Reference Section • 109
Chapter 5
Headstage Circuit
V-Clamp: Current noise decreases as the value of the feedback resistor (R
f
) is
increased. Thus, for minimum noise the largest R
f
should be chosen, subject
of course to range limitations. (See Chapter 5,
FEEDBACK RESISTOR.)
I-Clamp: Voltage noise decreases as the value of R
f
is decreased, but R
f
should be chosen so that it matches the load resistance (i.e. sum of electrode
and cell resistance) within a 1:10 ratio (a 1:5 ratio is optimal). Thus,
R
f
= 50 M will work optimally for loads between 10 M and 250 M. This
range limitation is determined by the effectiveness of the Capacitance
Neutralization circuit.
Compensation Circuits
V-Clamp: Adjusting Rs Compensation increases the current noise, because
the compensation circuit employs positive feedback that injects noise back into
the headstage. Further, the effect of Rs compensation is to reduce the
electrode series resistance, which reduces the effect of the RC filter mentioned
above (“Electrode and Holder”).
I-Clamp: Increasing Pipette Capacitance Neutralization increases the voltage
noise, for reasons similar to those just mentioned for Rs Compensation.
Although both of these compensation circuits increase the noise in the signal
of interest, they are most likely to be required in whole-cell recordings where
the dominant noise source is the cell. In any case, correction of Series
Resistance and Pipette Capacitance errors must normally take precedence over
noise concerns in whole-cell experiments.
Power Supply
Noise can arise from earth loops, power supply glitches and radiation from
nearby instruments. (See Chapter 5,
GROUNDING AND HUM, and POWER
SUPPLY
.)
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