High Frequency Artifacts in Dewe-43

We recently made some measurements using a dewe-43 that included a voltage pulse with a fast rise time. The measured voltage had an unexpected high frequency oscillation during the pulse with a frequency faster than the measurement bandwidth - we assumed there was a very fast oscillation in the voltage that the dewe-43 was undersampling, so that we were seeing the aliased measurement of a much higher frequency oscillation. However, when the measurement was repeated with a scope the oscillation was not there. Following this we did some lab experiments injecting fast rise time pulses into the dewe-43 and found that in each case there was an oscillation at half the sampling frequency in the measured signal. When we altered the sample rate, the oscillation frequency altered too. It looks like the Q of your anti-aliasing filter is rather high! What is the frequency characteristic of that anti-aliasing filter?

The ringing of the rectangual signal definitely comes from the antialiasing filter. From the theory the step function includes all frequencies and the filter of the AD tries its best to cut that. The filter is described in the user's manual of Dewe43. Since it is half of sampling frequency, an easy solution is to oversample and apply IIR or FIR filter to reduce this effect.