HEKA Elektronik EPC 10 USB, Hardware Manual, Page: 37 / 81

5.1 EPC 10 USB Window 31
In the upper box, the total C-fast value is displayed. τ -fast determines the time constant of C-fast (up to 8 µ s).
The value of τ -fast may be adjusted by dragging the mouse, or typing, or automatically by selecting the Auto
function.
<> : This will select both amplitude and time constant of C-fast . Values for each may be changed manually
depending on whether the mouse is moved horizontally or vertically.
Figure 5.9: C-Fast compensation
Auto C-fast: Selection of this button performs an automatic compensation of C-fast and τ -fast . The procedure
uses a routine that applies a number of small pulses (5 mV), averages the resulting currents and fits an exponential to
deduce the capacitance compensation values required to cancel the current. During this procedure some parameters
are changed temporarily:
Rs Comp is turned off, the stimulation is interrupted and the Gain is internally set to
an appropriate value (0.2, 2 or 50 mV/pA, depending on the effective Gain range). The values are updated in the
respective displays. If the automatic compensation routine fails to converge to ideal adjustment, the small box in
the
Auto button (E) will be darkened to indicate the error. Auto compensation works best with Sylgard
—
-coated
pipettes in the cell-attached configuration with C-slow turned off. The software allows for two ways of making the
compensation. The fastest (default setting) is to use a lookup table for setting C-fast (
EPC 10
menu
→
C-fast
Mode
→
Use Lookup Table
). Here, the lookup table created during the last calibration will be used. The slower
method is
Computed
(same menu) where an iterative compensation is performed.
C-slow: This is used to cancel slow capacitive currents that charge the cell membrane in the whole-cell con-
figuration. The 30, 100 and 1000 pF ranges actually allow capacitance values to be compensated in the ranges
of 0.12-30 pF, 0.4-100 pF and 4-1000 pF, respectively. The adjustment range is also limited by the program in
order to make the time constant
R-series * C-slow greater than 5 µs to prevent oscillations. The C-slow value
may be adjusted manually by dragging the mouse, or typing, or automatically by selecting the Auto function.
Auto-compensation will adjust C-slow as well as R-series .
C-slow Range: Selects the range for slow capacitance compensation:
ˆ
Off: Turns cancellation off.
ˆ
30 pF: Small cells.
ˆ
100 pF: Small and medium-sized cells.
ˆ
1000 pF: Large cells (low and intermediate gain range only).
Slow capacitance cancellation ranges (30/100/1000 pF) can be set to any desired value. However, in the high Gain
range (50 GΩ resistor) the 1000 pF range will not operate. If the Gain is set to value higher than 20 mV/pA,
while the 1000 pF range is selected, the capacitance cancellation range will automatically be reduced to the highest
possible setting (100 pF). Similarly, since the current clamp mode is not possible in the high
Gain range (50-
2000 mV/pA), the Gain will be reduced appropriately when selecting C-Clamp mode from high Gain range.
The program will prevent oscillations by limiting compensation according to the selected range and the effective
capacitance values (
R-series * C-slow > 5 µs ). Selecting the 1000 pF range will limit the Gain adjustment to a
maximum of 20 mV/pA.
Note: If you are using the Cap. Track feature (
Windows -> Configuration -> Display -> Show
Cap. Track
) to measure small capacitance changes, it is advantageous to use a higher C-slow range
when possible. Counter-intuitive as it may seem, the digital control of the C-slow circuitry is more
precise in higher ranges, resulting in higher resolution, consistent with the Gain range and capacitance.
Figure 5.10: C-slow Compensation
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