OPA2375: Input Impedance

Corbett Kaniff

Part Number: OPA2375
Other Parts Discussed in Thread: TLV6742,

Hello, I received the following question from a customer:

"I wanted TI to confirm and check the Differential Input impedance on the OPA2375 and TLV6742 datasheets. The input impedance for the Differential mode is 10MOhms and the Common mode is 10GOhm. 10MOhm for the differential input impedance seems low for the op-amps. Can this be confirmed to be true based on the datasheet?

Additionally, with the max 1mV max offset voltage / 10Mohm differential input impedance would have 0.1nA of leakage current. Would this leakage current be correct?"

Thanks!

over 2 years ago

0 Caro Walter over 2 years ago

TI__Mastermind 18937 points

Hey Corbett,

The input impedance is correct. I have found the following resources:

Input bias current vs input impedance e2e: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/356932/input-bias-current-vs-input-impedance

& the following blogs written by Bruce Trump:

Input Capacitance—common-mode?...differential?… huh?: https://e2e.ti.com/blogs_/archives/b/thesignal/posts/input-capacitance-common-mode-differential-huh

“I Need High Input Impedance!”: https://e2e.ti.com/blogs_/archives/b/thesignal/posts/i-need-high-input-impedance

Input impedance is not a very useful spec for CMOS input devices. The resistive part is always changing due to temperature.

It is possible to have "leakage current" as high as 0.1nA, it is called bias current in TI datasheets, here it is referenced in the datasheet:

All the best,
Carolina

0 kai klaas69 over 2 years ago

Guru 140200 points

Hi Corbett,

the footnote (4) in the absolute maximum ratings seems to indicate the presence of a protection scheme against too high differential inputs voltages, similar to what is used in mux-friendly OPAmps:

sbot040c.pdf

This can be responsible for the rather low differential input resistance of these two OPAmps.

The typical input current would be 0.15mV / 10M = 15pA which is close to the specified typical input currents. And 1mV / 10M = 100pA would not violate the datasheet specifications as for the input currents no maximum specification is given in the datasheet. I think this is the compromise you have to live with as result of the extremely low noise design of these OPAmps.

Kai

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OPA2375: Input Impedance