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I'm looking at using a NI PXI-6255 DAQ card and I noticed in the manual it says "Both inputs of the (diff amp) require a DC path to ground in order for the (diff amp) to work."

Is this a common design constraint to use a differential amplifier? If both inputs are floating, shouldn't it be able to reference each input to the amp's ground to take the differential voltage without needing a DC path?

SiegeX
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2 Answers2

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That advice is for measuring differential signals. The problem is that, even if the amplifier may have a high common mode rejection (CMRR), the signals have to be referenced to the ground, even if with a high-resistive path.

At the beginning of that section, it says:

It is important to connect the negative lead of a floating source to AI GND (either directly or through a bias resistor). Otherwise, the source may float out of the maximum working voltage range of the NI-PGIA and the DAQ device returns erroneous data.

So if the source is completely floating, the voltage between the ground of the instrument and the input signals may be randomly high. To prevent the PGIA to saturate due to common mode, it suggest to provide a path to ground.

Note

When using a PCB-mounted Diff-Amp (or also an In-Amp), it's not generally required, as the input signals and the Amp will share the same ground.

Community
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clabacchio
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Imagine you have a 1 volt difference between two inputs that you wish to measure.

Ground the most negative input. The more positive input is at 1 Volt.

You connect a differential amplifier with a gain of 1 (in this example) to the two inputs and it provides a 1 volt output.

Now, leave the amplifier connected but disconnect ground from the negative input so the inputs are floating but 1 Volt apart.

Now stand on a convenient high voltage insulator that just happens to be around and eg stand in the path of a beam of positive ions that happens to be passing by - so that your potential relative to ground increases continually.

Input voltage on two inputs 

Difference                --> Result    

2-1 = 1    

12 - 11 = 1    

102 - 101 = 1             --> 100 Volt common mode. Magic smoke   

1002 - 1001 = 1           --> 1000 volt common mode. Bright flash  

1,000,002 - 1,000,001 = 1 --> 1 million volts common mode. Br......

In all cases the differential voltage is 1V.
But the increasing voltage on the inputs after a while gets to voltages that cause interesting results.

NB - this is a genuine real world problem that needs to be addressed. "Static electricity" can easily produce thousands of volts on a floating input.

Russell McMahon
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