An isolation issue?

[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

PS: Sorry I have previously posted this message at a wrong board (Signal conditioning)

[AlanArmstead]

Hi Scottie,
A couple questions:
Does the DAQPad output the correct value when the pH meter is getting bad readings?
Do you have a recent version of NI-DAQ? (7.3.1 is the most recent)
You mentioned that you have access to two laptops, have you tried connecting both devices to each laptop individually?
If the DAQPad is operating correctly, you might need to contact the manufacturer of the pH meter.
-Alan A.

[E Murphy]

Hi Scottie:
It looks like an isolation problem as you state. Is your current converter referenced to ground?
You will have grounds on both USB connectors. Try using a square wave output from the 6020 and feed it to a frequency to voltage converter thru an optoisolator. There are several right off the shelve integrated circuits for this function (National Semiconductor, TI, Motorola...Radio Schack) and use a separate power supply for your voltage to current converter.
Regards and good luck
Eduardo Murphy

[SCOTTIE]

To Alan,

Regarding your questions, I use DAQ pad's +5V , AO_0 (working voltage, from -5 to 5), and AO_1 (fixed at -5). Once I put that +5V pin to the circuit, the pH meter goes crazy. At this time, the current it is supposed to generate (depends on working voltage passing through OPAMP circuit) is still correct. I also have tried to connect both devices to two laptops individually. Same thing happened.

If "sample <- pH meter(rs232 or usb) <- LAPTOP ->(usb) DAQ -> OPAMP circuit -> sample" forms a loop, pH meter can't work. So that's why I wonder if this problem can be solved by using an usb opto-isolator between pH meter and laptop.


SCOTTIE

[SCOTTIE]

Eduardo,

I don't quite get why you mentioned to generate a frequency and then convert it to a voltage? Is there any difference compared with my current setup(voltage from DAQ)? What if I connect each voltage generated from DAQ through an opto-isolator? Sorry I'm quite unfamiliar with this area. Thanks for your replay and patience.


SCOTTIE

[E Murphy]

Hi Scottie:
If you have a voltage to voltage optoisolator, you convert the input voltage to a frequency proportional to it. This wave is the one that turns on and off a led in an optoisolator. The phototransistor inputs this frequency to a frequency to voltage converter, for example, the old but still used LM2917 (National Semiconductor)
Instead of having to first convert a voltage to frequency, output your frequency directly from the 6020 and feed the optoisolator and then the LM2917. Use an independet power supply, for example 12 V from a wall outlet charger. You might find all that is required at your local Radio Schack store.
Regards
Eduardo Murphy