Front End Module Theory of Operation
2 - 16 0070-10-0441 Passport 2®/Passport 2 LT™ Service Manual
2.2.8 Signal Acquisition
ECG
The ECG signal acquisition consists of three differential amplifier channels, which can be
configured to support either 3 lead or 5 lead operation. These are connected to channels 0
through 2 of the MAX147 ADC. In the 5 lead mode, a dedicated electrode (RL) is used for
patient drive. This allows all of the remaining electrodes (RA, LA, LL, and C) to be used for
signal acquisition. There are 7 standard vectors (“leads”) which can be obtained
simultaneously from these electrodes. However, these vectors contain much redundant
information. In fact, only 3 channels of data are required to allow derivation of any or all of
the 7 standard vectors. The ECM module bus therefore caries only lead I, Lead II, and V as
the requisite data channels. In three lead mode, only three electrodes are attached to the
patient. Further, one of these electrodes must be used for the patient drive, precluding its use
for signal acquisition. Therefore, only vectors Lead I, Lead II, and Lead III are available, with
only a single vector being available at a time. Two of the 3 differential amplifiers are
configured to acquire Lead I and Lead II. The third amplifier is reconfigurable, by means of
an analog switch, to acquire either the V lead or Lead III. The configuration of the amplifiers
and the driven lead is controlled by the two ECG lead select bits, LS0 and LS1.
2.2.9 Lead Fault Detection
Lead fault detection is performed by observing the voltage on the ECG amplifier inputs. Due
to the bias networks included in the lead buffers, an open circuited electrode will develop a
strong negative voltage. The lead fault signal is then generated by the microcontroller, which
is actively monitoring the voltage via its A/D converter.
In leads I, II, and III, the operation of the lead fault detection is very simple. When all leads
are attached, both ECG amplifier inputs are approximately at the value of the drive amplifier
bias voltage, which is about +2.5 volts nominal. This voltage is not sufficiently negative, so
no lead fault condition is indicated. If one of the active electrodes becomes detached, the
voltage on the corresponding amplifier input swings to about -5 volts. This is well beyond the
threshold, so a lead fault condition is indicated. The effect is the same if either active lead is
detached. If the drive electrode becomes detached, this is equivalent to both active leads
being open, so the operation is the same. Note that the margins between the common mode
voltage established by the drive amplifier and the lead fault threshold are large enough that
the possibility of a 300 mV electrode polarization offset does not affect lead fault detection.
Note also that this lead fault detection scheme only tests those electrodes which are
necessary to view a particular lead selection. Therefore, to view leads I, II, or III, only RA, LA,
and LL are needed. The lead fault detection is indifferent to the state of the unnecessary RL
and C electrodes.
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