I notice on the specs they don't specify the frequency at which the

input impedance is '100Gohm'

If you look at the specs

http://www.measurementcomputing.com/PCI-DAS6014_6013specs.html
they don't specify the input capacitance (which would make a

significant difference).

Now, it's possible to raise the effective input impedance through the

roof by bootstrapping the inputs - and perhaps this is what has been

done, although 100G is awfully hard to believe *across the entire

frequency range of operation*. I can get really good amplifiers with

100s of fA of I(ib), which sets the input *resistance* easily in that

range.

At DC, it's perfectly possible. Consider an input of 3V - at 100G input

*R*, then I(in) would be 30pA (quite believable). It's possible this is

marketing tripe (hype) where the engineer stated the input resistance

is 100G, but that's not the input impedance across the range.

Even ignoring cables, there will be a minimum of a few pF input

capacitance, so at the stated rate of 200k Samples / sec (so one can

digitise up to 100kHz), the input would look like (for 10pF input C)

about 10G - j160k. [100kHz, 10pF input C]. At those differences in

range, the input impedance would be about 160k at almost zero degrees.

Apart from that, the *effective* input frequencies involved (due to the

sample and hold) would be higher, but at even these numbers, I don't

see the input *impedance* being above a few hundred K. (Note that those

numbers are quite superior in terms of general equipment).

Cheers

PeteS