ad7685 diode clamp required

The opam has power supply of 8, and -12VDC. 

In some conditions, the output can swing to 8 and -12v. 

Will that damage the AD7685? We have noticed issues with the readout is shifted downward. For example, 0vdc shows up as 60000, 5V shows up at 30000. 0-5vdc is the voltage input range. 

Do I need to put an external diode in front the input in when using AD7685. and AD4008?



Added tags for internal tracking purposes.
[edited by: tschmitt at 7:24 PM (GMT -4) on 23 Sep 2020]
  • 0
    •  Analog Employees 
    on Sep 23, 2020 7:23 PM

    Hello,

    Thanks for your query.

    The short answer is that driving the AD7685 analog inputs (IN+/IN-) to +8V or -12V would absolutely violate the absolute maximum voltage ratings given in the AD7685 data sheet and we would not be able to guarantee that the AD7685 would still be functional after such exposure. (See the Absolute Maximum Ratings section in the AD7685 data sheet for more info.) I would strongly recommend implementing protection diodes (i.e. Schottky) on the AD7685 analog input(s) if the amplifier output can swing to those potentials.

    For more background, the Analog Inputs section in the AD7685 data sheet shows how there are ESD diodes which connect the IN+/IN- pins to the VDD supply and to GND. When IN+/IN- exceed 0.3V above VDD or 0.3V below GND, the respective ESD diode will turn on and conduct current. The voltages you're describing are much higher in magnitude than what would forward bias the ESD diodes, so the amount of current would likely be very high and likely to damage the device.

    The AD4008 does implement an additional overvoltage protection clamp on its inputs to clamp the IN+/IN- pins to ~VREF, and are specified to support sustained currents of 50mA. However these overvoltage clamps only protect against voltages above VREF, and there is no such clamping action for voltages that go below GND. In your case, it is possible to design the circuit so that the AD4008 can reliably survive prolonged exposure to 8V on its inputs without additional protection circuitry, but there would certainly still need to be some external protection implemented to protect it against the -12V case.

    Thanks,

    Tyler

  • What Schottky diode do you recommend? 

    Since I did not see clamp diode on the user guide, I did not use one.