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ADA4851-1 : Input current and protection diodes

LPerbet
on Mar 11, 2019

Hello,

I'll be using an ADA4851-1 (supplied +5V) in a high-voltage (up to 60V during a small-time during which the OA isn't supplied, then max 38V in nominal operation) sensing application, with a bridge divider (1:11 with a 100kOhms and a 10kOhms which middle node is linked to the positive input of the OA used as a voltage follower.

I have two questions regarding my design :

-First, when the amp is not supplied (battery not connected to the DCDC converter supplying the OA in 5V), but that there is voltage on the sensed node, and given that I use a 5V Zener diode between the positive input and the GND, will the internal ESD diode of the ADA4851 send the (60V-5V)/100k = 550µA to the 5V node, thus potentially powering the 5V electronics ? Will the AO break down ? Or will it handle this problem properly ?

-Secondly, during nominal operation (ADA supplied with 5V), in the case where the "bottom" resistor of the bridge divider breaks in open-circuit, the Zener and the ESD diodes will limit the V+ voltage at 5V, but will the ADA support the (38V-5V) / 100k = 330µA input current ? Unfortunately there is no specification for the maximum allowable input current in the datasheet.

I hope that you will be able to help me regarding these two issues.

Best regards,

-Lucas PERBET-

  • 0
    •  Analog Employees 
    on Mar 18, 2019 3:49 AM over 1 year ago

    Hi Lucas,

    You are correct, the failure mechanism is not the over-voltage per se, but the current that the over-voltage current causes. Limiting the current flowing to the input by less than 5mA is the rule of of thumb for most of the op amps. No damage will be done but over-time, but overstressing the input can change its parameters such as bias current and offset voltage. Also, the things to consider in adding external protection for over-voltage is the value of the capacitance of your diode used, this may limit the frequency response of your circuit especially for hign speed application. Another thing to look to is the current leakage this should be minimized or taken into account when choosing the limiting resistor.

    Regards,
    Goz