Differential Mode in DAQ M series board

[Carmo]

Hello all!

I have a doubt concerning the Differential Mode in DAQ M series board. In the board's manual is saiing that the voltage limit is -10 to 10V, but somebody knows if this limit is always referenced to ground, or in differential mode is comparing with the +8 channel (ex. for channel 0 - voltage diference between ch0 and ch8).

Thanks in advance, best regards.

Paulo Carmo

[Carmo]

In attach you can find the schematic.

[ajangot]

Hi,

The voltage limit is always referenced to ground, so the voltage limit is between + and GND and - and GND. If you want wire more voltage, you need to use high-pass filterto reduce the common mode.

Regards,

[Norbert_B]

Paulo,

differential mode means that you measure the difference between two input channels. The channels used for this on E- and M-series boards is n and n+8. The difference between the channels might go up to +- 10V.  So there is no ground reference in this type of measurement. 

However, the device has a limited capacity against overvoltage. So if the offset of either of the channels succees a certain value in regard to the ground of the measurement system (the gnd of your PC), you could possibly damage your hardware.

Therefore it is recommend to use "BIAS-resistors" in such cases (or if you simply don't know the offset).

You can find a very good image on possible ) recommended connections in your DAQ-help. Open the DAQmx Help and navigate to DAQmx Help >> Measurement Fundamentals >> Signals >> Analog >> Connecting Analog Input Signals >> Measurement System Types. Just activate that entry (should have a "book" icon) and in the right side, you should see a something like a reference poster. I suggest you to print it out....

hope this helps,

Norbert 

[EDIT]: corrected a typo

Message Edited by Norbert B on 02-26-2009 08:03 AM

[Norbert_B]

Paulo,

that kind of connection is possible, if you can guarantee that the offset of your lines is not exceeding a critical value. The critical value is defined by the power on the lines. So it might be possible that the connection in the schematic already could damage the M-series device. Therefore, consider using BIAS or isolation (please note that there are isolated AI M-series devices available). Make sure reading the specs of the device that your device is capable of managing that input.

hope this helps,

Norbert 

[Carmo]

Hi Norbert!

Do you know what is that limit? The over-voltage protection? My voltage will not excedd 30Vdc.

Regards and thanks for your fast reply.

Paulo Carmo

[Norbert_B]

Paulo,

no, i don't know what device you are using therefore i don't know the specs.

To be on the save side, just use BIAS, the smaller the value, the greater the measurement error intrudced by them but the better you save the M-series device......but you'd possibly can calculate that error.

hope this helps,

Norbert