Measuring voltage with USB-6212

[s.gambuzza]

Good afternoon everybody,

I have a question about the feasibility of a measurement: I have an electrical DC circuit, in which the maximum current is going to be under 1 A, and I want to measure the voltage drop across a 5 Ω resistor under different load conditions.

In order to do that, I was thinking about connecting the two ends of the resistor to the center and sleeve of a BNC cable that then feeds into an USB-6212 DAQ board. From the manual, the equivalent impedance of the board is 10 GΩ // 100 pF, and from my understanding this will definitely limit the current in the BNC cable, and by consequence the power transfered to the DAQ. However, I am not sure if this is what's actually going to happen, and I am afraid I am going to burn the DAQ board somehow.

Sorry for the very noob-y question, but this is my first time using a DAQ to acquire data.

[crossrulz]

My recommendation is to get an amplifier circuit that is specifically made for current shunts.  These will have differential inputs that can handle a high common mode voltage and output a single ended value.  This will protect the DAQ as well as give you a low impedance source driving your DAQ which will help reduce "ghosting".

[s.gambuzza]

I'll keep that into consideration. However, from what I know about current shunt amplifier (which is, reading from the Wikipedia page), those work well in the case of very low voltages and high currents, which is the opposite of what I have (max. 5 V and 1 A). If the main concern is the potential spike in voltage that could damage the DAQ, wouldn't a low-pass filter be enough?

[crossrulz]

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@s.gambuzza wrote:

If the main concern is the potential spike in voltage that could damage the DAQ, wouldn't a low-pass filter be enough?

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My main concern is the common-mode voltage.  You cannot have an input that exceeds 10V relative to AI_GND, even if your differential is less than 10V.  Ok, the specification actually says 11V (assuming my memory is correct), but just stick with the 10V limit.  The shunt amplifier handles this issue.  You can probably find a buffer that has a gain of 1 or a simple op-amp configuration to handle this as well.

[Henrik_Volkers]

The 5 Ohm is bad idea, if 1A is flowing you have a voltage loss of 5V .

I suggest a 100 mOhm Resistor , measure in the GND line of your DC device , use differential voltage measurement.  The max voltage to be measured is 100mV , use the 200mV range (some overrange is always a good idea) , from your sense resistor add a 10kOhm in each sense wire and two anti parallel diodes at the AI inputs as clamping. Depending how your DC device is powered another 1k to 10k resistor from Device GND to DGND of your DAQ is needed. If you have a DMM measure the voltage between Device GND and DAQ GND first... while both devices are running 😉  than add a 1k and measure again...