: In the datasheet it is stated that the input impedance of Vocm is roughly 10kOhm. Furthermore, it is stated that a resistive divider with 10k or greater resistors is used. However, if I use a resistive divider with 10kOhm (ideally giving me half the voltage) one part is loaded with the input pin of Vocm and hence the voltage divider consists if 10kOhm and 5kOhm, giving a significant error. Increasing the resistive divider just exacerbraces the situation. Why is this?



  • Hi Eric,

    This discussion would help, ADA4938 common mode setting. To set the VOCM externally thru a voltage source, it is recommended that output impedance of that source should be low. A buffer would be recommended to ensure  more accurate VOCM. In any case where your source would have a considerable value of output resistance Rs, then the effective Vocm of ADA4938 is Vocm = Vsource {10k/(10k + Rs)}.



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