NOISE PERFORMANCE

PATCH VOLTAGE CLAMP

BILAYER VOLTAGE CLAMP

Rf

VO

Re

ee

-

+

Cp

Vm

Op Amp

Equivalent circuit analysis

2

0; 2 ; ; (1 )

; 1

O e m e mp m p m e m m e

f e e

O ee e p p m

f e

V e V e VsC V s j f sC V e V V s

R R R

V sR C sC V

R s

2 2 2 2 2O

2f e

e pep

VR 1+sτ

4π f e C=S =

Equivalent circuit analysis

Over bandwidths (B) important to patch clamping (<100 kHz - 1 MHz,i.e. < 1/2πтe),

we approximate Sep2

2

12

/2 2 2 2 2

2 2 2 2 33

4

(4 / 3)

amp Hz

amps rms

ep e p

ep e p

S f e C

and

i c e C B

• For pipette resistance 2 MΩ and a patch capacitance of 300 fF, 1/2πтe = 265 kHz and “Re-Cp” noise in a 10 kHz bandwidth (8 pole Bessel filter) is

nearly 0.3 pA rms

• On the other hand for a 10 MΩ and Cp = 10 fF, 1/2πтe = 1.6 MHz and Re-Cp noise in a 10 kHz bandwidth is

< 20 fA rms

Bilayer Analysisfor Andy Hibbs

Cp is the capacitance of the bilayer

Re is the access resistance (Raccess), to the bilayer

= convergence resistance plus small contribution from the bath and electrodes ee is the noise (in amps rms) of Re.

For a 200 μm diameter bilayer Raccess = 5 kΩ Cp = 314 pF. тe =1.6 μs. Bandwidth of 10 kHz (8 pole Bessel filter) giverms noise = 25 pA.

Noise improves as bilayer becomes smaller

Raccess varies linearly with radiusbut

Capacitance varies as a2

• 100 μm diameter bilayer Raccess = 10 kΩ bilayer capacitance ~78 pF.

Noise in a 10 kHz bandwidth is 8.8 pA rms.

• 10 μm diameter bilayer Raccess ↓ 100 kΩ, but

capacitance ↓ 0.8 pF.

• Noise in a 10 kHz bandwidth is then 0.29 pA rms

Conclusion

Wideband Low Noise Recordingis best done with

Patch Pipette

Unless one can make a stable gigasealed bilayer in a 1 µm aperture