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NI 9205 with both floating and grounded differential signals

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Hi All

 

I have a question about how to wire the 9205 (or if even what I want to do is possible/safe). 

 

I want to measure the output of 2 shunt resistors. The shunts are measuring battery current and alternator current on a vehicle. The first shunt lies between the vehicle chassis ground and the negative terminal on the battery. The second shunt is essentially in-line with the output of the alternator to the battery. My thinking is Alternator output current - battery current = current consumed by all the auxiliary engine equipment. So one is grounded and one is floating. 

 

The shunts are MuRata 200 A shunts with 50mV output difference between the terminals. 

 

This does mean that I have live 12-14 V being fed into the car and into the NI 9205. Along with a second cable which is the negative side of the battery. So yes I think I need to make sure that these cables never touch and that the live one also never touches the vehicle chassis. I should maybe fuse the live wire to stop large currents coming down. 

 

I have read that when using differential inputs you must also wire -ve side of the signal to ground. I guess this is to stop stray charge building up and messing your signals? However, if I wire the floating one into COM with a 1MOhm resistor but also have to wire the grounded one to COM as well, am I not essentially joining +ve and -ve on the battery? 

 

I do have a another solution which is just to leave the negative side shunt in place but measure the alternator current with a current clamp from Pico Technology. More expensive but is at least simple plug and play and safer.

 

Many thanks

 

Alex

 

 

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Accepted by topic author AlexMason

You do not want more than 10V relative to the COM to enter the 9205.  You will damage it if you do.

 

What I would do is use a current sense amplifier such as a TSC103IPT to turns your differential signal into a single ended.  This will also remove the common-mode voltage.


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Yes, I have spoken with some people who are more in the know and we have decided going with a current clamp on the "high side" of the battery is perhaps the more straight forward option. Not to mention the safety aspect... with the shunts simply being two brass blocks with a resistor bridge, I'm effectively pumping the 12 V from the vehicle battery into the cabin, in to the DAQ. It wouldn't take much to accidentally short the battery to ground and/or damage some equipment. 

 

With the clamp. I can leave everything as grounded differentials.

 

1 quick question though. Do I have to wire one side of each channel pair being used in to COM?

 

We have 2 inputs that are coming from charge amps (these are 0-5 V), 1 input from a pressure transducer (also 0-5 V) and will have 1 input from a shunt resistor (10-200 mV typical) and 1 input from a clamp (10 or 1 mV per Amp). 

 

Really the 9205 should have been wired as single inputs, but someone has wired it all up as differentials, but essentially all are 0 v to some other voltage.

 

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Since all of the signals use the chassis as the negative side, just wire one of those to the COM.


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Many thanks crossrulz

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Actually just thinking about this. They don't all use the same ground. 

 

The charge amps output 0-5 V on BNC style connectors. The charge amps are powered directly by 12V provided from a set of lithium batteries that are separate from the vehicle. 

 

The shunt will use the vehicle chassis ground. The other pressure transducer will also likely use that lithium 12V battery ground. 

 

The current clamp has its own 9V lithium PP9 source.

 

So 3 different grounds? Do I just wire them all into com with a resistor infront?

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AlexMason wrote:

So 3 different grounds? Do I just wire them all into com with a resistor infront?


Yep.

 

You might want to read this article: Field Wiring and Noise Considerations for Analog Signals


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Many thanks again for clearing this up. 

 

Alex

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