A/D module input resistances

I notice on the specs they don't specify the frequency at which the input impedance is '100Gohm'

If you look at the specs

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

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

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

That should have been not this: about 160k at almost zero degrees

But this: about 160k at almost 90 degrees (seen from the input) [the C/R are in parallel and the input impedance is dominated by the input capacitance]

Cheers

PeteS

Hmm, I wonder if 100G is a typo... 100M is pretty common, 100G is hard to believe. Anything that really is 100G is ludicrously difficult to produce and to maintain. Try finding 100G resistors for an op-amp feedback path, for instance. They are large glass tubes that have to be mounted on teflon standoffs and even then you are lucky if you can get anywhere near the rated value due to humidity, etc.

Do you really need a high value here? Even 100M is pretty high. Unless you are building a dedicated pH meter or electrometer or something, you will probably want some interface in front of the A/D anyway.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis