HEKA Elektronik EPC 10 USB, Hardware Manual, Page: 65 / 81

11. Low Noise Recording
The
EPC 10 USB
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, 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 8 on page 49.
The noise feature of the
EPC 10 USB
makes it easy for the user to identify important noise sources. When
the Noise radio button is selected in the
EPC 10 USB
dialog ( Amplifier window) of
Patchmaster
, the Noise
display shows the rms noise current present in the current monitor signal in the frequency band selected by Filter 2
(2.9 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
70-100 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-DA , 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
EPC 10 USB
’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 holder, and the combination of pipette immersion and patch
noise. The table below shows the relative contributions calculated in this way for the ”optimum” situation just
described.
Figure 11.1: Noise contribution
The contributions to the variance from the three sources are seen to be comparable in size, and improvements in
the amplifier noise level will not help very much, unless corresponding improvements are made in the other noise
sources. As it is, rms noise values as low as those quoted here are obtained only with considerable care. Some of
the important considerations are outlined below.
As we mentioned in chapter 8 on page 49, an unshielded holder is greatly superior to the shielded one for low
background noise. For low noise, the holder must be made from a low loss, hydrophobic plastic; polycarbonate is
one of the best, and plexiglas one of the worst materials. (For our purposes, low loss materials are those that show
little dielectric relaxation in the frequency range of a few kHz. Dielectric relaxation involves the reorientation of
dipoles within the material; since any dipoles will be in thermal motion, thermal reorientation in this frequency
range will result in current fluctuations coupled capacitively into the pipette.)
It is very important that the pipette holder be kept clean and dry. Noise can be coupled into the pipette from