usb-6008 limit switches analog I/O

[newbie09]

ya the limit switches are closed by the table, but the analog input you said was open, how would i close it?

[Dennis_Knutson]

Open - meaning open circuit. Your design has no voltage connected to the analog input when the switch is open. With my suggestion, the analog input always has a voltage present. An open circuit on an analog input is bad because the voltage read by the DAQ card will be undefined. Do a search for 'ghosting' to see the problems associated with leaving an analog input unconnected. Avoid all of those problems by using the pullup.

[newbie09]

could you please supply a sehematic of where the  pullup resistorwould be placed in relation to the rest of the circuit, or could i just use the +5V from the digital side??

Thanks

[Dennis_Knutson]

The 6008 has a dedicated +5 output on pin 31. Use that, an analog output, a digital output, an external source, etc. The +5 output can source 200ma. Since I = V/R, use pullup values that do not exceed the max (i.e. 1K). The analog and digital outputs can source much less current so pin 31 would work best.

[newbie09]

thanks! so just put a 1k resistor on the wire from pin 31, then put that to the switch, and then from the switch the AIo+ (pin 2)... ??

[Dennis_Knutson]

Yes, and ground the other side of the switch.

[newbie09]

chers dennis... very helpful!! thanks:manhappy:

[newbie09]

do i have to ground the common of the switch is it???

[Dennis_Knutson]

You have to wire the switch just like in the schematic.