LM7322: High bias current?

Per Bengtzohn

Part Number: LM7322
Other Parts Discussed in Thread: LM7321

Hi.

We have noticed a voltage (2.6V) on the IN+ when we apply 5V on the IN- and we got confused by this.
is this expected behaviour and is it safe to run it like this?

BR
// Per

over 5 years ago

0 Ron Michallick over 5 years ago

TI__Guru*** 145116 points

Per//

This is more current than I would expect for an input difference of 5V or (5V-2.6V).

What is the supply voltage for LM7322? Does this happen on more than one device sample?

This is IIB vs. VID sample data for LM7321. At 5V there is only about 1uA. (0.001mA)

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Per,

can C? discharge in time when the supply voltage of LM7322 goes down? Is the 5V applied during power down conditions? This would destroy the -input of LM7322 and could cause a huge leakage current running out of the +input of LM7322. See section 7.1 of datasheet.

Kai

0 Per Bengtzohn over 5 years ago in reply to Ron Michallick

Prodigy 115 points

Hi.

Supply voltage is 11.8V
We notice this behaviour on all we have measured on (3 different ones).

This doesn't show up in simulation but I did find an interesting post here: LM7322: LM7322 Equivalent circuit
(can't paste the link properly) but this indicates higher currents?

BR
// Per

0 Per Bengtzohn over 5 years ago in reply to kai klaas69

Prodigy 115 points

The 5V and the Supply voltage turns on/off simultanious but the 5V has more capacity so this could be on for a longer time.
We have tested this on new circuits as well as we suspected a fault in the circuit but this showed the same behaviour (we applied 5V after supply in this test).

0 Per Bengtzohn over 5 years ago in reply to kai klaas69

Prodigy 115 points

Forgot to mention we will add a series resistor (probably 10k) between 5V and IN- to decrease the current.
This should be enough to not put stress on the internal diodes I hope.

0 kai klaas69 over 5 years ago in reply to Per Bengtzohn

Guru 140200 points

Hi Per,

to attach a file, click the "Insert Code, Attach Files and more..." button. Then click the "Insert File" button in the header.

I think this post answers your question:

e2e.ti.com/.../2562948

I quote: Do the LM7322 NOT use as a comparator!

Kai

0 Per Bengtzohn over 5 years ago in reply to kai klaas69

Prodigy 115 points

Just to clarify the coupling shown is not the one we have but due to confidential information I can't share the full coupling.
It's not used as a comparator... :)
The most important question: Is it safe to run it with this currents going out from IN+?
According to the other post I assume it's ok but would like to get it confirmed.
Also a bit confused about the different Graphs Ron has shown (the one in this post, and a different one in the other post). And why isn't the behaviour shown in simulation? (where we get around 50µA current on IN+)

0 kai klaas69 over 5 years ago in reply to Per Bengtzohn

Guru 140200 points

Sorry, Per, but with the wrong schematic I cannot tell anything useful...

Kai

0 Ron Michallick over 5 years ago in reply to Per Bengtzohn

TI__Guru*** 145116 points

Per,

The 10k resistor between the 5V power source and the input will make a big difference. Without the resistor, there is nothing that limits the current that can flow from input pin to opamp supply pin. There is a diode from inputs to both supply rails.

0 Per Bengtzohn over 5 years ago in reply to Ron Michallick

Prodigy 115 points

Thank you Ron.
So with this we can feel confident that the op-amp will operate in safe conditions?

0 Ron Michallick over 5 years ago in reply to Per Bengtzohn

TI__Guru*** 145116 points

Per,

I don't have all the schematic details, but the worst case I see is 5V input with op amp power at 0V. This is (5V-0.6V)/10k = 0.44mA
I see no harm in this current magnitude.

0 Per Bengtzohn over 5 years ago in reply to Ron Michallick

Prodigy 115 points

Hi Ron.
Thank you for your response.

Just one final question, how come the behaviour isn't shown in simulation, is the simulation model faulty?

0 kai klaas69 over 5 years ago in reply to Per Bengtzohn

Guru 140200 points

Hi Per,

spice models do not model all parameters, usually. That's why always a thorough developement with own measurements must be carried out for each new circuit. Spice models can be very helpful, but they do not tell the whole truth...

Kai

0 Ron Michallick over 5 years ago in reply to kai klaas69

TI__Guru*** 145116 points

Per,

The model are marco models that show the expected performance in "normal" operation. Some models also include simplified effects for other parameters. They are not based on the exact schamic of the device.

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LM7322: High bias current?