calibrating NI6220 ADC with Comedi

[kraehe]

Moin,

we deceided for the Comedi drivers, but I wonder about the values of the ADC.

Zero Voltage seams to be 32900 instead of 32768, and +/-10V also have values lower/higher then expected.

Is this normal for an NI6220, or does the card need some extra calibration step?

Is there a datasheet to map the raw values into volts, if this is the default behavoir?

ciao,Michael

[Neil_S.]

I am by no means a Comedi expert...in fact I've never used it.  However, that being said, I can tell you that the PCI-6220 (an M Series board) uses M Cal software calibration.  This means that calibration is done in software instead of hardware.  See (http://decibel.ni.com/content/docs/DOC-3011).  To get accurate measurements you cannot simply do a linear conversion.  I'm pretty sure that Comedi already has implemented M Cal.  It also already has the self-calibrate procedure built in (see http://comedi.org/doc/x1781.html#AEN3784)  Read the paragraph that starts with "Autocalibration..."

Regards,

Neil S.

[kraehe]

Moin,

I think I got the software calibration running using comedi_soft_calibrate command and comedi_parse_calibration_file and comedi_to_physical library functions.

thanks,Michael