Appendix A — Technical Aspects
21
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.
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 M
Ω
).
AC/DC
coupling
control
Instrument
amplifier
Channel control bus
BNC Analog input
Grounding
and input
selection
1 M
Ω
AC/DC
coupling
control
Instrument
amplifier
Channel control bus
Analog input
To multiplexer
Grounding
and input
selection
Gain
control
1 M
Ω
1 M
Ω
BNC
Pod Input
Gain
control
To multiplexer
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.
Summary of Contents for PowerLab 2/20
Page 4: ...iv PowerLab Owner s Guide ...
Page 40: ...32 PowerLab Owner s Guide ...
