OPA828: Basic question about imput impedance

Takumi Suzuki1

Mastermind 8510 points

Part Number: OPA828

Hi,

Let me ask you some basic questions about input impedance.

There describes the value in the datasheet 5 page, 1Tohm.

Q1) What is the meaning of "Differential"(ZID) and "common"(ZICM)?

Q2) How do you measure the value?

Thank you for your support.

Best Regards,

Takumi

over 2 years ago

+1 kai klaas69 over 2 years ago

Guru 140200 points

Hi Takumi,

"differential" means between the two inputs and "common-mode" means from each input to ground.

See also this blog:

https://e2e.ti.com/blogs_/archives/b/thesignal/posts/input-capacitance-common-mode-differential-huh

Kai

+1 Thomas Kuehl over 2 years ago in reply to kai klaas69

TI__Guru* 93105 points

Suzuki-san,

The input impedance measurements are dynamic tests made as part of the op amp's parametric characterization. Specific Differential and Common-mode DC input voltage changes are applied to the op amp test circuit and the corresponding input bias current changes are measured. The resistance portion of the input impedance is found from calculating ΔVi/ΔIb. Additionally, when the product is in design the Differential and Common-mode impedances are first determined from high-level simulations to assure that it is in the expected range.

Thanks to Kai for providing the link to Bruce's blog on op amp input impedances.

Regards, Thomas

Precision Amplifiers Applications Engineering

0 Takumi Suzuki1 over 2 years ago in reply to Thomas Kuehl

Mastermind 8510 points

Kai-san, Thomas-san

Thank you very much for your kind answer !

I understood well.

Regards,

Takumi

Amplifiers

Amplifiers forum

OPA828: Basic question about imput impedance