
ECG1 • 7
When we measure the impulses electrically, the pattern will appear similar to
this:
Hopefully now the ECG patterns seem a little less “mystical ” than they were
when we began, and we have cleared up how the pulses are created and how
they correspond to the action of the heart muscle.
CIRCUIT DESCRIPTION
Getting back to electronics, let’s talk our way through the circuit we’re using to
monitor the minute electrical impulses that are present due the action of the
nerves of the heart muscle. These signals are in the microvolt range, and are
swamped out by other noise and static present on our body at any time (if you
don’t believe me, try and touch the probe tip of an oscilloscope sometime and
have a look at the output waveform). So what we need is a circuit that will
eliminate any common noise present on the inputs and amplify the difference
between two common voltages. The circuitry used on the ECG1 is a common
application for operational
amplifiers, or op-amps,
known as a differential
amplifier.
Have a look at the circuit
diagram in the figure, and
notice that we are using
two inputs to the circuit,
and that the circuit mirrors
itself with respect to the
two inputs. Without getting
too involved, notice that
the single ended output is
only taken off of one
transistor; this voltage with respect to ground is the difference between the
two input voltages. When both inputs change level together, we call that a
common mode input change. When the inputs are not exactly the same, we
P
T
QRS
Atrial
Depolarization
Ventricular
Depolarization
Ventricular
Repolarization
Comentários a estes Manuais