AD7124-4 input protection

Hi

I'm using AD7124-4 in my design. Input to this device is the output of an instrumentation amplifier whose supply is 31V and -5V. By any means if my instrumentation amplifier goes to saturation there is a possibility that ADC can get negative input / Very high voltage of the order of 31V at input.

1. Whether AD7124-4 can internally handle negative -4V input.

2. How can I protect my AD7124-4 from 31V/-4V saturation voltage of instrumentation amplifier.

Regards

Nidhi P Shetty

  • 0
    •  Analog Employees 
    on Apr 28, 2020 4:12 AM

    Hi,

    1. No, AD7124 has an absolute maximum ratings of -0.3V to AVDD+0.3V across the analog input voltage to AVSS. And AVSS can only go low as -1.98V.

    2. We usually suggest to add external Schottky diodes and current limiting resistor to the input that is likely to have an overvoltage or exceed the absolute maximum ratings. Connecting external schottky diodes from each analog input to AVDD and GND ensures that the external diode turns on before the ADC’s internal diodes so that the overvoltage is directed away from the ADC. The schottky diode must have a turn-on voltage of 300mV. It will clamp applied voltages to within ~0.3V of the supply so the majority of the current will be diverted via the external diodes (which can carry higher current) and not through the internal ESD protection diodes.

    Thanks,

    Jellenie

  • Hi Jellenie

    We are using BAT54S,235 so that we can protect from over voltage in both directions. Please let me know if any concerns.

    Regards

    Nidhi P Shetty

  • Hi Jellenie

    We are using AD7124-4 in unbuffered mode to achieve the minimum input voltage range currently.

    Since, the output voltage swing in my application is 35mV to 1.2V, we are planning to use gain of 2 in AD7124-4 to utilize the maximum dynamic range of the ADC.

    Is there any concern in doing so.

    As shown in attached schematics, for Ain0 and Ain1 pin protection clamping diodes are added between 0V and 3.3V.

    In gain=2 mode max. allowed voltage at analog input pin is 1.25V. Will there be any concern in doing so?

    Whether my ADC is still protected as per it's absolute maximum specification which is AVSS-0.3 to AVDD+0.3V (-0.3V to 3.6V)?

    Please help me out.

    Regards
    Nidhi P Shetty

  • 0
    •  Analog Employees 
    on Oct 16, 2020 3:27 AM in reply to nidhi.shetty@iwavesystems.com

    Hi,

    First, when the gain is greater than 1, the analog input buffers are automatically enabled. The PGA placed in front of the input buffers is rail-to -rail; thus, in this case, the absolute input voltage includes the range from AVSS − 50mV to AVDD + 50mV.

    Then at a gain=2, you are correct the allowable differential input voltage range is +/-1.25V. So your input range is within specification.

    In terms of the input protection, I think it is fine if you wanted to limit the analog input voltage at 3.3V as these diodes will only clamp if you have overvoltage meaning higher than the threshold. Might also worth trying to perform noise test when these diodes are connected across the inputs, just to be sure that it doesn't contribute errors in your measurements upon normal operation.  

    Thanks,

    Jellenie

  • Hi Jellenie

    Thanks for the confirmation.

    Regards

    Nidhi P Shetty