Input multiplexer settling problem

[ElbaTech]

NI Multifunction Cards have often a multiplexer before the A/D converter, which allows scanning multiple channels. NI observes that the settling time of this multiplexer may cause problems, especially when an input signal has a considerably higher amplitude than the nearby signal. Here the setlling time does not allow fast scanning because this results in "noise" on the smaller signal due to "memory" of the large one.
Is there a known solution to this problem ?
I tried to keep source impedances around 1 KOhm as suggested by NI, but this did not yield to a complete solution, rather to a slight noise reduction.
Is there a known way to use multifunction NI cards for input scanning with no inherent noise channel-by-channel ?
Thanks a lot
for attention,
Marco

Marco Sartore
ElbaTech srl
Via Roma, 1 57030 Marciana (LI)
Italy

[RIO-Todd]

Marco,

There are a few solutions that can help with the charge injection problem. The optimal solution is to decrease the impedance of your sources. The smallest source impedance is ideal. So, if you have sources at or above 1 Kohm, it would be good to use a unity-gain buffer to decrease the source impedance.

Other solutions involve increasing the inter-channel delay of scanning and/or grounding channels in between channels to allow the multiplexer to discharge. I have linked a couple documents below that go over charge injection in detail and the viable solutions.

How Do I Create a Buffer to Decrease the Source Impedance of My Analog Input Signal?
http://digital.ni.com/public.nsf/websearch/CF83426BC3AC514A86256C10005A4771?OpenDocument

Using a Unity G
ain Buffer (Voltage Follower) with a DAQ Device
http://zone.ni.com/devzone/conceptd.nsf/webmain/CD57A73721E0612586256BAE0055CDD9?opendocument

Regards,

Todd D.