Chapter 2
Setting Up
31
Inverting the Signal
The
Invert
checkbox allows you to invert the signal on the screen. It
provides a simple way to change the polarity of the recorded signal
without having to swap the connections to the recording electrodes.
For example, you might be recording from a force transducer where
an increase in force downwards gives a negative signal, but you want
to have a downwards force shown as a positive signal on the screen.
Checking the Invert checkbox will change the display to do this.
Offset Adjustment
Transducers almost always produce some amount of signal, usually
small, when in the equilibrium or rest state. Offset from a zero reading
need to be removed, in a process called zeroing. Commonly, the user
also wants to remove a constant term, for example baseline blood
pressure or initial tension in a muscle, from a measurement of interest.
This enables more accurate measurement of the changes in the signal
under stimuli. The offset controls in the Bridge Amp dialog can be
used to zero the reading manually or automatically.
Manual Zeroing
. The up and down arrows near the
Zero
button allow
manual adjustment of the signal offset. Click the up arrow to shift the
signal positively and the down arrow to shift it negatively. The shift
by clicking the arrow buttons depends on the range setting. At high
ranges the increments are larger to adjust the offset efficiently.
Automatic Zeroing
. To perform automatic zeroing, click
Zero
: the
program works out a corrective DC voltage that cancels, as closely as
possible, the output voltage from the transducer. Auto-zeroing may
take about 20 seconds to work out the best zeroing value at all ranges.
A dialog with a progress bar appears: click the Cancel button or type
Command-period to stop the zeroing process. If there is still offset
after auto-zeroing, then Option-click the up and down arrow buttons
to adjust the zeroing slightly, by the smallest increment at any range.
Figure 2–5
The Offset controls,
Windows (left) and
Macintosh (right)
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