10.5V voltage problem in cRIO-9215

[eeer]

Hi, everyone,

I have a cRIO-9215 and want to use it to measure a voltage from outside. The outside voltage comes from a MTL1132 Respiratory Belt, which is a piezo-electric device and is used to measure the respiratory amplitude. This voltage is usually about 20mV. However, as I found, when I plugged this MTL1132 device to any of the BNC port of cRIO-9215, the measured voltage is always increasing, at a rate ~ 1.8mili-volts/second, which is comparable with the normal output voltage of MTL1132.


I think this might come from the fact that, the BNC port of cRIO-9215 always has a voltage of 10.5 volts, and my MTL1132 has a resistance of ~100mega and a capacitance of ~2.2uF. When my MTL1132 is plugged into the cRIO-9215, the 10.5volts voltage coming from cRIO-9215 is charging the capacitance of the MTL1132.


Does anyone know how to solve this problem? Is there any idea to solve the problem of 10.5 volts voltage from cRIO-1132? Thanks very much!

[AdnanZ]

I am not sure as to what exactly is going on over here but let's try a couple of things. I assume there must be some sort of grounding issue involved. How are both of your devices grounded? Also, what sort of readings do you get if you attach a pull down resistor to one of the channels of the cRIO?

[eeer]

Hi, Adnan,

Thank you for your reply! Actually my cRIO-9215 is a cRIO-9215 with BNC terminals, so I do not assume that there is a grounding problem with this cRIO. Only cRIO-9215 with screw Terminal has problems of grounding. Also my MLT-1132 is just a small signal-source, which is a piezo-electric device. It can produce a signal ~20mV depending on how much press it feels.

When I connect this MLT-1132 with my cRIO-9215 with BNC terminal, I think they should have a common ground, right? The problem may be because that the piezo-electric device is also a big capacitor (~2.2uF) simultaneously, and the cRIO-9215 can also be treated as a +10.5volts source. When they are connected, the +10.5volt source will charge the piezo-electric capacitor, this might be the reason why my measured signal is constantly increasing. But, do you have any idea to solve this problem?

Thanks very much!

[AdnanZ]

I don't think there is anything we can do with our devices which can solve the issue. We know the problem is being caused because of high capacitance and you might just have to include something in the circuit which can resolve that. Maybe a breaker circuit in parallel which mirrors the voltage but reduces the capacitance?