THS4631: input bias current

user1939811

Prodigy 60 points

Part Number: THS4631

Hello Support team,

I want to know more detail regarding below post.

THS4631 input bias current - Amplifiers forum - Amplifiers - TI E2E support forums

I've built a circuit similar to the one in the previous question.

I'm attaching a document in which I checked the operation.Inquiry_THS4631_input-bias-current.pdf

When the frequency of the AC voltage applied to the THS4631 was increased (1MHz),it was confirmed that a large DC offset voltage was generated.

Is the cause of this really an increase in the net current, the AC component?

Why does an increase in the net current of the AC component cause an increase in the DC offset voltage?

If net-current is an AC component, I imagine that net-current swings to the same magnitude on the positive and negative sides, centered at zero.

I suspect that increasing the net current (AC) does not increase the DC offset voltage when it flows into the outer impedance of the THS4631.

Best Regards,

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

1M at the +input of THS4631 is abnormely high for a 325MHz OPAmp. You should decrease the impedance by at least a factor of 100, better more. Use a big bipolar electrolytic AC coupling cap to get the same corner frequency. Or use a back-to-back circuit of two unipolar electrolytic caps.

Kai

0 user1939811 over 3 years ago in reply to kai klaas69

Prodigy 60 points

Kai,

Thank you for the suggestion of countermeasures.

It is true that if the resistor constant is reduced, the DC offset voltage can be reduced even if the DC current becomes large.
I would like to try it.

However, I still do not understand why the DC offset voltage increases as the net current, the AC component, increases..

0 kai klaas69 over 3 years ago in reply to user1939811

Guru 140200 points

See also this thread:

The only remedy is to keep the source resistance low.

Kai

0 user1939811 over 3 years ago in reply to kai klaas69

Prodigy 60 points

Kai,

Thanks for your reply.

Yes, I guess the solution is to keep the source resistance low.

By the way, when an AC voltage with a higher frequency is applied to the THS4631, does the bias current of the DC component also increase?
It is not only the net current of the AC component.

And isn't this increased DC bias current flowing through the large source resistance, creating a large DC offset voltage?

I want to know if the increased current is not only for AC but also for DC.

Sorry for the repetition.

0 kai klaas69 over 3 years ago in reply to user1939811

Guru 140200 points

Unfortunately, I'm not an employee of TI. So I cannot fully answer your question. But keep in mind that the THS4631 is part of an older chip generation. Newer chips won't have this restriction :-)

Kai

0 user1939811 over 3 years ago in reply to kai klaas69

Prodigy 60 points

Kai,

Thank you for your answer.

I am glad to know that the new generation of chips has the potential to reduce such currents.

0 SimaJalaleddine over 3 years ago in reply to user1939811

TI__Mastermind 22400 points

Hello,

We can suggest a new generation of a JEFT Input amplifier. What are your key specifications for your application?

Thank you,
Sima

0 Hirokazu Takahashi over 3 years ago in reply to SimaJalaleddine

TI__Expert 8420 points

Hello Sima-san

I'm communicate with the person post at first in this thread.

I want to discuss about your suggestion by e-mail directly.

Best Regards,

Hirokazu Takahashi