Regarding this information,

enter image description here

regarding the bias resistor the page says:

The resistor value is equivalent to about 100 times the source impedance. If the source impedance is less than 100 Ω, you can connect the negative polarity of the signal directly to AI_GND, as well as the negative input of the Instrumentation Amplifier.

1-) Why a bias resistor needed if the source impedance is less than 100 Ω and otherwise not?

2-) What is so special for the source impedance being about 100 Ω in this context? It is not product specific since the same 100 Ω recommendation can be found in another source like this one at page 26 under Floating source and differential input.

  • \$\begingroup\$ With 100 ohm or less source impedance the instrumentation amp sees very little difference across its inputs relative to ground. With a large impedance the source must "float" with just a static reference to ground, hence a high value resistor. As a general rule the lower the source impedance the better the signal quality if it has to travel many yards/meters to the amp. \$\endgroup\$ – Sparky256 Jan 23 '18 at 1:46
  • \$\begingroup\$ I have both single ended and diff ended transducers. What do you think about this wiring: What do you think about this wiring: i.stack.imgur.com/KUAsh.png ? \$\endgroup\$ – Genzo Jan 23 '18 at 1:52
  • \$\begingroup\$ That wiring diagram (.png) is correct. \$\endgroup\$ – Sparky256 Jan 23 '18 at 1:55
  • \$\begingroup\$ Thanks for feedback. I wasn't sure about 10k resistors use. Some articles dont mention it some do. And also in that png I drew the AIGND connects to the earth at the end which I wasn't sure it needed to be. What also worries me if the transducers I will use have source impedances and unbalance can cause magnetic pick up. See my second question here: electronics.stackexchange.com/questions/351483/… \$\endgroup\$ – Genzo Jan 23 '18 at 2:08

From this page: http://www.beam.org.uk/opensource/pci-6143/docsource/S_Series_Help.chm/Differential_Connections_for_Nonreferenced_or_Floating_Signal_Sources.html

DC-Coupled, Low Source Impedance

You must reference the source to AI GND. The easiest way to make this reference is to connect the positive side of the signal to the positive input of the instrumentation amplifier and connect the negative side of the signal to AI GND as well as to the negative input of the instrumentation amplifier, without using resistors. This connection works well for DC-coupled sources with low source impedance (less than 100 Ω).

DC-Coupled, High Source Impedance

For larger source impedances, this connection leaves the DIFF signal path significantly off balance. Noise that couples electrostatically onto the positive line does not couple onto the negative line because it is connected to ground. Hence, this noise appears as a DIFF-mode signal instead of a common-mode signal, and the instrumentation amplifier does not reject it. In this case, instead of directly connecting the negative line to AI GND, connect the negative line to AI GND through a resistor that is about 100 times the equivalent source impedance. The resistor puts the signal path nearly in balance, so that about the same amount of noise couples onto both connections, yielding better rejection of electrostatically coupled noise. This configuration does not load down the source (other than the very high input impedance of the instrumentation amplifier).

100Ω seems to be fairly arbitrary crossover between 'low' and 'high' source impedance.


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