OPA991: IN+/IN- 30V when V+/V- =0V

Hi Koji,

this is usually accomplished by using a protection circuit at the input of OPAmp, a diode clamp to the supply rails with a current limiting resistor and TVS from the supply rails to signal ground, followed by an additional voltage divider.

Kai

Hello Ikeda-san,

There are three methods. The first two methods are high impedance.   

1) Find an op amp that allows input voltage without device power.

2) Add a MOSFET to input path that turns off when supply power is off.

3) Redirect input current to supply; supply absorbs this current.

Kai-san, Ron-san,

Thanks you for your support. But, I'm not clear.

Could you provide suitable OPAMP for #1?

Could you provide reference schematic for #2? 

Could you explain more for #3? This is irregular case for customer, they can't bias power on VDD in this case.

Thanks,

Koji Ikeda

Hi Koji,

examples can be found here

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1168812/tlv1701-device-keeps-burning

and here

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1097514/tlv9002-can-we-connect-ct-to-tlv9002

Kai

Kai-san,

Thank you for your support. But, customer and I don't have time to check these post fully. Could you provide recommended schematic for OPA991?

And could you provide OPAMP solution without any protection external circuit?

Thanks,

Koji Ikeda

Koji-san,

The solution also needs to consider what is expected in the normal state when op amp power is supplied.  What is the input source? What does to op amp do when power is on?

LM358B is an op amp that allows (positive) input voltage with op amp power off 

Ron-san,

This is irregular case, they need to support no failure even if irregular case. So they need +/-30V bias to INPUT even if no Vs. How about negative bias on LM358B?

Thanks,

Koji Ikeda

Hi Koji,

the simplest approach is to disconnect the input signal from the OPAmp as long as the supply voltage is down. The supply voltage of OPAmp itself can be used as a control signal for this relay: "Full supply voltage" -> "connect", "no full supply voltage" -> "disconnect".

Kai

Kai-san,

Your explanation is not clear for me, could you share schematic? And if you have any OPAMP which can survive this usage without external circuit, could you share?

Thanks,

Koji Ikeda

Koji,

I understand,  bi-polar input voltage.

OPA991 and most other op amps won't be destroyed.