External voltage excitation NI 9237

[pho_nox]

Hello,

I'm using a CompactDAQ Chassis with 2 NI 9237 modules for strain gauge measurements.

The maximum excitation voltage of my gauges is limited to 1V...

The 9237 supports internal voltages from 2,5 to 10V.

I want to connect an external voltage supply. Now I found the restrictions of 2V-10V in the datasheet for external devices. Is there any way of getting my 1V voltage into this system?

Karsten

[TobiD]

Hi Karsten,

When external excitation is enabled on the NI-9237, the excitation ±terminals (pins 6 and 7) disconnect from the internal cDAQ backplanesource and connect to the 4 terminal EX ± AI Bridge on the front of thedevice.  With external excitation enabled, a 2-10 volt external powersupply needs to be connected in order to provide excitation. 

If the external power supply voltage is less than 2 volts,you will see inconsistent voltages measured across pins 6 and 7 (EX±).  For example, with 0.5 volt excitation applied, the measuredvoltage might be 4.5 volts.  There is no workaround for this; theminimum voltage the 9237 accepts is 2 volts.  This is a constraintplaced on us by the ADC.

regards,

Tobias 

[pho_nox]

I also tried to work with 2V external source... but then I get about 5 V between Pin 6 and 7? How could this happen?

I connected the external source to 2 of the 4 ex-Pins

I switched the settings in the vi to external source and excitation value = 2

[TobiD]

Karsten,

this happens because of the limiation of the ADC in the NI-9237 which requires at least 2V.

regards

Tobias Daub

[pho_nox]

OK ...

up to 2V - I understand... but 

why does it happen with my external source with 2V? Or with 2.1 V or 2.2 V?

The described effect disappears after an external voltage of 2.5V