I am using the NI 4 channel Sigma Delta A/D with 40MHz clock. What kind of convertor is it - switched capacitor or continuous time modulator?
There are 2 design methods for sigma delta converters, switched cap
and continuous time. The switched cap variety often directly sample
the input pins onto internal caps that might be fairly large ( up to
10 pf). In this case the input sampling will directly alias signals
near the over sampling clock rate with little attenuation.
The other type of converter (continuous time modulator) presents a
resistive input to the user and has an inherent sinc(x) roll off near
the over sampling clock rate. So it is easier to drive and requires
less anti-aliasing than switched cap designs.
I need to know which method Ni uses.
