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Figure A–3
Block/schematic diagram of
the input amplifier circuitry for
BNC-only inputs.
Figure A–4
Block/schematic diagram of
the input amplifier circuitry for
dual BNC and pod inputs.
Appendix A — Technical Aspects
ground. Differential signals can only be recorded using a pod
connector; the BNC analog inputs on the PowerLabs are all single-
ended. Inputs 3 and 4 of the PowerLab 4/20 are single-ended, but can
be set by the software to be either positive or negative (inverting or
non-inverting).
It is important to note that the PowerLab grounds the inputs to
amplifiers not in use. It also grounds each amplifier and measures the
DC offset voltage when the gain is changed. In this way, the software
corrects for any DC drift or offset in the circuits that may develop
over time or between readings. The operation of the input amplifiers
is illustrated by the block diagrams below.
AC/DC
Grounding
coupling
control
1 MΩ
Analog input
AC/DC
coupling
control
The input amplifiers can be set to pass both DC and AC signals, or to
pass only AC signals without passing DC signals. Input impedance is
one megohm (1 MΩ).
Gain
and input
control
selection
Instrument
amplifier
Grounding
and input
control
selection
1 MΩ
Instrument
amplifier
1 MΩ
Channel control bus
Channel control bus
Gain
To multiplexer
To multiplexer
21
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