Trying to set an analog output line to high impedance

[adams156]

Is there any way for me to set the impedance high on an analog output channel?

I am currently using a NI 7854 FPGA card to send out analog signals.  I have one analog output channel on the card that I sometimes have to let another external source drive.  I'm trying to find a way to just set the impedance high on my analog output so that I can let the external source drive this line without my FPGA interfering.  I've seen the "Set Output Enable" method node that is used to set high impedance on digital lines, but I've seen no mention of using that method on an analog line.  I also thought about just removing that output line from my project, but I'm not exactly sure what the result would be. 

[nathand]

As far as I know there's no such option for an analog output.  My understanding of electronics is somewhat limited but I expect that the two analog sources would interfere and decrease the output accuracy.  Could you use a relay driven by a digital output to select which source is connected?

[johnsold]

nathand is right.

Depending on the source and load impedances of all the devices in the circuit (if you are lucky) you might be able to select some series resistors which would allow you to keep everything connected. Another, much more complicated, option would be to sense the voltage at the load and set the FPGA output to match. Similar to a guard drive circuit.  It is not really that simple, but in principle it could work.

Lynn

[smercurio_fc]

Connecting two voltage sources in parallel? Bad idea. I wouldn't count on trying to get lucky with series resistors, since they may change with temperature, unbalacing quicker than you might expect. A relay is by far the safest way, and you want one that is break-before-make.

I like the picture in the first response here: http://electronics.stackexchange.com/questions/23943/what-happens-if-i-connect-two-different-dc-volt...

[adams156]

Thank you all for the quick replies.  I should probably describe my situation in more detail.  I will have the FPGA card normally driving a signal across an interface board, which will connect to the destination circuit board.  There are some circumstances where I will hook up another entirely separate board to my destination board.  On the days that I have that separate card hooked up, I'd like to have my FPGA completely cease driving its analog signal.

I'm not really looking to set the impedance high during execution.  This would be a configuration that I would set up ahead of time and would maintain that setup for days at a time.  The relay is definitely an option that has come up, but I'm trying to avoid it if possible.

In response to johnsold, I don't really want to set the voltage low, just because I would still be driving a voltage of 0 rather than basically just disconnecting.  It may be that there simply is no method to do this through software, but I definitely wanted to pursue the software route.

[johnsold]

If it will only get changed occasionally and by personnel who are at least somewhat technically competent, put a switch on the interface board to select the source.  If you need to monitor the status of the switch, make it a two pole switch and use the second pole for the status circuit.  This works with relays also.

Lynn