I was hoping someone here could shed some light on a strange issue I'm having at the moment testing the FMC116. To put it in just one sentence, I'm observing different amplitude responses over frequency of sampled MHz
signals, when I feed them to different channels.
In the pictures attached, you can see the normalized amplitude over frequency
for channel 1 (blue) and channel 2 (green). The signals fed to the two
channels are supposed to be identical. A sweeping sinus from a Rhode&Schwarz
signal generator is split using two active buffers, whose input and output
impedances are matched to avoid reflections (50 ohms).
The different pictures correspond to different amplitudes of the input signal,
ranging from 75 mV to 750 mV. We see how the normalized amplitude of ch1
remains similar for the different input amplitudes, which does not happen for
ch2. Ch2 shows a stronger fluctuating behavior that dissapears using small
input signals (100 mV, 75 mV). This behavior seen in ch2 seems to be the one
present in all other channels. In other words, ch1 seems to be the exception.
And at the moment, using the information from the FMC116 user's manual I can't
find an explanation for this. Maybe a different layout or configuration of the
DC offset correction?
This is a big issue for us, since apparently this "artificial" variation of the
amplitude affects the phase difference between channels, which is critical for
If you are wondering about the jumpy amplitude in ch1 (and in ch2 for small
input signals) that seems to be a problem of the signal generator, the ADC is
not causing them.
Thanks a lot in advance.
Yes, the signals fed to chs 1 and 2 were checked in the oscilloscope and no apparent amplitude changes over frequency were visible there. I will check that again.
As soon as I have results from the reference app I will post them here.