Measuring Current with NI6250

[Rafi2003]

Hi,

I'm using the NI 6250 for some Analog input analysis.  In addition I want to measure the current I supply to Unit under test.  The current is approximately 300ma.

I thought to use a 2 ohm shunt resistor, thus to lower the supply voltage by 0.6V and not having too low voltage for measurements.  I plan to adjust the supply votage to 9.6 V so that after the shunt resistor, I'll get 9V as I'm suppose to provide.

1. Any remarks or better suggestions for that?

2.  I will calculate the current by measuring the voltage over the resistor.  The question is shall i use 2 single inputs or a differential input?

3.  Looking in Table 4-3, page 4-12 of the DAQ M Series - User Manual, I see a recommended application of a differential input voltage.  There, they put 2 pull down resistor to ground.  Is this what I need to do as well?  Can't I just apply the 2 lines from the resistor, directly to the diff. input?

4. Am I suppose to connect the AGND to my supply GND?

Thanks

Rafi

[Henrik_Volkers]

Well, depending on your load and your source I would try a current measurement with a 0.2 Ohm shunt (4 wire) in the ground line with differential measurement in the +/- 100mV range.( gives you a 500mA current range)

With 9V you might use the shunt in the high side and use a differential channel.Take a look at the specs.

If your source has sensing capabilities you might even compensate the voltage drop on the shunt.

[Rafi2003]

Hi Henrik

Thank you for your response.

Actually, I had difficulties to obtain a 9V power supply (15W).  Therefore, I decided to take a 12V ps, trimmed down (by the supplier) to 9.6V.  Now, all I need is to use a 2 ohm resistor with 300mA current and I get a nice 9V for the Unit under test.  My PS does not have sense lines.

Another consideration in selecting the shunt resistor was not to use a too low voltages.  With 2 ohms I get 0.6 Volts.  My UUT can tolerate the little voltage drop since, eventually, it regulate it down to 3.3V.

I was looking for advice whether to use a differential measurement or 2 single ended measurements...and whether I need pull down resistor if I use a differential measurement.

Thanks

RK

[Henrik_Volkers · Accepted solution]

In your case the diffential measurement is the way to go. You don't need any more bias current (pulldown) resistors if your PS is connected to (A)GND somewhere anyway.