# FieldPoint Family

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## AI-110 COM terminal question

Hi,

The AI-110 manual clearly states that the inputs are single-ended, and all channels share a common reference terminal, COM.  If I'm just using a single input, can I just treat the input as a differential input?  Can the COM terminal float freely relative to the backplane?  If not, what are the restrictions?

Thanks,

Anton

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## Re: AI-110 COM terminal question

Anton,

A single ended input will always be single ended.  No matter how many are used.

What exactly are you trying to measure?  Depending on the circumstances, there may be a way to use a combination of 2 single ended inputs to create a differential.
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## Re: AI-110 COM terminal question

I just want to measure the difference between two voltages, neither of which is at ground potential...

Anton
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## Re: AI-110 COM terminal question

Anton,

If the two inputs aren't referenced to ground, it will not work.   What exactly are you measuring?

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## Re: AI-110 COM terminal question

Anton,

Let me rephrase that.  The two inputs need to be referenced to the common of the power supply feeding the COM of the AI-110.

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## Re: AI-110 COM terminal question

I am measuring the voltage across a shunt resistor.  My intention was not to use 2 input channels, but to connect COM to one side and channel 0 to the other side of the resistor.

Anton

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## Re: AI-110 COM terminal question

Anton,

If the voltage across the shunt is isolated from GND, you should be able to do it with the one channel.

I mentioned the two channel because you asked about a differential measurement.  Depending on the wiring and circuitry involved, a differential measurement can be made by using two single ended inputs and taking the difference of the two measurements.  This can be done when the common of the circuit being measured can be tied to the common of the cFP-AI-110 and the two individual input voltages don't exceed the maximum in spec.  This allows other measurements that are referenced to the same common to be made on the other channels of the module.

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