voltage signals connection to SCB 68

[rrd]

i am working on a smell sensor(e-nose), using an array of FIGARO TGS sensors. i use DAQ card 6225 connected to the sensor chamber through an SCB 68 and to the computer. I have problem with the readings i got from the MAX test board. Readings appear to be mere noise, and sometimes when i connect a single sensor from the array, a constant value is reached (10.8V). I have the feeling that this should be a floating point differential signal. Checking the SCB68 manual i suppose i am to connect some resistors but not sure how. I understood page 5-2  to mean that the SCB board should have all the resistors and capacitors indicated on table 5-1 on page 5-2 added.

what do i do?

rrd0204@yahoo.co.uk

[devchander]

First of all, what is the voltage range that your sensor gives??

Check if you are reading that channel on MAX, using the same mode in which mode had you connected the sensor on SCB 68 ( Diffferential, RSE, NRSE)

Have you selected appropriate AI range for that channel??

[rrd]

Thank you for your reply.

I am using a sensor array, actualy. There are 6 sensors arranged on bases- 2 bases have no sensors attached. When measured at output pins, the sensors (which are mere voltage droppers) gave 1.5 - 2 V on the average. I have built a circuit to give amplification of 2 or 3 times.

I have been using MAX for my investigations so far. I chose differential as my AI input mode. The range selected was -10 to 10 V. My observation is that i got same noisy values on all chanels - including ones without sensors

[Erik_J]

Hi rrd,

You said that you are using differential as your AI input mode.  What channels are you using?  When using differential as your AI input mode, you need to wire the positive terminal to channel x and the negative terminal to channel x+8.  The sensors give 1.5 to 2 volts and you amplify 2 or 3 times with a custom-built circuit.  Have you tried to measure the noise without the amplification circuit?  It is possible that the circuit is introducing additional noise into the system.  You may want to try not amplifying the signal.

Here are some great resources regarding noise:

Developer Zone: Field Wiring and Noise Considerations for Analog Signals


Best Regards,
Erik J.