How To Test 0-20mA Bricklet

[thicks]

Hi,

 

I am trying to accurately test the Industrial 0-20mA Bricklet for a 2 wire application.  My application will be using 24VDC for excitation with a 0-20mA Pressure Transducer.  (Pin 1 +24V Power Supply, Pin 2 0V Return to Power Supply, Pin 3 to Transducer Excitation, Pin 4 Signal from Transducer). 

 

To test the accuracy of the Bricklet, I was thinking of renting a Keithley 6220 Current Generator BUT I am not sure how I would wire thise into the bricklet.  A Current loop Simulator might not be accurate enough.  I am looking for an input source that will allow we to select current at 16 bits or 0.00061mA.

 

If anyone has a good idea of how to accurately test the Bricklet then let me know. Please send wiring diagram of setup.

[borg]

Generating a very precise current is a harder then measuring current precisely.

 

So one way may be to just connect a sensor or similar and to measure the current with a good multimeter or through something like a uCurrent for the actual value. This way you should be able to get a few Bricklet/real measurement pairs and to calibrate offset/gain if necessary.

 

The Bricklet itself relies on the accuracy of the used parts (datasheets can be found on the Bricklet page) and is not calibrated additionally.

[thicks]

I might try the uCurrent.  Thanks for the advice borg.

 

I was able to set up a voltage divide circuit with a 1.2K Resistor and a 5K Potentiometer.  I can adjust the potentiometer and see the current change between 4-20mA with "Brick Viewer". 

 

Is there an easy way to modify the 0-20mA bricklet firmware so that I could do the following:

See more digits in "Brick Viewer"

Change the resolution to 18bits on the MCP3423 ADC.

Output just the raw Analog to Digital Converter data.

 

I have the processes for modifying the Bricklet firmware working.  I am hoping that I could be directed to the place in the code where I could make these modification.  Also, is it possible that a Red Brick could help me with this?

 

Any help would be appreciated.

[borg]

To change the resolution to 18 bits you just have to change the samples per second to 4 (which is default).

 

To get the Analog Value you have to change line 212 of 020ma.c from

ret_na = measurement*15625/91;

to

ret_na = measurement;

 

Alternatively you can just multiply the current that you get by the Bricklet with 91/15625 to go back to the analog value.

[thicks]

That worked fine.  Thanks!