HEKA Elektronik EPC 10 USB, Hardware Manual

Page: 29 / 81

4.1 Testing the EPC 10 USB with the Model Circuit 23
In a similar way as you explored the C-slow compensation, you could now have a closer look into the Rs com-
pensation. Turn the compensation on by setting an appropriate compensation speed, 2, 10 or 100 µs (9), and
gradually increase the percentage of compensation from 0 to 95% by clicking and dragging the mouse upwards
(10). As soon as you are overcompensating the series resistance typical oscillations will occur in the
Oscilloscope .
Series Resistance Compensation is a more complicated topic and is therefore treated in more detail in chapter 7.1
on page 43.
The steps listed above can be automatically executed by clicking the Whole-Cell button or pressing the key ”3” on
the numerical keypad ( Amplifier Window has to be active). This will execute the following protocol (see Protocol
Editor ) that sets the right Gain and does a C-slow compensation with reasonable values:
3 : WHOLE-CELL
E Mode 3 ; Whole Cell
E Gain: 11 ; 10 mV/pA
E CSlow: 50.00pF ;
E RSeries: 20.0MOhm ;
E AutoCSlow: ;
E AutoCSlow: ;
E Beep ;
4.1.5 ”Whole-Cell” - Current Clamp Recording
If C-slow has been compensated so far, switch into the current clamp mode by selecting C-Clamp from the
Recording Mode pop up. This automatically selects the voltage monitor (Vmon) as the active channel displayed
in the Oscilloscope . Note, that the unit of the Test Pulse amplitude changes from ”mV” to ”pA” as soon as you
switch from voltage clamp (VC) into current clamp (CC) mode.
Patchmaster
uses two different amplitudes for
VC and CC modes, therefore the Test Pulse is set to ”0 pA” initially. Now you need to inject current into the
circuitry, 100 pA should be a reasonable value. The current injection will charge the ”membrane” of the ”model
cell” at a time constant
τ = R
m
∗
X
C
m
= 500 M Ω
∗
22 pF = 11 ms
to a final value of
V
max
= R
m
∗
X
I = 500 M Ω ∗ 100 pA = 50 mV
Due to the slower time constant compared with voltage clamp conditions it takes much longer to reach Vmax ,
therefore you should increase the duration of the Test Pulse to a more appropriate value of 100 ms.
Note: In contrast to voltage clamp conditions, were τ is proportional to the access- or series resistance
( R
s
) of the pipette, in current clamp experiments τ depends on the membrane resistance ( R
m
).
The normal setting of the Oscilloscope scales the voltage monitor at 250 mV per division. You should therefore
increase the gain of the Oscilloscope to 16 which scales the display to be 16 mV per division. Please remember
that the Oscilloscope gain is different from the amplifier Gain and only scales the display, not the acquisition of
data. Using a very high Oscilloscope gain together with a low amplifier Gain allows you to determine the digital
resolution of the analog-to-digital converter.
Note: The 500 M Ω setting of the model circuit is not a good method for testing the fast clamp speed
of the
EPC 10 USB
due to the long time constant of 11 ms which the amplifier can easily follow. If
you want to have a better estimation of the amplifier’s speed under current clamp conditions you should
do the same test as above with the
10 M Ω setting. This results in a much shorter ”membrane” time
constant of only 60 µs .
http://www.heka.com