The ADA4661-2/ADA4666-2 has a low voltage noise density of 14nV/√Hz at 10KHz.
Placing a 10KΩ resistor with thermal noise of 12.6nV/√Hz at the input of the
amplifier increases the total input referred noise to 18.9nV/rtHz. With a low
source resistor, the amplifier voltage noise dominates. As source resistance
increases, the resistor thermal noise increases. As the source resistance
further increases (>1MΩ), current noise dominates the total noise.