Measuring the voltage across resistors using the 6225

[NicholasWittebol]

I need to monitor the voltage on at least 48 resistors, and was considering using two PCI-6225's to accomplish this task (since I need at least 48DI input channels). Is this recommended, or is there a better way to do this?

[280Z28]

If you can find 9 resistors that share a voltage on one side (8 if that voltage is ground), then you can do it with a single 6225. Keep in mind that your maximum sampling rate could be pretty low if you are sampling all channels at once: 225kHz/80 = 2.8kHz.

[NicholasWittebol]

Well, the truth is that this is being done for the purpose of current sensing, so no resistors are grounded, and none of them share the same voltage. They are connected in series with a transformer, and then based on the voltage across the resistor, we will calculate how much current a connected device is pulling.

[280Z28]

If the transformers have a supply and ground, could you place the resistors on the ground side? They would have a common terminal then. Based on what you said though, I assume each device has its own transformer and each device is using a +/- supply instead of +/0?

A 6225 and with a 6220 would also give you the required 96 inputs for a bit cheaper. It's exactly 96 inputs though so you couldn't expand without new hardware.

Message Edited by 280Z28 on 05-23-2006 11:16 AM

Message Edited by 280Z28 on 05-23-2006 11:18 AM

[NicholasWittebol]

That's right, each resistor has it's own little transformer, and they are +/- not +/0.

[Tom_W_[DE]]

Hi Nicholas-

I would recommend that you keep in mind the working voltage specs as you choose hardware.  Since it sounds like your signals are not referenced to a common ground you will need to use differential input mode if you plan to use a DAQ board.  This will help to overcome ground loops in your system.  However, you must make sure that no input pin exceeds +-11V of AIGND (according to the NI 622X Specifications).

Since your signals do not share a common ground, a channel-to-channel isolated solution would probably be a better bet.  The SCXI-1125 may be an option if you think your signals may lie out of the working voltage limits for a DAQ card alone.

Hopefully this helps-

[NicholasWittebol]

I'm hoping that my voltages will be in the +- 200mV range, so this card is suitable as far as that goes. I'll get good resolution at that level too (6.4uV I think). I have a max of about 30A going through a 0.05ohm resistor, so that's only 150mV.