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LT1818 sustained differential input

Hi,

in a scenario where I want to use the LT1818 (output stage of scope differential probe) it can happen that the inputs stay at a differential voltage of about 4V for longer (when a wrong attenuation is selected or to high signal is measured). The inputs however always stay within V+ and V-. In front of the inputs are 100Ohm resistors. Amp runs in Gain 11 config (1k feedback). LTspice shows input currents in the range of under 10µA. The datasheet says to avoid this scenario because the amp may draw excessive current in the range of 50mA; however LTspice shows only 7.3mA and 13mA at +/-5V (output is 1Meg loaded). Is the simulation simplified there and doesn't show the excessive current draw problem, or is there no inherent danger of damage in my scenario? 

Thanks,

Simon

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  • Hi Simon:  You can believe the data sheet.  Apparently the fault condition wasn't modelled.  The high current is due to the fact that the opamp is trying to slew, when in fact it is clipped.  It may be possible to limit the supply current at the supplies, for example with 33 Ohm resistors, so that the supply voltages collapse somewhat during the fault condition.  Also, you can put protection diodes across the inputs to reduce the draw, but this will also reduce the maximum slew rate achievable if that's okay.   I believe the real threat to the part is thermal.  If you can use these methods to reduce the power dissipation, that should protect the part.  Don't rely on the opamp to limit to 50mA, as that is just a typical number.

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  • Hi Simon:  You can believe the data sheet.  Apparently the fault condition wasn't modelled.  The high current is due to the fact that the opamp is trying to slew, when in fact it is clipped.  It may be possible to limit the supply current at the supplies, for example with 33 Ohm resistors, so that the supply voltages collapse somewhat during the fault condition.  Also, you can put protection diodes across the inputs to reduce the draw, but this will also reduce the maximum slew rate achievable if that's okay.   I believe the real threat to the part is thermal.  If you can use these methods to reduce the power dissipation, that should protect the part.  Don't rely on the opamp to limit to 50mA, as that is just a typical number.

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