CRIO NI9205 current shunt differential measurement

Trying to make differential measurement of the voltage drop of a low side current shunt to monitor battery current.
The battery is isolated from the cRIO NI9205 measurement common. Is it necessary to add a 100k resistor to common to ensure that my common mode is within spec's.
What measurement accuracy would you expect if a 0.2% shunt resistor was used ?
Attachments:
currentShunt_a.jpg ‏14 KB

Some remarks to the accuracy:
Tempco of the shunt? The current will heat up the resistor and the value will change ....
The absolute value of your shunt can measured by a calibration...
I hope you use 4 wire technique
If you are not only measuring a DC current the impedance of the shunt is important. ( inductivity and capacity ... yes even a shunt can have a resonant frequency )
Usually the datasheet of the shunt will give you an answer.
Greetings from Germany
Henrik
LV since v3.1
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I would like to do something similar to what is discussed in the following thread using multiple channels of a PXI-4141 4-channel SMU:
http://forums.ni.com/t5/Digital-Multimeters-DMMs-and/SMU-used-for-current-measurement/td-p/1297856
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hello,
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is it ok to use either a current shunt or should a hall effect sensor be used such as this http://ie.farnell.com/honeywell-s-c/csla2cd/sensor-hall-effect/dp/1082269 . which of them would be more accurate or are both of them fairly accurate.
when i am measuring voltage can i connect two resistors between the positive and negative of the wires going to the battery and connect two wires from either side of the resistor directly into the analog inputs of the scc-68 and measure the 12-15 volts directly would the pci6221 and scc-68 be able to measure the voltage drop across the resistor.
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How to measure high currents accurately (upto 24A) using FPGA

I need to measure high currents accurately. I have only a FPGA board and TBX 68 terminal block. Do I need any other NI hardware?

The answer will depend on your specific situation, such as the voltage levels you are working with, the source of the current, the influence some additional components such as a shunt resistor may have, etc. Can you provide more details about the application, the source of the current, the voltage levels, how quickly the current will change, the stimulus for a current change, etc.
The most common method to measure a current is to add a shunt resistor in the current loop and measure the voltage across the shunt to calculate the current based on the voltage and resistance. You would need to make sure the absolute voltage levels (relative to the ground of the FPGA board) do not exceed its specifications (+-10V). The value of the shunt will depend on the current and the accuracy you need in the measurement.
If you use a 5 milliohm shunt, then you will have a 0.12V drop across the shunt resistor at 24A. The 7831R has a relative accuracy of about 300uV, this would translate into a relative accuracy of about 61mA for a 5 milliohm shunt (absolute accuracy will be less). If you cannot tolerate a 0.12V drop at 24A, then you will have to use a smaller shunt resistor which will give youless accuracy in the current measurement.
The shunt resistor value will have to be precisely measured ahead of time using a good DMM and a 4-wire resistance measurement. There will be some error associated with this measurement. The resistance will also change due to self-heating as the current through it changes (0.12V * 24A = 2.88W) adding some non-linearity to the measurement.
This is probably the simplest approach. If this setup is not feasible (e.g. the voltage in not within the 7831R input range) then you will need some additional signal conditioning. There are some off-the-shelf components for measuring larger currents that would translate the current into a voltage that you can measure with the FPGA board. Look at the following component as a place to get started.
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NI Consulting Services
Christian Loew, CLA
Principal Systems Engineer, National Instruments
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How can I measure a current continually after switching it to a DMM?

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小木虫

Hi 小木虫,
Ignore my first reply. I misunderstood your question and provided an example of how to create a "make before break" scan list entry. You do not need this for your application. I think "break before make" will suffice.
I now understand that you want to make multiple current measurements after creating a switch connection. The scan list entry you provided: r2->c0 & r3->c1; will create the connection required for the DMM to measure the current flowing between c0 and c1. However, the DMM will send a trigger back to the switch after making its first measurement.
One way to make multiple measurements is to create multiple connection entries in the scan list. Each scan list entry triggers the DMM to make one measurement. I don't like this idea much, as it will actuate the relay many times (once for each measurement).
Is hardware-timed operation necessary? If you used a switch software trigger, you could make the scan list connection (r2->c0 & r3->c1; ) and then begin a DMM multi-point acquisition. Once you've collected all the required current measurement samples, you could send another switch software trigger to continue with the next scan list entry.
Will this work?
Chad Erickson
Switch Product Support Engineer
NI - USA
Message Edited by Chad PSE on 02-20-2009 01:51 PM

CRIO NI9205 current shunt differential measurement

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