Self calibration of a PXI 4472 DSA card

[gammalight]

Hello, I am looking at the gain specs on the PXI 4472 card and have a question about how much the self calibration will help, if any, in improving this spec.  In the manual it says the 4472 has a gain (amplitude) accuracy of +/-0.1 dB, which equates to around 1.0% error.  This spec seems marginal at best, but in the manual it also talks about a self calibration routine that can be carried out.  Will the self calibration improve this accuracy at all?  If so how much will it help?  What does the self calibration do for this card?

[Norbert_B]

I just copied a paragraph from the 447x calibration manual:

NI 447X devices should be externally calibrated at a regular interval as
defined by the measurement accuracy requirements of your application.
NI recommends that you perform a complete calibration at least once every
year. Based on your measurement accuracy needs, you can shorten this
interval to 90 days or six months. Self-calibration can be performed as
needed or when the temperature varies by 5 °C or more from the last
external calibration.

hope this helps,

Norbert 

[gammalight]

Thank you for your reply, but that does not answer my question.  I am wondering if I perform the self calibration is that the absolute best gain accuracy i can achieve with this card?

[Norbert_B]

Self calibration, as stated in the manual, mainly compensates thermal drift. So it does not guarantee by any means the best possible measurements.

In order to get the best measurements, you have to externally calibrate the whole system including all components. This is very expensive and normally not really necessary. More often, a good signal conditioning is cheaper and better.....

This is true for any kind of parameter within the acquisition, so also for AI gain accuracy.

hope this helps,

Norbert 

[mikealder]

I have a system with 7off 4472 cards in it along with some voltage attenuation cards, it doesn't take too long to calibrate the whole lot as an end to end calibration - it is the only way to have any level of trust in your measurements. A couple of vi's were created to simplify the calibration which calculate the offset needed for each virtual channel to go through all 56 channels takes around a full shift - Mike

[mallorim]

Hi Gammalight,

Norbert is correct that Self-Calibration is meant to compensate for thermal drift over time. As such, it will take measurements against a temperature resistant lead on the card, and use calibration coefficients then stored on the EEPROM to compensate in software. On the other hard, Gain error is caused due by nonlinearities in the ADC of the device. These are really only addressed by external calibration. For more information about calibration (FAQs and offerings) I recommend seeing the Calibration Services page at ni.com.

Something to note is that the 0.1 dB specification is followed by "max" in the specifications page. This means that this is the maximum gain error that you can expect within one year of external calibration.  If the board has recently been calibrated or is new, then you can expect gain error to be much less. As the calibration of the device ages, the gain error can be expected to increase, but within one year of claibration it should not be more that 0.1 dB.

Regards,

Mallori