Hi
I am in the process of planing a measurement-PCB.
It will amplify a voltage in 6 different "channels" of different magnitude.
Since the voltage i measure is battery-based (I measure a shunt-voltage), i should use differential measurement accoding to NI's "Field Wiring and Noise Considerations for Analog Signals" @ http://zone.ni.com/devzone/cda/tut/p/id/3344
See: Measuring Floating (Nonreferenced) Sources, Just below "Table 1":
Single-ended input connections can be used when all input signals meet the following criteria.
Input signals are high level (greater than 1 V)
Signal cabling is short and travels through a noise-free environment or is properly shielded
All input signals can share a common reference signal at the source
Differential connections should be used when any of the above criteria are violated.
And since i may have quite small voltages on some channels much below 1V, i should use differential, Nonreferenced.
OK so far.
But now those bias-resistors come into play.
Since i have 6 different channels, all sharing a common ground, i am unsure how many and where i have to put those bias-R's in.
If i put a bias-R on each +Line to each diff-Inpout of my NI USB-6216-Box thats no Problem since they are all regulated with Opamps. So no real voltage-error will occur.
But what about the bias-R's on the -Line of the diff-Inputs of my box? If i put one on each input, i de facto wire them in parallel.
Now what? Do i need only one bias-R at the -Line (At the common ground-point) or do i need a 10x bigger bias-R at each -Line of the box as close to the box as possible? (10x bigger because this way i end up with a bias-R 1x the value in total because i would wire them up in parallel).
Sorry for my english, but it is not my first language...
If you dont understand something, dont hesitate to post questions! I also can try to upload an scribble/note on how i want to wire the stuff up...
Thanks a lot!
