This alog summarizes the measurements done with an ISC whitening chassis that implements ISC whitening boards D1001530-v7 using a 9V battery at the input.

Several issues were found:

1. Due to a BOM error resistors R15 and R21 were stuffed with 3kΩ thick film resistors instead of metal film. Plot 1 shows the output referred noise measure at test point TP1. The blue curve was measured with a grounded input, whereas the red curve was measured with a 9V battery at the input. The later clearly shows the flicker noise of a carbon film resistor at roughly 600nV/√ f.
2. The power supply input of the whitening board eomploys 10µH series inductors. Together with the bypass capacitors this forms a resonant circuit near 10kHz which shows up as a noise peak in the plots. Plot 2 shows the output noise with no whitening engaged for the original board (blue curve), the board with fixed 3kΩ resistors (green), and with the power supply indcutors shorted out (red).
3. The gain stages will generate excess noise when over driven. The whitening board employs 4 switchable gain stages with 24, 12, 6, and 3dB of gain, respectively. With 9V input both the 24 and 12dB stage are over driven with their ouput voltage limited by the power supply rails. This will in turn lead to a large differential input voltage at the inputs of the OpAmp which then triggers the protection diodes. The current is limited by the 300Ω resistors (R11) in series with the + input. Even so, it is safe to operate like this, it can generate execss noise as seen in Plot 3. The blue curve shows the output noise with the input grounded, whereas the red curve uses a 9V battery at the input. The green curve shows the noise of the SR785 and the battery alone.

Plot 4 shows the 9V input noise using the optional whitening stages. A whitening stage consist of a 1Hz zero and a 10Hz pole.
Plot 5 shows the same plot but with the 300Ω resistors removed, the 3kΩ resistors replaced with metal films, and the inductors at the power supply shorted out.
Plot 6 is an overlay of Plots 4 and 5.