TL074: input impedance when no power

Hey Zach,

Additionally you may want to explore some shutdown versions of op amps. The function is slightly different than high impedance when not powered, but if there is power applied and shutdown is active your inputs and outputs are high impedance. I've provided a list of all our quad amplifiers with shutdown.

https://www.ti.com/amplifier-circuit/op-amps/general-purpose/products.html#p480=4;4&p2192=Shutdown 

Best,
Jerry

1. Most opamps (CMOS or others) have clamping diodes to V_CC. The low input bias current is what allows CMOS opamps to have a power-saving mode without having to disconnect the inputs.

2. The purpose of the clamping diodes is to protect against ESD.

3. When there are clamping diodes, the absolute maximum ratings say that the input voltage must not excced V_CC + 0.? V. Opamp with OVP would allow higher voltages.

Hi Zach,

to believe that everything is fine only because the input impedance is very high and no current can flow is not a good approach. The higher ohmic the input, the easier to destroy it by ESD or any other overvoltage.

The usual way to handle this is to use an external diode clamp to the supply rails and to limit the current through the diode clamp by a suited series resistor. With a 10k resistor and a 10V input voltage only 1mA is flowing through the diode clamp into the rails which can easily be absorbed by the TL074 itself because it draws a supply current of 5.6mA totally. So the supply voltage will only rise to about 1...2V until the TL074 starts to draw 1mA.

And ESD which runs through the 10k resistor and the diode clamp onto the supply rails is usually absorbed by the decoupling caps of TL074.

Kai

Zach,

The input pins will be high impedance when the part is in shutdown, but you are still not able to exceed the Vcc of the op amp. This will then forward bias the diodes to V+.

Shutdown was proposed as an alternative approach if you want an input signal but no output while the supply is still present.

Best,
Jerry