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I need to connect a DAQ to an existing system which uses a DAQ. The idea is to connect the DAQs in parallel so the existing system is not changed. It is likely that only one DAQ will be used at a time. The DAQs will be configured to output a 0-10V signal. I believe this will be fine, but I'd appreciate any clarification.

Thus, I cannot interface with DAQ0, so can I introduce DAQ1 in parallel with DAQ0 without compromising the signal or either of the DAQs?

Would there be any benefit to keeping the systems isolated?

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  • \$\begingroup\$ Do you mean DAC (digital-to-analog converter) rather than DAQ (data acquisition module, which is actually a kind of analog-to-digital converter)? \$\endgroup\$
    – Dave Tweed
    Mar 18, 2014 at 15:03
  • \$\begingroup\$ @DaveTweed I mean DAQ as in data acquisition. I suppose my question could also be "can I connect two DACs together?" because I'm concerned about connecting two analog outputs together where only one would be powered at a time. \$\endgroup\$
    – Derek
    Mar 18, 2014 at 15:23

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Probably not without some means of isolating DAQ0. Even though it may be disabled, it could still take a significant amount of current from DAQ1's output and possibly one or both could be damaged. This is the worst case scenario.

I'd use DAQ1 to announce itself to a relay which breaks DAQ0's connection thus allowing DAQ1 to have full control over the target.

I'd use a relay that switched both ground (0V) and hot too. This keeps them isolated and fool proof.

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  • \$\begingroup\$ Thanks for your input. Interesting idea with the relay. Then you would say that it would be best to disconnect DAQ0 if possible? \$\endgroup\$
    – Derek
    Mar 18, 2014 at 15:19
  • \$\begingroup\$ If you have two outputs trying to drive one input, then you will have to disconnect the unwanted output, otherwise the two outputs will fight each other, and the "active" output will not be able to drive the input as desired. \$\endgroup\$
    – Peter Bennett
    Mar 18, 2014 at 15:54
  • \$\begingroup\$ @PeterBennett Regardless of whether or not the unwanted output is active? The unwanted DAQ0 will be completely powered off. \$\endgroup\$
    – Derek
    Mar 18, 2014 at 16:09
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    \$\begingroup\$ @Derek: if the unwanted source goes to a sufficiently high impedance when powered off, you may not need to isolate it - but an unpowered output may be damaged by the voltage from the active output. \$\endgroup\$
    – Peter Bennett
    Mar 18, 2014 at 16:22
  • \$\begingroup\$ @PeterBennett I think that is my answer then. \$\endgroup\$
    – Derek
    Mar 18, 2014 at 16:29
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If the second driver is powered down, you're almost certainly going to violate its maximum acceptable ratings.

There do exist ICs which are tolerant of moderately-high voltages wrt ground, independent of the power rail being present. But semiconductors naturally have parasitic PN junctions to the power rail, which would become forward biased in your scenario and risk latchup. The cheapest way to protect against latchup is to augment these parasitic diodes with intentionally-built protection diodes.

You would need a system that doesn't have either the protection diodes or the parasitic PN junctions. Not only that, you need the existing system to be already using such, which is highly unlikely. You could check the datasheet to be sure.

And note that all of the above is also true for input stages. Normally you can fanout a single low-impedance signal to multiple ADC inputs, but not when some of them are unpowered, as the protection diodes will cause clipping of the signal seen by all the rest.

In both cases series resistance can help.

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