Help setting up USB 6009 for experiment

[ahsan2]

Hi,

I am a new LabVIEW user. I have purchased a USB 6009 and I need it to acquire data from a pair of electrodes and a differential pressure sensor. Below is a schematic of how I think everything should be hooked up. I just have some questions regarding the setup before I run my experiment.

I will be using the electrodes to measure voltage drop. However, I am wondering whether I need a ground reference connected to the bottom GND pin. This is because I will be operating the electrodes in the +/- 20 V range and according to the user guide, each pin has a maximum voltage of +/- 10 V with respect to GND. I assume this means that if the +ve electrode measures +15 V wrt to GND and the -ve electrode measures -5 V wrt to GND, the DAQ will provide a reading of 15 V instead of 20 V. So if I need a ground reference, what do I connect to the bottom GND pin?

During this experiment, I will also be collecting data from an Omega PX26-030DV differential pressure sensor, which has a maximum voltage output of 100 mV. It will be powered by a Circuit Specialists CSI3005T power supply at 10 VDC. I believe I have made the right connections, but do I also need to connect the terminal 4 GND on the DAQ to the power supply GND?

[johnsold]

You definitely need grounds!  The internal circuitry of the DAQ device is referenced to the DAQ Ground. Since the device is USB connected, the DAQ ground is also the computer ground.  All grounds should be connected together, ideally on only one place to avoid ground loops.

1. On the 10 VDC power supply shown in your diagram what is the relationship of + and - to ground? The ideal situation for a strain gauge pressure transducer is +5 V and - 5 V because that reduces the nominal common mode voltage to zero. Regardless, there needs to be a reference connection between the power supply and AI GND.

2. What is the circuit (or equivalent circuit) to which the electrodes are connected? That also needs a reference connection to AI GND.

3. What is the source impedance of the electrode circuit? The USB-6009 has a relatively low input impedance (144000 ohms) and it is terminated to a non-zero voltage. I do not have the manual readily available but I think I recall the equivalent voltage is about 1.4 V.  Both of these can cause significant problems for some measurements with "floating" electrodes.

4. The +/-10 V limit with respect to ground is absolute. You cannot have +15 V to ground on one electrode and - 5 V to ground on the other electrode and expect it to work. The only way a 20 V differential rangs is possible is with one signal at +10 V and the other at -10 V. A 15 V signal will not be measured accurately and could result in damage to the DAQ device.

Give us all the information about your system so we can provide appropriate answers.  You may need some external signal conditioning for parts of your system.

Lynn

[ahsan2]

Thanks for the helpful response. I'll add more detail so you can get a better understanding of the experiment. Below is a new schematic of the experiment.

Regarding the 10 VDC power supply, the relationship of + and - to ground is +5 V and -5 V. So do I connect the GND terminal of the power supply to the GND terminal of the DAQ?

In the experiment, the electrodes will be measuring voltage drop in a salt solution. So the equivalent circuit would be the blue salt solution in the schematic. To ground the salt solution, I have submersed a wire in the electrolyte tank and connected it to a metal part of the computer PSU. Is this correct, or should I connect it to the DAQ GND terminal?

I'm not sure what the source impedance of the electrode circuit is. I just know that the voltage drop measurements will vary between 25 mV and 15 V depending on the concentration of the salt solution.

Also, could you recommend an amplifier that I could use with the pressure sensor that provides 10X gain?

[Bob_Schor]

My temptation would be to tie the "solution ground" (which you show as going to the PC Chassis) to the Power Supply Ground, instead.  There is a potential Ground loop through the USB-to-PC connection, but I'm guessing this won't be too serious.  Does your PC and Power Supply plug into grounded (3-pin) outlets on the same circuit (so they could be expected to have the same "building ground"?  That would be a good thing to do.

Bob Schor

[Bob_Schor]

P.S. -- I recently met Johnsold -- if you get different advice from the two of us, disregard mine (I bow to superior knowledge).

BS

[johnsold]

Do what Bob said.

Lynn

[ahsan2]

Thanks for the help Bob and Johnsold.

So just to conclude, I should connect the solution ground to the GND terminal of the 10 VDC power supply and also have another wire connecting this same GND terminal to the DAQ GND terminal?

Also, do you know of any amplifiers I could purchase from NI that provides 10X gain for the pressure sensor. It operates between 0 - 100 mV and I want to amplify it to 0 - 1 V.

[johnsold]

Yes. The grounds all need to be connected.

I am not familiar with any amplifier like that, but I usually design my own.  Look for "signal conditioners" or something similar.   Depending on the concentration of the salt solution, it is very likely that you will need some kind of amplifier or buffer on the electrodes as well.  The USB-6009 analog inputs could look like a 10 uA source to the solution!  

Lynn

[Bob_Schor]

Making a gain-of-10 amplifier with a pair of resistors and a 741 op-amp (I'm probably dating myself!) is not that difficult (even a biologist can do it).  It's also something that anyone with a little EE experience can advise you.  I don't think you have "severe" requirements,so you should be able to cobble together two buffer amplifiers for your electrodes and a gain-of-10 for your A/D input stage with little dfficulty.  Do you have access to a "breadboard" (a piece of plastic with holes every 0.1" electrically connected in one dimension that makex it easy to pop in ICs, resistors, and wires to (as they say) "breadboard" a circuit)?.

Bob Schor

[ahsan2]

Thanks for the help again.

I have put together a signal conditioning circuit. Each output from the pressure sensor is connected to an op amp with 10 X gain. Each electrode is connected to a unity gain buffer amplifier followed by an op amp with 10 X gain. The outputs on the left go to the USB 6009 DAQ terminals and the grounds to the right go to the 10 VDC power supply GDN terminal. Are there any issues with my design?