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# LMP7721 input impedance

Other Parts Discussed in Thread: LMP7721, LMP7711

What is the input impedance of lmp7721 or lmp7711?

I cannot find it in the datasheet.

Gabriel

• Hello Gabriel,

"Input Impedance" is not very useful spec for CMOS input devices.

It is going to be very high....hundreds of Gigohms. Because of the femtoamp currents, the resistive part of the impedance will always be changing slightly due to temperature.

2.5V/3fA = 833e12 ohms...or 833 Tera-Ohms.

The "capacitive" part of the impedance generally dominates for anything above DC. That would be 11pF for the LMP7721 and about 12pF for the LMP771x family.

The LMP7721 is based off the LMP7711 architecture  - so the input impedance behavior will be similar. The LMP7721 has a bootstrapped ESD protection stage, so the input capacitance is slightly lower due to the reduction of  ESD structure capacitance.

So... 833 Tera-Ohms//12pF

Regards,

• Thank you very much.

But please explain the 2.5V...

Gabriel

• Hi Gabriel Koutilellis,

Ohms law(http://en.wikipedia.org/wiki/Ohm's_law) states thtat resistance(impedance) = potential difference/current

When V+ = 2.5 V, input bias current is typically 3fA as given in data sheet. So input impedance=2.5/3fA =8.33*10^(13) ohm

• ok, I didn't think of the datasheet...

So if V+=2.5V and V-=-2.5V in a bipolar power supplied setup then it is 5V/3fA = 1.66*10^(15) Ohm

I believe for your example you misplaced the comma, it is 8.33*10^(14) Ohm

Thanks again

• Hi Gabriel Koutilellis,

It is not so. Input bias current is not the same for positive and negative terminals. And it can also be negative. Usually input impedance is not referred in data sheet. It is different for both terminals. It is not much useful parameter as it is always very very high.

Sorry. It is 8.33*10^(14) Ohm only.

• Hello Gabriel,

2.5V is the mid-common mode voltage for a single supply. The single supply voltages are referenced to the V- pin.

The datasheet is written for single supplies. You are using split supply - so your "0V" is actually +2.5V single supply. Confusing? Yes. Welcome to our world..:^)

The bias current is specified at mid-supply (+2.5V, or "0V" on a split supply).

So the "equivalent" resistance looking into the input of the amp is:

2.5V / 3e-15 = 833.33e-12 or 833 Tera-Ohms.

As Deepak mentioned, the bias current is ±, so it does not always look like a "resistance". It is better to think of it as a tiny leakage current - which it is - and the current can be into or out of the input.

Because the "bias current" of a CMOS device is actually the miss-match in leakages of the ESD structures (there is usually just aA of actual gate current), the bias currents will be the lowest in the center of the common mode range, and worse near the rails (the 7721 avoids this effect near V- by bootstraping the diodes to within millivolts of the negative rail).

Regards,

• Not confusing at all...

Thank you all.