Loop Impedance Meaning -- Data Logger Troubles

[katiencbabe]

Hi!
I am trying to measure the voltage through a "DaqPro", basically a data logger. I've used a voltmeter to get a reading, but then when I use the DaqPro it's totally off. So my question is--what does loop impedance mean, especially if it reads:

(Input Channel Measure: 0 – 50.000 mV)

Range 0 to 50mV
Resolution 3µV
Accuracy ±0.5% of reading
Loop impedance - 50?

Please let me know if you understand!

 

[analogkid2digitalman]

I think it is a typo, that spec is probably for the 0-20mA input module. Input impedance appears to be 200Kohm.

See:

http://www.fouriersystems.com/products/8-channel/sensors.php

200K is still fairly high, but when compared to a typical Fluke DMM with 10Mohm input it seems low.

Could it be loading down the source of voltage you are trying to measure? What is the nature of the voltage source?

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"It's the questions that drive us"
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[katiencbabe]

So basically, it's a rather large mixer that is being monitored. The millivolt reading that I got using a fluke meter was around 200mV, so I used the daqPro reading (in Volts) and was unable to get a substantial number. I have plenty of resistors in all different sizes. Do I need to use a resistor to try to get a better reading?

If it were loading down my voltage source (which I don't think it is), would a resistor help?

Thank you!!

 

[OperaHouse]

I you have lots of resistors try this. Put a 470K resistor in series with your input. Also add a 100k resistor in parallel with the module input and common. This will make the input in the range of 300 mV. If you get acceptable results, then this voltage divider can be constructed with a little more precision or calibrate it for the mV/count.

 

[Madcow]

http://www.fouriersystems.com/products/8-channel/data_logger.php

It may not be a typo, if you check the data logger tab
It says 50 ohm input. The spec. sheet (pdf) seems to
leave the input impedance off.

 

[katiencbabe]

OperaHouse, I'll try that.

I don't understand the difference between loop impedance and input impedance. Does loop pertain to simply the resistance/impedance throughout the circuit? And does input mean the impedance just from the mixer?

 

[OperaHouse]

Before someone corrects me, I just want to say I was a little dazed because my wife woke me up in the middle of the night to make her pinapple pancakes. The calculation I did in my head was for a 470K/220K voltage divider to give about a 100K at the input. The 100K resistor I specified will give you pretty close to a 400mV input.

This all assumes a 200K input. What you actually have will become evident with this experiment.

 

[waross]

The high input impedance values are typical of voltage measuring equipment.
A loop impedance of 50 ohms may be typical of a current loop.
Each device in a current loop will have a fairly low impedance. The current loop will have a maximum impedance value. The maximum impedance value of a current loop is determined by the voltage of the loop power supply.
Are you sure that your module is not intended for use with a current shunt?
respectfully

 

[OperaHouse]

This company definitely needs a proof reader! On one page the 0-20mA has a loop resistance of 21 ohms and on another page it is 50 ohms, 10 volt input is 125K and another page is 200K, 50mV is 50 ohm and on another page it is 200K. Frankly, I wouldn't believe anything this company says. 50 ohms on a 50mV input does not seem realistic for a general use data loger. That still leaves a question about the input being 125K or 200K which can only be determined by experimentation.

 

[analogkid2digitalman]

Ignoring any resistances, if you are trying to measure a 200mV signal with a device whose upper measurement spec is 50mV, well, that just won't work.

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"It's the questions that drive us"
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[katiencbabe]

Well, that was after I took the first resistance off and tried to measure in regular Volts (I believe it has a sensor that measures 1-10 V). Instead, it just read .000 V.