hi manga,
I post you this information that could be helpful:
"When external excitation is enabled on the NI-9237, the excitation ± terminals (pins 6 and 7) disconnect from the internal cDAQ backplane source and connect to the 4 terminal EX ± AI Bridge on the front of the device. With external excitation enabled, a 2-10 volt external power supply 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 measured voltage might be 4.5 volts. There is no workaround for this; the minimum voltage the 9237 accepts is 2 volts. This is a constraint placed on us by the ADC.
With internal excitation, there's a 150mW maximum power draw limitation. With external excitation, we didn't calculate or test for the maximum power draw as we couldn't think of a valid customer use case that would put the module at risk. As long as the customer is using strain or bridge modules, we should never draw too much power through the module. "
