Should I isolate DAQ with other instruments?

[SCOTTIE]

Hi,

The main instruments in my project are: a 6020E DAQ Pad and a pH meter. They are both hooked up to a laptop by usb interface. The pH meter keeps reading pH into LabVIEW program from a biochemical sample. Depending on conditions required, the LabVIEW program asks the DAQ Pad to output a certain analog voltage. The voltage is then converted to a current via a constant current source circuit (two-OP circuit) and the current is passing through the sample.

PH meter obtains pH valus by detecting potentials of acid concentration in hundred "mV" range. However, the pH meter always gets wrong pH values (out of measurement range) after that DAQ card is connected and turned on. I'm thinking this is happening due to "isolation issue" between two usb slots because it won't happen like this if I hook up DAQ and pH meter separately to two laptops. I wonder can I eliminate this issue by connecting an opto-isolator device, such as an "usb opto-isolator", to the pH meter instead of connecting to the laptop directly? I don't have quite understanding on this isolation issue. Can anyone give me a clue? Thanks in advance.

BTW, this problem also happens if the pH meter is connected by RS-232 interface.


Scottie

[Buechsenschuetz]

There is an excellent article on how to ground/reference analog signals and how to avoid noise:

http://zone.ni.com/devzone/conceptd.nsf/webmain/01F147E156A1BE15862568650057DF15

In your case, without knowing your setup in detail, I would guess that your problem is caused by improper grounding. The USB as well as the RS232 link have a ground line, and if you have another connection to analog ground of the DAQ board you will have a ground loop which might cause noise or - as in your case - minor level shift. Probably using an opto isolator will help. But if your sensor is powered by the USB line you will need an external power supply, too.

[SCOTTIE]

Thanks for reply.

In my case, both of the sensor(pH meter) and DAQ card are powered by a power isolator. So I think grounding issue doesn't exist. The only reason I can think of is that the sensor (via usb) is influenced by the other slot (either usb or rs232) where causes a voltage deviation to the sensor. As I said, if I connect the sensor and DAQ card separetely to two laptops, sensor(pH) reading is ok. So do you think I can solve this problem by using a usb opto-isolator which can ensure that sensor won't be influenced by voltage shift from the other slot? Thanks again!


Scottie