Protect Analog Inputs from overvoltage X series

[FrankHS]

I want to protect X series (6320) analog inputs from overvoltage.  The inputs have a maximum impedance of 100k (usually lower).  It can happen that one or two of the inputs are connected to 70 volts. This can happen with the computer on or off. I want to protect the DAQ card from overvoltage damage. It doesn’t matter what the card returns when the voltage is out of range.

I don’t want to load the input when the voltage is between 0 and 10 volts.

http://www.ni.com/datasheet/pdf/en/ds-152

From this document, the specifications are:

Maximum working voltage for analog inputs (signal + common mode)     ±11 V of AI GND

Input impedance  Device on  AI+ to AI GND                                                         >10 GΩ in parallel with 100 pF

Input impedance  Device off  AI+ to AI GND                                                        1200 Ω

Overvoltage protection (AI <0..31>, AI SENSE, AI SENSE 2)  Device on      ±25 V for up to two AI pins

Overvoltage protection (AI <0..31>, AI SENSE, AI SENSE 2)  Device off     ±15 V for up to two AI pins

Input current during overvoltage condition ±20 mA max/AI pin                  ±20 mA max/AI pin

How can I protect the DAQ card?  A 10K resistor in series with the AI input, would limit current  to 7 ma. This should handle the device off situation. 10K should be insignificant compared to the 10 GΩ input impedence of the DAQ. A Zener diode could clamp the voltage but I am concerned about the leakage current.

[crossrulz]

There is also the option of using an Op-Amp to act as a buffer between your circuit and the DAQ.  In power inputs of the Op-Amp will limit the output voltage, protecting the DAQ.  Just find one that can handle high out of range voltages.