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Input Protection for INA116

Other Parts Discussed in Thread: INA116

Hi guys, I'm building an electrometer that measures the difference voltage across a ~100pF capacitor, and needs to avoid significant leakage for at least 30 seconds. Therefore the input/leakage specs are as follows:

Input impedance > 1TΩ

Input bias current < 100fA

Therefore, I'm looking at the INA116.

The INA116 says it has "INPUT OVER-VOLTAGE PROTECTION: ±40V". But it doesn't say anything about what happens if this is exceeded, or how the protection works. Are there protection diodes to shunt current away from the inputs, or does the device just blow out suddenly? If I put say 100kΩ or 1MΩ in series with the inputs, will that be sufficient against HBM type ESDs?

  • Just to clarify a little, I did see the other post suggesting 10kΩ and 47pF. The reason I'm bringing the matter up again is:
    1. It doesn't clarify what type of protection the INA116 actually has.
    2. It's not too clear what "reasonable precautions to minimize ESD" actually means. The electrometer I'm building is intended for use in a classroom with a carpeted floor, and people will be touching the inputs.
    3. I don't want to add much capacitance to the system (certainly not >10pF), since that will interfere with the intended use of the electrometer.
  • Hello Colin,

    Please understand that there is a difference between ESD and EOS. Here is a link to some videos on the topics:

    www.ti.com/precisionlabs

    For the INA116, the +/-40V input protection circuitry is there to guard against EOS which thereby protects the ESD structures. It does this by limiting the input current based on the input voltage. Please refer to the "Input Over-Voltage V/I Characteristics" plot on page 5 of the data sheet. As far as what happens outside this range, we neither test nor characterize the device beyond this specification.

    Concerning the ESD structures, how they work exactly is proprietary. Some description, however, is discussed in the following post:

    e2e.ti.com/.../578825

    Typically ESD robustness is a system-level solution and ideally does not depend on the devices' ESD performance themselves, though it sounds like your application is a special case (classroom with carpeted floor). The ESD specifications for devices are mainly there for manufacturing and handling purposes while building a system. I interpret Bruce's comment about "reasonable precautions to minimize ESD" as adding the 10kohm/47pF RC on the input and ensuring that the equipment is handled by people with appropriate ESD protection (e.g. ESD wrist straps, ESD mats on table and/or floor, etc.).

    Hope this helps!
  • OK, so after reading your post and looking at the datasheet page 5, my conclusions are:

    - The INA116 has ESD diodes that conduct when the inputs go past the supply rails
    - The EOS circuit limits the input current through the ESD diodes to about 2mA as long as the input is <40V
    - Therefore, if I put protection resistors in front of the input, I could go to much higher voltages as long as I limit the current to <2mA. For example, I could put 1kV on an input as long as there was a series resistor >500kΩ.

    Is this a reasonable conclusion?
  • Hello Colin,

    Your understanding of the ESD diodes and EOS circuit is correct, but I do not recommend placing such large series resistors on the inputs. Perhaps a better approach is to use ferrite beads instead of resistors on the inputs. Ferrite beads have low impedance at low frequency but go to high impedance when presented with a large voltage transient.
  • OK, that makes sense. I'll do some experimentation and figure out the best approach for my setup.

    Thanks so much for your help!

    Best,
    Colin