Operating Modes EPC9 Manual 57
stable with pipette and series resistance down to 1 MΩ. The large capacitance will
however produce an extra load on small cells, slowing the time course of membrane
potential changes.
Test Mode
This mode is only available in E9Sreen. It is very much like the voltage-clamp mode
(see figure below) except that the Test Integrator signal is used to inject small test
currents into the I-V converter input. Internal stimulation can be used for this
purpose by connecting DA 2 (Test Output) to the Test Input.
The scaling of the current that is injected is fixed at approximately -100 pA per volt
applied at the Test Input. This scaling is independent of the Stim. scaling setting. The
exact scaling is measured in the calibration program and is stored in the “Scale.epc”
file. The signal is not DC-coupled, but square waves of >20 Hz, pulses >30 ms in
duration, and sine waves at >1 Hz are reproduced faithfully.
Note: The test integrator inverts the externally-supplied signal. For accurate frequency
response and to avoid degrading the sensitivity of the I-V converter, the test currents
are injected through a capacitor (rather than a resistor) into the probe's input
terminal. The applied signal is integrated by an auto-zeroing integrator before being
applied to the capacitor. The integrator may require several seconds of recovery time
when a signal with a substantial DC component is applied to the input.
The Test mode is particularly useful for measuring the step response of the entire
recording system, which is a necessary step in high-resolution channel analysis, as
described by Colquhoun and Sigworth (1995). The current-injection circuitry has a
useful bandwidth greater than 100 kHz in the high range of gain settings and can be
used for quantitative measurement of the step response.
V-membrane
Ref. Pot.
V
0
-Offset
Pipette
Current
Monitor
Test. In
Bath Electrode
IV Converter
Test
Integrator
Σ
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