I have some strange phenomena with the operation amplifier AD8099. I have designed a 4-layer board with 4xAD8099. The first three is placed as a instrumental amplifier and the fourth is an additional buffer/amplifier before connecting to MMCX connector.
When I have one of the amplifiers connected and tested it works fine although up to 25MHz (my generator range). When I connect all the amplifiers it does not work so good anymore. The differential stage in the instrumental part have a square wave out , as saturated. I feed in a 200mV, 1kHz sine wave signal so that's not the problem.
I use the LFCSP chip that has an internal feedback connection to the output. If I bypass the non-function differential amplifier from pin 2 to pin 7 the amplifier starts to work and output a nice amplified sine wave. Is the internal connection sensitive to soldering? What am I missing here?