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TMP117: removing noise, question about the old and obsolete TMP007 , not the 117 ...

Part Number: TMP117

Hello, 

first, i know that the TMP007 is obsolete but we are still using it, and would love some advise.

we have a noise from the TMP007 that is in jumps of 300Khz all through the frequency range at least to 100Mhz (didnt check further)

i made a patch, cut the board and seperated the DGND and AGND with a resistor and i can see clearly that the dgnd is the source from my noise.

pic below shows scope and spectrum images:

 

i am now in the process of a new layout which in the DGND and AGND are separated with a ferrite bead of 1Kohm@100Mhz and 0.8ohm@DC

this seems to solve the noise problem, but my question is this:

i did not see any mention to special care to the DGND signal and in all application notes and layout guidelines i saw the instructions are always to directly connect to AGND.

is there a reason for this? should the separation by FB ruin the device operation or accuracy for some reason?

thanks,

Roy Barak

ClearCut Medical