LMP7721: We are looking for the lowest input capacitance (differential) in your product line

John kimura

Prodigy 70 points

Part Number: LMP7721

Also very low bias current.

Goal to measure voltage across a sensing capacitor, displacement of charge.

1 pA bias current or less

Cin 1 pF differential

very low noise

We wish to use this methodology to reduce the Cin

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/600269/opa659-how-to-reduce-the-input-capacitances-via-power-supply-bootstrapping

And would like recommendations on a Neutralization circuit of the Cin.

Goal is for <100 femto farad

Thank you

over 2 years ago

0 kai klaas69 over 2 years ago

Guru 140200 points

Hi John,

and common mode input capacitance doesn't play any role? Can you show a schematic of your application?

Kai

0 John kimura over 2 years ago in reply to kai klaas69

Prodigy 70 points

we need ultra low Cin differential capacitance bc probe C is ~100pF

so the Cin dif needs to be made to be by neutralization <500 fF

0 kai klaas69 over 2 years ago in reply to John kimura

Guru 140200 points

Hi John,

hhm, I don't understand what you are saying. In the schematic above due to the operation of feedback loop the -input of OPAmp is sitting on the same potential as the +input. So the differential capacitance doesn't become charged at all and because of this does not dampen the input signal?

The common mode input impedance, on the other hand, plays a much more relevant role.

Kai

0 kai klaas69 over 2 years ago in reply to kai klaas69

Guru 140200 points

john_lmp7721.TSC

Kai

Amplifiers

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LMP7721: We are looking for the lowest input capacitance (differential) in your product line