Banding in the derivative of a lock-in amplifier signal I'm analysing several measurements taken with a SR830 lockin amplifier.

These measurements look similar to this one.
Since I'm interrested in the derivative of the signal I took the numerical difference. What I found was this. 
One can clearly see two bands in the signal. I'm no expert, but I'm guessing it might be related to digitalisation in the lock-in amplifier.

So this leads me to two questions:


What could be the origin of these two bands and how to avoid them in the first place?
How should I correct the data already taken?
It might not be so straight forward to simply take the derivative of the output from a lock-in amplifier. If you think about how a lock-in works, the final stage of a lock-in amplifier is the low-pass filter. So if you then simply take the sample-to-sample derivative, you will simply be undoing the work of the low-pass filter and getting noise. Instead, you need to decimate your time domain signal, and then take the derivative.