# Multifunction DAQ

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## If i sample multiple Differential channels, do i need multiple individual bias-resistors?

See attached PDF.

Trying to measure 3 pairs of a synchro..floating.  When I measure diff S1>S2 (AI0 rel to AI8) I have a biased resistor ( r3 and r2 to AIGND ).  Now I want to measure differentially S2>S3 ( AI1 rel to AI9).  Is my setup regarding the biasing resistors legit?  Can you "short" to analog channels together , AI0 and Ai10, AI1 and AI8 , AI2 and AI9 to get the 3 pairs of differential measurements I want??  Should the biasing resistors be as close to the DAQ as possible or doesn't it matter?

Thanks..

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

S1, S2, and S3 should each have a biased resistor.

It might be easier to just use 3 referenced single ended channels to measure your S1, S2, and S3.  You can then do whatever math is needed in the software.

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

crossrulz wrote:

S1, S2, and S3 should each have a biased resistor.

They do as shown in the (earlier) attached drawing.

It might be easier to just use 3 referenced single ended channels to measure your S1, S2, and S3.  You can then do whatever math is needed in the software.

Don't understand this one.  I need to measure ..say..S1 relative to S2.  If I tie S1 to an analog in channel and reference S2 to an analog ground...How would I later be able to measure ..say..S2>S3 ??  S2 is tied to ground to get the RSE configuration.

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

Clint1000 wrote:
It might be easier to just use 3 referenced single ended channels to measure your S1, S2, and S3.  You can then do whatever math is needed in the software.

Don't understand this one.  I need to measure ..say..S1 relative to S2.  If I tie S1 to an analog in channel and reference S2 to an analog ground...How would I later be able to measure ..say..S2>S3 ??  S2 is tied to ground to get the RSE configuration.

You don't tie S2 to GND.  You wire S1 to AI0, S2 to AI1, and S3 to AI2.  With the bias resistor, they should all have a voltage relative to GND.  So the idea is to measure S1, S2, and S3.  Then in software you can do S1-S2, S2-S3, S1-S3, etc. to get your differential measurements.

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

So the configuration and setup would be a "hybrid" from the document attached to obtain the required measurement you just described?? Analog In1 would be tied to S1 with a biased resistor tied to Analog In gnd.  Analog In2 would be tied to S2 also w a biased resistor tied to analog in gnd and the same for S3??  In my DAQ configuration in SW I would set the configuration to "RSE" for each channel I wish to measure then do as you say and take the difference??  ie S1 minus S2; S2 minus S3 etc??

thxs..

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

That is correct. You would set the configuration to "RSE" and do the differential math in software, as crossrulz stated.

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

So I'm a big visual guy and drew up this simplified drawing ( included) based on everyone's comments.  I want to make sure I understand this concept because it is far different than what I had read on and understood.  If someone would be so kind as to comment on my statements.  I don't understand cruz's comments if this dwg is correct.  From the sketch on the bottom left I measure AI1 (relative to AIGND) which electrically is the voltage across R1...also across S1/S2/R2 combination...figure to far right.   If I now measure the voltage at AI2 (relative to AIGND) its the voltage across R2..electrically the same as S2/S1/R1 combo.  This is the same potential as at AI1 isn't it??  What am I missing?

Thanks..

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Message 8 of 11
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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

Have you tried wiring one end of s1 to ai1 and the other end to a bias resistor that goes to ai gnd? Same for s2, s3, etc

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## Re: If i sample multiple Differential channels, do i need multiple individual bias-resistors?

yes.  I just want to make sure what I'm doing is correct before I build/wire a test station.  Maybe I'm seeing what I want to not what I should.  That's why I asked the questions.

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