Mindray Passport 5-Lead, Service Manual

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Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 25
Theory of Operation Block Diagrams
The three remaining A/D converter inputs are used for self-test functions. Input 0 is connected
to a 2:1 ratio voltage divider (R10 and R11) which monitors the +7.5 volt supply, providing
a nominal 3.75 volt test point. The software monitors this input for a low supply condition (-
10%) only. The -7.5 volt rail is monitored with a divider, consisting of R8 and R9, connected
from the +5 volts to the -7.5 volt rail. This provides a n.83 volt test point, which is
monitored by A/D converter input 7. The software is designed to sense a 10% drop in the -
7.5 volt rail. Both of these A/D converter inputs have diode protection, and are furnished
with small capacitors to filter any transients. The +2.5 volt supply is directly connected to
input 6, and is tested by the software for both high and low conditions. The A/D converter
also generates an internal, half-scale, test voltage, which is similarly monitored. A failure of
any of these supply voltage tests is reported in the STATUS field of the data packets. Because
extreme accuracy is not needed in these measurements, the software checks only the upper 8
data bits. Significant measurement uncertainties can result from the resistor tolerances,
converter input leakage current, input charge, and reference (+5 volt supply) tolerance.
These factors are taken into account in the setting of the limits, to prevent false reporting of
errors. Additionally, a supply fault must be detected by the software on two successive tests
before it is reported.
The A/D converter has a pipelined structure. Each time the SPI communicates with the
converter, it writes the multiplexer address for the next conversion to the converter, and reads
the result of the previous conversion. The next conversion starts as soon as the address is
written. The software then engages in other activities while waiting for the conversion to
finish. Thus, the data read from the A/D converter lags one sample behind the current
multiplexer address. The software takes this delay into account when formatting the data.
The A/D converter has two +5 volt and two ground connections. Although these eventually
connect to common points, they are run on separate traces to minimize noise and errors due
to trace impedance and interaction with other loads. Operating power for the A/D
converter, on pins 10 and 20, is supplied from the digital branch of the +5 volt rail, and
returned via the digital portion of the ground plane. Capacitor C 120 directly bypasses these
points. The reference voltage, at pins 14 and 13, is supplied from the analog branch of the
+5 volt rail and the analog ground plane half. The same reference is used excite the pressure
transducer bridge. The temperature function makes use of the ratio of two A/D converter
readings and is thus independent of the exact reference value.