Signal Conditioning

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Not receiving any signal input with LabVIEW SE, USB DAQ board

Hello all, good afternoon.

I am having a little trouble with my DAQ device, I hope that someone will be able to help me.

I'm plugging a BNC pH sensor to a NI USB 6251 M-series board, to try and obtain the electrical response of that particular sensor, measuring the voltage output for each buffer solution (pH 4 and 7). Have 9.2.3 mx (lastest) drivers.

The pH sensor (floating) is plugged through a female BNC plug to the circuit board, signal is going through a signal conditioning  circuit (composed by a voltage follower on each of the wires to lower the output impedance of the sensor, and 2 bias resistors from AI+ and AI- connected to AI GND with the board case wired to the power supply ground, as seen in http://zone.ni.com/devzone/cda/tut/p/id/4494)

I'm measuring in differential mode, as I said both wires of the sensor are going through a voltage follower each, and into the DAQ board. The Op-amps are being powered by +-15v as seen, again, in http://zone.ni.com/devzone/cda/tut/p/id/4494

I did take the time to read the manuals and tutorials I found, did quite a bit of investigation over the last few days, and wired the system carefully.

In labview signal express, I choose to take an analog voltage read, and select input range from -500mv to 500mv, because the sensor should output at most -400mv to 400 mv


Now, much to my dismay labview registers in that channel absolutely nothing, only a little noise (~1 mV). I unplug the sensor and realise that whether the sensor is connected or not, it doesn't make a difference in the presented output - it's as if it the sensor wasn't there at all. And yes, I am measuring the correct channel. I tried connecting the wires to different AI channel pairs as well, could be a faulty one, but no success either.

If I plug a voltimeter to the wires that are going into the analog inputs (after conditioning), it measures about 50 mV, which is a lot more than the 1 mV the board is measuring, but a lot less than the ~180 mV I should be measuring with a pH 4 buffer solution.

 

I know for a fact the problem is not the sensor, and the DAQ board seems to be fully operational.

 

I think it might have to do with the signal conditioning I'm doing, or some sort of configuration with the hardware of the DAQ board.

 

 

So, any idea what I might be missing?


I'm a week behind schedule because of this, hopefully I will find my answer here.

Thank you very much in advance!

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Hello,

 

First of all, the USB-6251 needs an external power supply. Do you have it?

Did you also try an other USB port, or with an other computer?

 

Finally did you try the same acquisition with a voltage generator. The objective is to try a measurement of a known source.

 

Can you tell us the manufacturer and the model of your pH sensor please?

 

 

 

Jean-Philippe C.
National Instruments France

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Good afternoon.

 

Yes I have tried all of that. The board responds very well when a flat 5V DC voltage is fed into a channel.

 

However after much investigation I believe to have found the problem yesterday, it probably has to do with the operational amplifiers I was using. The input bias current was in the magnitude of nanoAmps, while for pH measurements an IBC of picoAmps or less is necessary due to the high impedance of the sensor. This was causing a drift of over 100 mV on my output which is about 2 pH and thus unnacceptable (even though I had the Bias resistors at the input to ground, like suggested in the NI tutorials, but apparently they were not enough to compensate for the high IBC).

 

In the lab all available op-amps suffer from this, even Instrumentation op-amps like AMP04 from Analog Devices. I still haven't had a chance to acquire new op-amps with the necessary IBC but hopefully it will do the trick. Makes sense since I checked the datasheets for every op-amp in the lab, and they all had IBC around 400 nanoAmps (and none of them worked), while every single op-amp for every pH meter circuit I found have IBC's of pico/femtoAmps.

 

The sensor I'm using is from AquaMedic, I'm not sure what the model is, but it's a cheap plastic sensor that comes bundled with their pH Computers.

 

Thank you very much for your input.

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