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NI9215 Single ended connection

Dear All,

I am using NI 9215 to collect 3 voltage singles simultanousely. I know I suppose to connect the AI- to the COM terminal. However, when I do so, the signal seems to be wrong. When I just connect AI+ and AI- (sharing the same reference by connect all AI- together), the signals are good.

I am worry that it may cause damage if I don't follow the instruction. Please advice.

Jake
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Message 1 of 10
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Hi Jake,

The NI 9215 only supports a differential measurement mode, so you should be connecting only the AI+ and AI- across your voltage source.  This measurement mode will reject common mode voltages, but check the specifications to ensure that you are operating the device within range.  The signal+common mode voltage should not exceed 10.2 V during normal operation although the device does have an overvoltage protection of +/- 30V for transient spikes.  As long as your signal is within specifications, you should operate it with only the AI+ and AI- connected.  Is the voltage source a floating source?  What is it's range?

The specifications for this device can be found in the user manual:

NI USB-9215 Series User Guide and Specifications
http://www.ni.com/pdf/manuals/371568e.pdf

Also, see this document on wiring analog signals and noise:

Field Wiring and Noise Considerations for Analog Signals
http://zone.ni.com/devzone/cda/tut/p/id/3344

Let me know if this helps!

Regards,
John Bongaarts
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Hi John,

Thanks for your reply!!!

My signals are ac voltage (5 V Max),.....They all share a common reference, which is the groud of the external (+ and - 12Vdc) power supply. The signals are floating since they are not grounded to the earth.

My question is whether I should connect my reference terminal to the COM terminal of the 9215.

Best,

Jake
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Hi Jake,

With a floating voltage source, the differential measurement mode should work well.  This means you would only need to connect the AI+ and AI- to the signal + and signal- for each channel.  The grounds for the 9215 and the power supply do not need to be connected.  They may be at slightly different potentials with respect to earth ground, but the differential measurement mode will remove any common mode noise introduced by this.
Regards,
John Bongaarts
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Hi John,
I totally agree with what you saying. However, on page 13 of 9215 manual, it clear indicates that the ground signal MUST connect to COM.

http://www.ni.com/pdf/manuals/373779e.pdf


Also, if I use two 9215 on a chassis to measure voltage signals (single Ended), do I have to connect both COM together?

Best,

Jake
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Hi Jake,

That page refers to measurements made in single-ended mode.  You are making measurement in differential mode, so this connection is not necessary, although I can see where the confusion comes from now.  When connecting multiple modules in a chassis, the ground is shared via the chassis backplane so there is no need to connect them externally.
Regards,
John Bongaarts
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Hi John,

Now I am confused. I just check the 9172 with DMM. The COM of two 9125 are not connected, when the chassis is active (turn on).

Best,

Jake

PS: Sorry for so many questions. I am newbee in NI product.
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Hi Jake,

The 9215 is isolated, so the COM terminal on the front is not connected to anything on the backplane or any other module.  The user manual is correct, which I believe means the answer to your questions are:


My question is whether I should connect my reference terminal to the COM terminal of the 9215.

Yes.

Also, if I use two 9215 on a chassis to measure voltage signals (single Ended), do I have to connect both COM together?
Yes, you should, though if they're both connected to the same source, you've already done this.


Since both your source and the measurement circuitry on your 9215(s) are floating, it might work without connecting the references, but the references may drift apart and the measurement could become incorrect.

It sounds like you may have had trouble with this configuration initially, though.  If you post some more details on what kind of signals you're seeing in this configuration, we can probably help sort this out.  It can be quite confusing to sort out the first time!

Regards,
Kyle

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Hello Jake,

As Kyle mentioned, the NI 9215 is isolated, so its ground would not be connected to the chassis backplane and my answer earlier is incorrect in that respect.

Regards,
John Bongaarts
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Dear John and Kyle,

thank you all for helping me in this regard. I have figured out the problem and my results run perfectly (i just tested)!!!!!!

I now know where to find my questions. NI forum!!

Best,

Jake
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