In a specs of a data acq. device the input impedance is shown as "20 MΩ differential". Does that mean the measured impedance in diff input case? If so how can I calculate the input impedance for single input case? Here is the device: http://www.mccdaq.com/PDFs/manuals/USB-1616HS-BNC.pdf
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\$\begingroup\$ Halve the differential impedance usually \$\endgroup\$– Andy akaCommented Nov 7, 2013 at 15:56
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\$\begingroup\$ Do you mean differential as "use 2 inputs and measure the difference" and this is 20MΩ. So the question becomes is What is the Single input impedance? 10MΩ is not unreasonable (Single = centre pin of BNC to shield of BNC) \$\endgroup\$– SpoonCommented Nov 7, 2013 at 17:00
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2 Answers
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It depends on how the inputs are designed. The instrumental amplifiers have very high input impedance (almost infinity) so, these 20MOhms are result of the input dividers. And what will be the impedance, if you connect one of the differential inputs to the ground, depends on the schematic.
There are generally two variants, simplified shown on the below schematic. If you ground the negative input, the left schematic will have a half of the differential impedance - i.e. 10MOhm, but the right one will have the full 20MOhm.
simulate this circuit – Schematic created using CircuitLab
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\$\begingroup\$ Would it be worthwhile to mention a third variation, with two resistors that tie not directly to ground, but to a third resistor which is then tied to ground? Such a design may reduce common-mode noise resulting from impedance mismatches. \$\endgroup\$– supercatCommented Dec 9, 2013 at 17:03
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In this case the 20Mohm is the (single ended) input impedance of any of the BNC's as measured between the pseudo-floating shield and center conductor.
The word "differential" is probably left in there because that is what it said in the ADC chip datasheet and no-one cared too much about this. And you have inputs that are pseudo-floating in that you can have different voltages on the shields of two BNC's (within the common mode range - 6V or 10.5V).
Does that help you?
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\$\begingroup\$ Hm, are you sure about it? I read the datasheet and have an impression, that the shields of the BNCs are insulated from the ground (and there is separate common ground connector). They claim 16 differential inputs that can't be the case if you need to pair two inputs in order to get the difference. \$\endgroup\$ Commented Nov 7, 2013 at 16:43
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\$\begingroup\$ I have to agree with @johnfound. They say multiple times in the manual that the inputs are measured differentially. There's also this line in the datasheet on the input connector connections: imgur.com/9rFd7es . Of course it would be much better to have an equivalent schematic for the input circuit...For OP, it wouldn't hurt to call the vendor and ask. \$\endgroup\$ Commented Nov 7, 2013 at 17:53
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\$\begingroup\$ It's hard to be sure here - maybe the right question is: Why does it matter to you? Notice the spec is given without any accuracy, which in itself is a sign of some level of sloppiness. So don't build something that relies to heavily on this parameter. \$\endgroup\$ Commented Nov 7, 2013 at 20:23
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\$\begingroup\$ I am not sure we disagree btw. This is how I read it: 16 inputs with at least 20M input impedance. The signal and shield can have any voltage within the 6V or 10.5V range relative to the separate GND connection marked "Analog Common". \$\endgroup\$ Commented Nov 7, 2013 at 20:35
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\$\begingroup\$ If you are happy with the answer, please mark it correct so it does not "hang" in the unanswered list. \$\endgroup\$ Commented Nov 19, 2013 at 12:31