AN206
Vishay Siliconix
DG406 Multiplexer Optimizes Medical Simulator
Introduction
A patient monitoring system collects information from several
transducers connected to a patient. The transducers convert
physiologic electromechanical activity into electrical signals
which are routed to a data acquisition module for digitization.
The digitized data is then processed and displayed on the
monitor screen. Additionally, bedside patient information can
be relayed to a central nursing station, a hospital information
system, or a doctor’s office.
Patient monitors must provide reliable data at all times. For this
reason, it is imperative to have an accurate means for
calibration and troubleshooting of patient monitoring
instrumentation. This function can be accomplished by using
a patient simulator which generates electrical signals similar to
those produced by the transducers.
For this application note, a patient simulator was designed and
built. The simulator consisted of a personal computer
peripheral device and had to be capable of producing the static
display of all numerics and graphics associated with the
following three human parameters: a) body temperature,
b) blood oxygen saturation level, c) heart pulse rate. Having
these capabilities greatly enhances the ability to perform fine
calibration and troubleshooting.
Performance Goals
The simulator design goals were as follows:
a)
b)
c)
Body temperature: from –1_C to 46_C in 0.1_C
increments, with an accuracy of
"0.1_C.
Blood oxygen saturation level: sixteen independent
values in the range of 60% to 100%.
Heart pulse rate: fifteen independent values in the range
of 30 to 300 pulses/min.
Theory of Operation
The commands to modify the simulated parameters are sent
from a PC over an RS-232 line to the peripheral device. A
microcontroller containing a UART receives these commands
as 8-bit frames and processes the data. After processing, the
microcontroller uses its I/O port bits to send data to the
parameter generation hardware. The parameter’s signal is
modified as specified by the down-loaded command. The
modified signals are then output to the patient monitor to
produce the appropriate visual displays.
Central Nursing Station
Bedside Monitor
Network
Controller
Doctor’s Office PC
From Patient
FIGURE 1.
A Typical Medical Information System
Document Number: 70604
10-Aug-99
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AN206
Vishay Siliconix
PC
Temperature
RS-232
Micro
Controller
Oxygen
Saturation
To Patient
Monitor
Pulse
Rate
FIGURE 2.
Patient Simulator Block Diagram
Design Considerations
The temperature parameter was simulated by using a discrete
resistor array and a pair of analog multiplexers as illustrated in
Figure 3. A discrete resistor value representing a known
temperature was placed between the same channel of two
analog multiplexers. Using its I/O lines, the microcontroller
simultaneously selects one of the 16 resistors. A know current
is sent through the resistor. The medical monitor reads the
voltage drop across the resistor and translates it into a
temperature reading.
Since r
DS(on)
affects the output voltage, in order to obtain
accurate results, it was necessary to use the multiplexers with
a low on-channel resistance. The temperature simulation
section of the design utilized the DG406, a 16-channel
multiplexer manufactured by Vishay Siliconix. Thus, it was
possible to simulate 16 discrete temperatures by using a pair
of multiplexers. The DG406 was ultimately chosen for its low
r
DS(on)
of approximately 40
W
typical. This value was
measured and confirmed several times. To get the best results
possible, lower resistor values were used and a digital
potentionmeter array was used to get the exact resistance
required including the 40-W r
DS(on)
.
Simulating the saturated oxygen level was accomplished
utilizing another DG406 multiplexer. An analog signal was sent
through the selected channel of the multiplexer and attenuated
in a predetermined manner. The resulting signal is then output
to the medical patient monitor modifying the saturated oxygen
parameter. Again an array of digital pots was used to
compensate for the already low on-resistance and to get the
exact resistance values desired.
R
1
Digital Pots
R
2
V
OUT
to
Monitor
I
R
16
DG406
DG406
From mC
FIGURE 3.
Temperature Simulation Circuit
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Document Number: 70604
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Vishay Siliconix
R
17
R
18
V
REF
Programmable
V
OUT
to
Monitor
From mC
Counter
Frequency
Out to
Monitor
R
32
Clock
DG406
From mC
FIGURE 4.
Saturated Oxygen Simulation Circuit
FIGURE 5.
Pulse Rate Simulator
The pulse rate parameter was simulated by varying the
frequency of a square wave obtained from a programmable
timer/counter. The timer/counter was programmed using
some of the microcontroller I/O lines. The square wave
frequency represents the patient’s pulse rate.
Although this design used lower resistor values and digital pots
to compensate for the r
DS(on)
, this may not be necessary in
other less sensitive designs.
The critical components in this design were the
microncontroller for its speed and numerous on-board
devices, the analog multiplexer for its low r
DS(on)
, and the
digital potentionmeters. Considerable time was spent
shopping around for these three devices. Viewing from the
better than expected results of the overall design, it is obvious
that the time spent in search of these devices was well worth
it.
Conclusion
The finished simulator met all design goals. The DG406
worked as documented in the Vishay Siliconix data book.
Document Number: 70604
10-Aug-99
www.vishay.com
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FaxBack 408-970-5600
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