NI 9219 is not usable with full bridge sensors in ADC mode "high speed"

[Matthias_H]

Hello,

I'm using the NI 9219 with the CompactDAQ system to measure a force with a force sensor. It's a full bridge sensor with a range from 0 to 2 kN with 1 mV/V at 2 kN.

The sample rate is 50 Hz and the ADC timing mode is set to "high speed". I'm using the internal excitation of the NI 9219.

The attachment shows my input signal.

However the force is constant (about 30 N), the input signal is a sine wave with an amplitude of about 10 N.

I tested different sensors and the input signal always shows the same behavior.

With the ADC timing mode "best 50 Hz rejection" I'll get the correct value of about 30 N and no sine wave (supply voltage 230 V / 50 Hz). The problem is that the NI 9219 has a sample rate of only 7,7 Hz in this timing mode and this is too less for my application.

Why does the NI 9219 not work correct in this timing mode?

Could there be a problem with the grounding? I can't see any problem here.

Best regards

Matthias

[HoussamK]

Hi Matthias,

The NI 9219 has 4 different sampling modes: High Resolution, Best 60Hz Rejection, Best 50Hz Rejection, and High Speed. The default mode for the device is High Resolution. The 9219 User Guide and Specifications contains additional information about each sampling mode.

NI USB-9219 User Guide and Specifications

If your hardware is configured in the Best 50Hz Rejection mode which configures the Conversion Time for the ADC to be 130 ms. The maximum sample rate possible for the hardware is limited by the ADC conversion rate, which means that in the Best 50Hz Rejection mode the maximum sample rate is 7,69 Hz.  In order to use the NI 9219 at a rate above 7,7 Hz, it can be configured programmatically using a DAQmx Property located on the Functions palette, as shown below 

For more information about configuring the sampling mode, please go to:

How to Set the Sampling Rate of the NI-9217/9219

I hope this will help

Best Regards,

Houssam Kassri

[Matthias_H]

Hello,

thank you, but this is clear and I read the manual before. This was the first thing I did. And I also read the page you linked before.

Look at the attachment above. There you can see a sine wave at a constant force if the timing mode is set to "high speed". I want to have a sample rate of 50 - 100 Hz and so, I can't acivate the 50 Hz rejection filter in the ADC timing mode, because the ADC is too slow in this mode.

Is it normal to have this behavior in the timing mode "high speed"?

Measurements with strain gauges and a sample rate higher than 7,7 Hz are not possible with this card although it has a max. sample rate of 100 Hz.

Maybe the 100 Hz are only possible for the other modes of the card (no bridge circuits).

Regards