How to wire NI 9205 COM terminal for 16 differential voltage measurements?

[ping2002]

Hi,

I have a NI 9205, I need to measure 16 differential voltages from 16 sensors, which do not share a common ground, some of them are electronicly isolated from others, some share the same circuit, their values range from 10mV to 10 V, which need different inpput range for each channel.

How to connect the COM terminal of this NI 9205 to get accurate reading for all the16 differential channels?

I thought about the following methods, but I doubt that any of them will work, please advise:

1. Connect all the negative leads of the 16 differential voltage to COM through a 10Mega ohm resistor? (I can not directly connect them to the COM, since that will cause shorting in the circuit).

2. Connect the cabinet which host the test equipment to COM?

3. connect the cable shield to COM? (the shielded twisted pairs cable carries the 16 differential voltage signals)

Thanks a lot

Ping

[Now_With_Underscores]

Hey Ping,

With the 9205, all of your inputs must remain within 10.4 volts of common, but since you're using differential mode the common line of each device under test does not need to be at the same reference.  The two main considerations are that all of the inputs are within 10.4 volts of common and that no voltage exceeds 60 volts over earth ground.  If you need a device that has isolated channels (voltage between different DUTs is greater than 10.4 volts over COM), I would recommend the 9225, 9229, 9219, or 9239 modules. 

Page 12 of the 9205 manual shows the pinouts to wire up differential connections.  If your DUTs are floating signal sources (battery powered, isolated, etc.), you will need to add 100K resistors from AI+ to COM and AI- to COM.  This will keep your voltage inputs within the +/- 10.4 volt range.  If your DUTS are grounded, you will not need to hook anything up to Common.

To answer your questions specifically:

1.  If you connect this way, you will be using Reference Single Ended (RSE), not differential.  In this case, all devices must share a common ground.  This is probably not what you want.

2.  If you connect the COM to the cabinet, you will remove the 60 volt isolation built in to the card.  This is probably not what you want.

3.  If you connect to the shielding and the shielding connects to earth ground, just as in 2 you will remove the 60 volt isolation.

Hopefully this information gets you up and running.  

[Dan_K]

Hello Ping,

The COM connection of the NI 9205 is meant to be a common ground reference.  In order to prevent damage to your device due to floating voltages going beyond the range of the device.  What would be a good idea would be to put some sort of isolating circuit, even a single resistor, between each of the negative terminals and the COM pin.  This COM pin should also be connected directly to earth ground, such as the cabinet holding your test equipment.

For more information about the NI 9205 and the different ways to connect signals to this device see the NI 9205 Operating Instructions and Specifications on ni.com.

Message Edited by Dan_K on 11-14-2008 12:12 PM

[ping2002]

Thanks, John and Dan.

I found out that this task might be simplified by dividing my sensors into two groups, using two NI 9205. 

1. Actually, I have two NI 9205 for voltage input measurement, and one NI 9264 voltage output as power source for 8 sensors.  8 of the 16 sensors measure signals which are powered solely from this NI 9264 voltage output module. so I can directly connect the COM of NI 9264 to the COM of NI 9205 module, the output voltage of this NI 9264 is within 10V, that should satisfy the input requirement for NI 9205, this NI 9205 will be solely used for these 8 sensors. This will work, right?

2. for the other 8 sensors, they are not powered from NI 9264, but powered from 8 separate KIKUSUI PAN70-2.5A DC power supplies (powered from AC line) with floating output voltage varying from 0V to 70V (in other case, either the positive or negative of the output can be connected to the DC power supply ground, which is connected to the AC line ground). the voltage signals of the 8 sensors are all within 10V differentially, but the positive and negative leads of these sensors to the ground of the DC power supply will surely exceed 50V, so I can not connect the DC power supply ground to the COM of NI 9205 even through a 1Mega ohm resistor, right?  

In this case, I am planning to connect the negative lead of each sensor to the COM of NI 9205 through a 1 Mega ohm resistor, respectively. This NI 9205 will be solely used for these 8 sensors. Will this work? please advise.

Regards

Ping

[ping2002]

In another thought, for the 8 sensors powered from 8 separate KIKUSUI PAN70-2.5A DC power supplies, maybe I can do it in the following ways.

1. Since the sensor's positive and negative leads are all winthin 10V to  the positive output terminal of  KIKUSUI PAN70-2.5A DC power supply, I can connnect the positive output terminal of DC power supply to the ground terminal of the DC power supply, because all the ground terminals of DC power supply are connected to the AC power line ground, so these ground terminals are connected to the same ground. Then I connect one of the DC power supply ground to the COM of 9205 via a 10K or 60K, or 1Mega ohm resistor, will this configuration work? what resistor value is best for this configuration? (the sensor and load of DC power supply are all below 100 ohm)

or 

2.  The sensor's positive and negative leads are over 50V to  the negative output terminal of  KIKUSUI PAN70-2.5A DC power supply, I  connnect the negative output terminal of DC power supply to the ground terminal of the DC power supply, because all the ground terminals of DC power supply are connected to the AC power line ground, so these ground terminals are connected to the same ground, but this ground can not be connected to the COM of 9205 because the hight voltage between this ground and the sensor leads. Then I connect the negative leads of each sensor to the COM of NI 9205 via a 1 Mega ohm resistor, respectively. This NI 9205 will be solely used for these 8 sensors.  what resistor value is best for this configuration? (the sensor and load of DC power supply are all below 100 ohm)

which is more accurate?

thanks.

ping

[Dan_K]

Hello Ping,

Sorry for the late reply. Looking between the two cases you are describing I would recommend the first. Whenever dealing with any measurement devices reducing the amount of potential being dealt with is almost always the best thing to do. If all of the sensor leads are within 10V of the power supply ground there shouldn't be any problem connecting the power supply ground directly to the NI 9205 COM terminal. This should prevent any damage to your device and give you good measurements.

[ping2002]

Thanks, Dan,

I will do it, connecting the DC power supply ground directly to the 9265 COM.

ping