Setting nominal voltage?

[D-Three]

Hi,

I want to connect a piezoresistive sensor (strain gage bridge) directly to my USB-6216 BNC. I realize this device doesn't have an internal excitation source so I will have to provide this myself. So I believe the best thing to do then, is measuring the output voltage of the sensor with one of the analog inputs. The full scale output voltage of the sensor is 300mV +/- 100 mV but for our application the working range is below 200 mV. On page 5 of specifications of the USB-6216 I could find that the sensitivity is 5,2 µV if the nominal range is between 0.2V and -0.2V. Now my question is how do I set this nominal voltage? I use Signal Express 2012 but I'm a bit confused. If I add a step and choose 'DAQmx Axquire'->'Analog Input'->'Voltage' then I am able to choose the 'Signal Input Range'. Is this how the nominal voltage is determined?

Also the sensor has a very high frequency response so I want to do some measurements at maximum rate of the device (400 kHz). We don't expect such high frequency signals but it's something we like to check. Has the device some built-in anti-aliasing or filtering?

[crossrulz · Accepted solution]

You set the range.  That sets the sensitivity of the DAQ.  In your case, set it to 0.2.

As far as the frequency is concerned, that DAQ can only sample at 400kS/s.  Which means you can't really capture any signals above 200kHz (Nyquist).

[D-Three]

Thank you for your answer! I'll set the range to 0 and 0.2 then.

[Dennis_Knutson]

Just remember that the actual range is not necessarily what you set it to. DAQmx uses the numbers you provide and selects the best range that the device actually has.

[D-Three]

Thank you for you reply. That was actually what wasn't entirely clear to me since I was able to choose any voltage range between -10.0 and 10.0 while the specifications only mention 4 nominal voltage ranges. So I assume (and hope) if I set 0 and 0.2 as signal input range, the system will choose the nominal range of -0.2 and 0.2 then.

But what happens then if I add a second analog input to the DAQmx acquire and I set the voltage range of that to -10 and 10 and I leave the first one to 0 and 0.2?

[Dennis_Knutson]

Sorry, I have no idea how SignalExpress does it. With LabVIEW, you can do it channel by channel as well as the DAQ Assistant. You should post to the SignalExpress board.

[ThiCop]

Hello D-Three,

Can you post what you currently have?

This way I can double-check it and verify what will be done.

[D-Three]

It's nothing special actually, just acquiring data of two analog inputs. 🙂 I included an example.

[ThiCop]

For each of your defined analog inputs it will try to set the correct range.

Example:

- You specify range 0-60mV for the first AI and range 0-20mV for the second AI

- Imagine the card only allows -0.05 V to 0.05V and -0.10 to 0.10V (and up..)

- The first AI will get a range of -0.10V to 0.10V

- The second AI will get a range of -0.05V to 0.05V

[D-Three]

I didn't know it setted the range for a complete data acquisition or per channel. So if it's easy if it's really per channel, thank you for your answer. Everybody 🙂