Front End Module Theory of Operation
2 - 18 0070-10-0441 Passport 2®/Passport 2 LT™ Service Manual
The respiration signal is isolated from the baseline by a fixed analog highpass filter at 0.1
Hz, and a lowpass at about 3 Hz. The signal is amplified and acquired by channel 3 of the
MAX147 ADC. Although the QSPI scans all channels every 2 ms, the respiration is to be
sampled at only 100 Hz. Therefore, the data processing software takes data from the QSPI
buffer in only 1 out of every 5 scans. The highpass filter is equipped with a reset function
activated by a control line in Port A. The respiration reset is controlled in a manner similar to
the ECG reset, in that it is activated whenever the respiration signal appears stuck offscale,
and must remain asserted for a settling period after the signal is brought back in range. The
baseline impedance is also monitored, by a channel in the 68HC16 internal ADC. When the
baseline impedance exceeds about 4000 ohms (patient plus cable resistors), a high
impedance flag must be set by the software. This flag indicates that the respiration signal is
no longer reliable.
When monitoring neonatal patients, it is desirable to raise the highpass corner frequency to
0.2 Hz. Since the analog filter is fixed, this is done by refiltering the signal with a digital filter
in software. In the adult mode, this digital filter remains active, but is set to a very low corner
frequency, 0.0125 Hz. Therefore, the analog 0.1 Hz filter dominates the response. This
relaxes the design constraints on the respiration filter and DC amplifier circuits, as any
residual offset voltage present in these stages is now removed by the digital filter, even in the
adult mode. In the neonatal mode, the digital filter is set to 0.155 Hz, which produces a 0.2
Hz 3 dB point when cascaded with the analog filter. A Mathcad document fully documents
the digital filter. Note that these filters are based on the 100Hz sampling rate. Whenever
software resets the analog filter, the cascaded digital filter must also be reset, or the baseline
will not settle properly. The assembler code shows the method of resetting the digital filter.
Note also that the detection of an offscale condition for reset generation must be done on the
raw ADC data, before the digital filter.
The respiration can be configured to sense impedance in either a lead I (RA/LA) or lead II
(RA/LL) axis, by means of the Respiration Lead Select bit in Port A. The respiration can be
disabled by the Respiration Enable bit in Port A. It is necessary to disable the respiration
when the carrier might cause interference with other electromedical devices. The table below
summarizes the respiration control bits in Port A.
2.2.12 Invasive Blood Pressure
An interface is provided for two standard 5mV/mmHg/V transducers. This consists of a
common excitation power source and a pair of differential amplifiers with lowpass filters,
one for each channel. The excitation source is monitored by the 68HC16 ADC, while the
pressure waveforms are acquired by the MAX147. The frequency response can be modified
by digital filters processing the acquired signals.
The excitation voltage is nominally +5 volts, but tracks the ADC reference. In this way, the
actual value of the ADC reference does not affect accuracy, since the transducer is a
ratiometric device. The excitation source has current limiting, which prevents the module
power supply from being disrupted if the transducer excitation becomes shorted. The
excitation regulator’s error amplifier output is monitored by the 68HC16 internal ADC. This
signal will abruptly jump to almost zero when the excitation source begins to current limit.
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