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THS4531: Input over voltage protection

Prodigy 150 points

Replies: 6

Views: 91

Part Number: THS4531

Hello all,

I have a requirement for my analog inputs to protect it against overvoltage up to 60V.

The continuous input current in THS4531 is 10mA so can I put a series resistor to limit the current in 60V overvoltages?

The required gain is 0.5V/V

So can I put these resistors

Rg=7.5k (0.5W)
Rf=3.75k

If I want to use a smaller resistor in input can I use :

Rg=15k (0.25W)
Rf=7.5k

Rf is some kind of big then do you think if it is acceptable?

Best regards,




6 Replies

  • Hello Mehri,

    While this may help with your input current limitation, you will still need to be wary of exceeding the input pin ratings of (VS–) – 0.7 to (VS+) + 0.7.

    Best,

    Hasan Babiker

  • In reply to Hasan Babiker31:

    Dear Hasan,

    Then what is the purpose of internal protection diode?

    It does not make sense to me. If we limit their current they should not burn meanwhile we are safe.

    regards,

  • Hi Mehri,

    can you show a schematic?

    Kai

  • In reply to Mehran:

    Morning Mehri

    The first set of R's should be adequate to protect the current into the input pins for 60V input. That spec is set by the ESD steering diodes from each pin up and down to the supplies. You do need to be a little careful about losing phase margin due to input C and the feedback network, I tried to explain all that in this designing attenuators article, 

    https://www.planetanalog.com/are-inverting-attenuators-with-voltage-feedback-op-amps-inherently-unstable-and-why-might-you-want-to-apply-them-insight-11/

    Michael Steffes

  • In reply to Michael Steffes:

    Thanks, Michael,

    Can I always consider that there is a protection diode on the inputs when I see the absolute maximum input voltage as below?

    Vin max = VS(+) + 0.2 V or 0.7V and VS(-) - 0.2 V or 0.7V

    I think this 0.2V or 0.7V implies on the presence of diode, isn't it?

    Best regards,
    Mehri

  • In reply to Mehran:

    Hello Mehri,

    This is correct, the 0.7V limit on the abs max implies diodes on the pins. I would not recommend thinking of these as protection diodes though as these are diodes aren't designed to handle continuous currents greater than 10mA (as shown in the abs max). The diodes are designed for ESD signals, typically meant for higher currents in the range microseconds or less.

    Best,

    Hasan Babiker

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