Measure Analog Signal in Millivolts

[Peejay]

Hi,

   I am using a E6052 DAQ card and a SCB 68 Connector Block. I am currently working in RSE Mode, using analog 

  channel ai7.  I am trying to measure the voltage across a shunt resistor (0.025 ohm). I have a 1K resistor in series with

 shunt resistor as suggested by NI document on how to measure current. When I suppy a 100 Volts i do not get the reading in

 millivolts. May you give me some insight on what might be the problem.

Thank You

PJ

[Seth_B.]

PJ,

Is this schematic consistent with your setup?

 

Also, what do you mean when you say that you do not get the reading in millivolts?  Where are you trying to take your reading (e.g. Measurement and Automation Explorer, LabVIEW, Visual C++, etc.)

Message Edited by Seth B. on 12-16-2008 12:09 PM

[AnalogKid2DigitalMan]

The circuit in the above post would yield 2.5mV across the 0.025 ohm resistor.

-AK2DM

[Peejay]

Hi Scott,

           The drawing is consistant with what i am doing. My sine wave is supposed to generate 2.5mV p-p but

           i am not getting that result. The question arises am i suppose to get a sine wave across the shunt resistor?

           Another question i have is should it matter whether i am using RSE or Diff mode? Because from my understanding

          you use Diff mode when you measure voltage under 1V.  I tried to switch over to Diff mode.  I get a flat line at negative  -1?

          Which is the best mode to use in the situation at hand and would that make a difference?

Thanks

PJ

[Peejay]

The reading im trying to take is in labview.

[Peejay]

Sorry Seth. I thought your name was scott due to message at the bottom of the screen.

Once again i apologize.

The reading im trying to take is in labview.

[Rialter]

Hi Peejay;

Just as you say, if you have asine wave input you should obtain a sine wave meassurement in the shunt resistor. And regarding your other question, there is something you should take on account, the connection diagram of a differential mode is different from a RSE one, because in RSE you use an AI channel and AIGND and in a diff mode you use two different AI channels. This is because Differential mode is obtained by substracting one signal from the other one, and is commonly used in applications where your signals are very small because since you're making a substraction, the noise signal that can be generated in the wires is nearly eliminated and you obtain a clearer signal.

I hope this information is useful, and i suggest you use differential mode, but check the connection you should follow, because yo u need to connect another AI and not the AIGND.

Good luck

Francisco Arellano

[dousley]

Hi PJ,

I noticed that you are using a 100V source. Have you verified that the 100V source is actually producing what you think it's producing? That could be difficult because the 6052E has an input range of only +/-10V max. Furthermore, it's only protected to voltages up to 25V. Hopefully the inputs were never exposed accidentally to your 100V source since that could cause damage to the device.

It should also be noted that using a 1k ohm resistor as your current limiting resistor will allow a fairly high current to flow in the system. You have 100VAC (since it's a sine wave that would be about 70Vrms) and 1k ohm...

V = I*R, so

I = 70Vrms / 1k ohm 

I = 70mA

Not only that, but you are going to dissipate that voltage and current over your resistor.

P = I*V

P = 70mA * 70Vrms

P = 4.9W

So your resistor is dissipating about 5 watts. Normal resistors that you find at radio shack are probably going to be rated for about 1/8W or so. If you are not using a special power resistor you might have burned it already. That could result in a broken connection between your 100V source and your sense resistor. So there wouldn't be any current flowing in the sense resistor anymore. That would mean that there would be no voltage on the sense resistor.

Hope that helps.

Dousley 

Conditioned Measurements