Post on 18-Jan-2016
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