Low-Noise Recording EPC9 Manual 80
11. Low-Noise Recording
The EPC9 amplifier has a particularly low background noise level. The noise level is
in fact low enough that in most experimental situations it can be neglected in view of
other background noise sources that make larger contributions to the total. As we
consider these other sources, first let us make it clear that in this section we are
concerned with random noise, which is fundamentally due to the thermal motion of
electrons and ions; we assume that any user who is interested in low-noise recording
has shielded and grounded his setup sufficiently well to take care of any
synchronous noise due to line-frequency pickup, computer power supplies, TV
cameras, etc. Synchronous noise can be readily identified as stationary features on an
oscilloscope trace when the oscilloscope is triggered by the appropriate signal
source, for example, line-frequency triggering. Grounding and shielding is discussed
in Chapter 10. Using the Patch Clamp on page 75.
The Noise feature of the EPC9 makes it easy for the user to identify important noise
sources. When the Noise button is selected, the I-mon display shows the rms noise
current present in the Current Monitor signal in the frequency band selected by Filter
2 (3 kHz is recommended). For noise measurements, the standard setting of the Gain
control is 50 mV/pA. With the probe placed in a shielded enclosure and with
nothing connected to the input, the Noise reading is usually 80-100 fA. If you get a
reading higher than this, try varying the C-fast control. If you have a noisy stimulus
source connected to Stim. In, the induced current noise will vary with C-fast, with a
minimum occurring with the control set to 1-2 pF, i.e., for best cancellation.
Starting from the intrinsic noise reading of 80-100 fA, one observes increments in the
noise level when the holder and pipette are installed and when an actual recording is
made. By analyzing these increments, you can see where there is the most room for
improvement in your technique. Under the best conditions (i.e., with a clean holder,
an aluminosilicate pipette, etc.), we have observed the noise reading increase to
about 130 fA when the holder and pipette are present, and 160 fA when the pipette
tip is in the bath, sealed on a cell. These are rms current values, which means that
they are equal to the standard deviation of the fluctuating current.
Since the noise sources in the patch clamp amplifier, pipette holder, pipette and
patch membrane are statistically independent, their contributions to the total noise
do not add linearly; instead, their variances (the squares of the standard deviations)
add. This means that the rms reading on the EPC9's display represents the square
root of the sum of the squares of the rms currents from each source. Taking this into
account, one can calculate the relative contributions from the amplifier, pipette
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