Axon Axopatch 200B, Теория работы

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INSTRUMENT OPERATION • 63
Chapter 6
Output Section
All the controls in the Output Section affect only the signal on the Scaled Output.
Filter
The filter is a 4-pole lowpass Bessel filter. The attenuation of signals and noise above the -
3 dB frequency is 80 dB/decade (24 dB/octave). The Bessel characteristic is suitable for
patch and voltage clamping because it introduces < 1% overshoot. The lowpass bessel
filter control allows you to select a bandpass of 1 kHz, 2 kHz, 5 kHz, 10 kHz or 100 kHz.
All lowpass filters slow the rise time of the signal. For filters with < 10% overshoot the
10-90% rise time is:
t
r
≈ 0.35
f
c
where f
c
is the -3 dB frequency in Hertz. For example, the 10-90 % rise time of a 1 kHz
filter is approximately 350 µs.
Note: If you use an external filter: Some manufacturers specify the -3 dB frequency based
on the phase response of the filter instead of its amplitude response, or based on a straight
line approximation to the filter characteristics instead of the actual characteristics. You
should check your external filter by checking t
r
for a step signal applied to its input.
When a signal with 10-90% rise time t
1
is passed through a filter with 10-90% rise time t
2
,
the rise time of the output signal is approximately:
Output Gain ( α
αα
α )
There are ten output gain settings in a 1, 2, 5 ratio. These are: 0.5, 1, 2, 5, 10, 20, 50, 100,
200, and 500
.
In patch clamp amplifier design, there are conflicting concerns when deciding where to
place the Bessel filter and the gain amplifier in the signal pathway. On the one hand, to
minimize the noise contribution of the circuitry of the Bessel filter itself, the gain amplifier
should come first. On the other hand, if the gain amplifier comes first, the peaks of the
amplified signal and noise might get clipped if the gain setting is high. After filtering, it is
difficult to distinguish the clipped signals from biological channel currents.
t t t
r
≈ +
1
2
2
2