How are A to D settling times determined and/or changed?

[tjscientist]

We have a NI USB-6218 and I am trying to understand how the A to D settling times are determined. In the product specification there is graph showing the relationship of input impedance to settling time for settling errors, but how is this determined? Mainly, what I am interested in is how the device "automatically" determines the correct input impedance and allows for the correct amount of settling time before doing an A to D conversion. We are currently using the device on lines with a typical 1 Meg impedance and I am questioning if the settling times are correct. Can I obtain that data per channel? Can they be changed? Thanks in advance for any help,Jeff Swan

[tnek]

Hi Jeff, Thanks for posting your concern. There is lots of good information about this issue. I would recommend <a href="http://zone.ni.com/devzone/cda/tut/p/id/2825" target="_blank">this</a> article as a good place to get background information about settling time. As for ways to minimize the errors due to settling time, I can think of two ways to go. The first is to reduce the impedance on each channel. <a href="http://zone.ni.com/devzone/cda/tut/p/id/4494" target="_blank">This</a> article explains how this can be accomplished. The second method is extending the amount of time between each conversion. <a href="http://digital.ni.com/public.nsf/allkb/42484E84DA98053686256D32006E0494?OpenDocument" target="_blank">This</a> article explains how the convert clock is determined in the driver and how this value can be manually set. Hopefully this information is sufficient for your needs. Let me know if you have any other questions. Regards,KentApplications Engineer