Hardware protection of Analog OUT

[WhiteCloud]

Hi,

my question is about the Analog output (0-10V) of the myDAQ device.

In the specs, you can read:

"Overdrive Protection +-16 V to AGND"

Lets imagine the worst case: The DAQ outputs 0 V, but from outside a battery or anything else is connected with 15 V to the Analog out.

Does a "unlimited" current flow into the DAQ, or does the "Overdrive protection" work here and the hardware is safe from destroying?

The background: I want to protect the AO from voltage transients, and thouht about a simple zener-diode or a tvs diode....

Thank you all, Markus

[Alex_Pi]

Hi Markus,

The analog output Impedance is specified at 1 Ohm.

The overdrive Protection should kick in once this limit is reached. Still the myDAQ is susceptible to ESD to the outputs. A Zener diode as you proposed would be ok for protection.

[WhiteCloud]

Hi Alex Pi,

thanx for your answer!

I will connect a zener diode parallel to the AO for protection. But still I didn´t really understand the physics behind that and I would like to understand...:-)

If there would be a spike which the zener-diode will limit to lets say 11 V and the AO of the my DAQ is 0 V there is a difference of 11 V. If the Resistance of the DAQ AO is 1 Ohm, the current would be I = 11V/1Ohm = 11 Ampere.

I guess that would probably destroy this device. Where is my error in reasoning?

Greetings, Markus

[johnsold · Accepted solution]

Markus,

To use a zener diode as protection you also need to have a current limiting impedance in series with the source against which you are protecting.  The manual for the maDAQ device indicates that the AO lines are driven by OPA1642 op amps.  The TI data sheet for this device shows internal current limiting to ~36 mA when shorted to ground and a thermal shutdown circuit which tries to protect against overpwer conditions.

However, under your 15 V battery, 11 V zener, and 0 V programmed DAQ output, the situation may be different.  The current from the battery through the zener will be limited by the impedance of the wires or any resistance in that line.  This has no direct effect on the myDAQ device, but will likely destroy the zener unless the resistance limits the current to a value less than the maximum rated zener current.  As long as the voltage at the output of the myDAQ is less than the internal power supply voltage (+/-15 V), the current limiting on the OPA1642 should apply.  With +11 V applied to the output and the programmed value set to zero the device would probably try to sink the maximum current to the -15 V supply.  That results in a power dissipation of more than 900 mW, which exceed the rating of the op amp. The themal protection should, in principle, reduce the current being drawn to a level which does not exceed the thermal limit.

Testing this may be an expensive process. The device could be destroyed.  Since the specified maximum current for an analog output channel is 2 mA and the maximum voltage is 10 V, I would consider a series resistance of perhaps 1000 ohms and clamping schottky diodes to the +15 V and -15 V power supplies.  This will limit the current to 10-20 mA under all the conditions you have mentioned and would also provide some protection for the case where the battery is connected when the myDAQ device is powered off. It alsoe does not care about the polarity of the external source.  It will drop the output voltage according to the load impedance.  If this is to be used in a student lab, the calcualtion of that drop is something they should be doing anyway and would be a small price to pay for the protection.

Lynn