measuring multiple voltages with NI-9205

[sol_pow]

Hello

For our project we are using the NI-9205 module to measure the voltage with 3 chanels. According to the Labview DAQ-assistant, we can use either RSE or differential mode. But no matter which one we use, the following problem arises:

Example RSE:

-We need to use 2 chanels to measure 2 voltages independently (but fed by the same battery): chanel (1) and (2) 

-The com-chanel (29 or 10) is connected to the ground or not at all (both version tested).

-When connecting a battery (3V) between (1) and (10/com) and nothing between (2) and (10/com), the chart/graph shows, that both chanels get 3V. Why is that? It makes sense for chanel 1 connected to the battery, but not for chanel 2, which is free/not connected or short cutted.

-When short cutting (2) and (10/com) (while keeping 3V between (1) and (10/com), it shows 0V for this chanel and 3 V for the other.

Since we want to use these signals for on-off controllers (trigger=3V), we dont want, that both actions get triggered, if only one chanel effectively has 3V.

When using the differential settings, we get the same problem.

I hope, our problem is clear, if not feel free to ask.

Thanks in advance for any advice!

Nic

[Henrik_Volkers · Accepted solution]

Usual crosstalk due to multiplexing ....

If a 100k Ohm load doesn't hurt, just add it, a floating input can have any value..

Or have one channel with a short cut (say AI08) , read a channel list like (AI01,AI08,AI02,AI08,AI03,AI08)  and only use AI01-AI03....

[sol_pow]

Thanks a lot for your answer! 

It works much better than before and we could work with it. However, there is one strange thing with this method:

I short cutted AI01 with the com and connected AI02, AI03 and AI04 to the battery (+) and the battery(-) to the com.

-Connecting only A02 to the battery indicates that all channels (except for the short cutted one) give 3 volts.

-When connecting only A03, it tells me that A01 and A02 are 0 and the others 3 volts.

-Connecting only A04 to the battery, only A04 has 3 volts and all the others are at 0.

Does it react as you would expect? It would be nice to have 3 volts only for the connected chanel, but we could work with this, if it is not possible.

Again thanks a lot for your help!

[Henrik_Volkers]

Since I always expect the bad with multiplexed inputs, I try to avoid them 😉

Think of a capacitor (small but there) at a high impedance input. Now you (dis)charge it to a certain voltage (battery or short cut or...) .. if you leave it open it will only chage its potential by the current due to input impedance (>1GOhm) and the input bias current (to laisy to look up ... if it's even stated in the specs.. NI tends to write less and less in the specs...:(  )