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A/D module input resistances

J

Joel Moore

Jan 1, 1970
0
I've been shopping around lately for remote (USB/serial/ethernet) A/D input
modules and so far the highest input resistance I've found is 100M Ohms
(Measurement Computing's USB-1608FS). The next closest contender is around
20M Ohms

Out of curiosity I checked out one of their PCI analog input cards (PCI-
DAS6013) and was surprised to see its input resistance is rated at 100G
Ohms! That's 3 orders of magnitude larger.

How do they achieve such a number? And why aren't there any remote modules
that offer numbers even close to that?

Thanks for any insight.

Joel Moore
 
B

Bob Masta

Jan 1, 1970
0
I've been shopping around lately for remote (USB/serial/ethernet) A/D input
modules and so far the highest input resistance I've found is 100M Ohms
(Measurement Computing's USB-1608FS). The next closest contender is around
20M Ohms

Out of curiosity I checked out one of their PCI analog input cards (PCI-
DAS6013) and was surprised to see its input resistance is rated at 100G
Ohms! That's 3 orders of magnitude larger.

How do they achieve such a number? And why aren't there any remote modules
that offer numbers even close to that?

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
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator
 
P

PeteS

Jan 1, 1970
0
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
 
P

PeteS

Jan 1, 1970
0
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
 
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