What is the pratical resolution of the SCC-CI20 when used in conjunction with a 16-bit DAQ card (PXI system)?

[Silbah]

Hi,

Theoretically, the combination of an SCC-CI20 and a 16-bit DAQ board would yield a resolution of aproximately 0,3 uA for the 0-20 mA range (20 mA / 65536 steps)

But taking into consideration the practical constraints of the SCC-CI20 (gain error +/- 0,1%, offset error +/- 0.6mV, temperature drift of gain errors, offset error, and temperature coefficient) what would be the worst case current measurement resolution?

I'm asking this because I have an application where I need to measure a current that's typically around 10mA, but I need a resolution better then 1uA.

Can I use an SCC-CI20 for this? If not, what would you recommend for this application, applicable to a 16-bit DAQ card.

Regards,

Habil Silva

TESTWISE, Portugal

[Remi.D]

Hi Silbah,

To determine the worst case operation with this configuration (SCC CI20 + DAQ card), you need to study the combination of this two board in the measurement process. 

In fact the SCC-CI20 converts current to voltage by passing it through a precision 249 Ω resistor and sending the resulting voltage to the DAQ device as a 0 to +5 V signal. The SCC CI20 and the DAQ board have their own resolution (board depending).

To know the worst operation case you must take in mind either the SCC CI20 absolut accuracy and the DAQ board absolut accuracy.

In the following link you will find a VI to calcul the absolut accuracy of a DAQ Board:

Absolute Accuracy Calculator

https://decibel.ni.com/content/docs/DOC-9912

Best regards,

[Silbah]

Hi Remy,

Could you please help me with the values that I should input into the VI in order to calculate the accuracy?

From what I gather:

Positive Full Scale: 5V

Negative Full Scale: 0V

Residual Gain Error: 100 ppm (+/-0,1 % of max, according to SCC-CI20 datasheet)

Gain Temperature Coefficient: 6 ppm/ºC (0.006% / Cº, according to SCC-CI20 datasheet)

Reference Temperature Coefficiente: (don't know what to input here)

Residual Offset Error: 120 ppm of range (+/-0.6 mV offset error relative to 5 V, according to SCC-CI20 datasheet)

Offset Temperature Coefficient: 4,2 ppm/ºC (21uV relative to 5 V, according to SCC-CI20 datasheet)

INLError: ppm of range (don't know what to input here)

Random noise: should I leave the default value of 153uV? (don't know what to input here)

The DAQ device that I want to use is the PXI-6220.

Please let me know what the other values should be.

Anyway, even if I set ReferenceTempco = 1, INLError = 0 and Random Noise = 0 I get a best value of 621uV which corresponds to aprox. 2,49 uA.

Apparently the accuracy of the SCC is not good enough for my application.

I probably need to go for a DMM, right?