Voltage tolerances on PXI-6704 output lines

[SpinDoc]

One voltage output channel on my PXI-6704 analog output module is no longer functioning and I'm trying to determine the cause of the problem.

1) The channel was regularly used to provide +5 V across a 300 Ohm load.

2) Our lab occasionally generates noise on the signal line amounting to an exponentially-decaying sine wave with a peak amplitude of 20 V, a decay time of about 10 microseconds, and a period of about 1 microsecond. After the pulse train dies away, there is an overall voltage step of about 0.4 V.

3) The 6704 gives its protection as "short-circuit to ground" and can provide up to 10.1 V at 10 mA with an output impedance of 0.1 Ohms, but the spec sheet doesn't list tolerances for back voltages. For comp
arison, the 4351 DMM lists "Over-Voltage protection" of 17 V when off and 42 V when on.

Can anyone tell me what over-voltage protection's on the 6704 or if the above-described signals are likely to have caused trouble?

Thank you.

[rgharrison]

Hi,

If all you have connected to the output pin is a load of 300ohms you won't be dealing with back voltages and the need for over-voltage protection. The Voltage analog outputs are rated for +/-10mA maximum. You are trying to draw 17mA from the card, which is greater than the spec'd maximum. This is likely causing the internal (automatically resetable) fuse to open. If you remove your circuit from the pin it should start operating normally again.

What I would use instead is the Current output pins. Since you want 5V across a 300ohm load, you need a current output of 16.666mA. Since the current outputs are rated to +/-20mA you will be within specs for these outputs. If your circuit really is 300ohms and creates a path to ground then this approac
h will work for getting you the 5V you need and not blow the current specs.

Hope that helps.

Ron
Applications Engineer
National Instruments

[SpinDoc]

First, as noted in part #2 of my original comment, the load isn't simple but occasionally picks up brief 20V noise from EMI in the lab. I was hoping to learn the voltage tolerances so that I could see if this was a problem.

Second, I tried what you suggest and the channel's still dead - even across a MOhm input impedance multimeter.

Thanks

[rgharrison]

Hi,

We unfortunately don't spec what the overvoltage protection or the backvoltage protection is on these cards.

If the spikes are just that...short duration spikes...then the outputs can handle a decent amount of voltage on the pins without suffering permanent damage. So if something caused the outputs to bump up to 15V for very short periods of time, then I think any AO will be fine. Usually, the problem with short circuit protection and things like that is the heat dissipation that occurs over a longer period of time. If the AO were trying to output -10V and you tie that pin to +15, then quite a bit of power gets dissipated in our AO buffer and it will eventually overheat and fry. But if the spike is really short then things don't get too hot.

The other condition you want to avoid is latchup. If output pin gets driven hard enough above the (15V) rail, then you can induce parasitic transistors in the silicon that cause a rail-to-rail short. In that case, removing the offending voltage does not reset the situation (you have to cycle the power in oder to turn off the parasitic transistor) and the part will fry shortly. The following helps explain this situation:
http://www.analog.com/library/analogDialogue/archives/35-05/latchup/

So the bottom line is we don't have a spec for overvoltage and I don't know what those numbers are without measuring them, but if you stay within +/- 15V, then short-duration spikes are okay.

If your device needs repair, the repair procedure is as follows:
1. Go to www.ni.com/ask to create your own Service Request number (SR#). Choose the Phone NI option. Fill in the form with the appropriate information.
2. Call the support number that is given to you and enter in your SR#. You will be directed to an Applications Engineer (AE) that will assist you through the process. The AE will be able to answer any questions you may have about the repair process.

Anyway, sorry I couldn't be of more help. Have a good day.

Ron