DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 120

START
CONVERSION

*CS

>0.25 µ s

123456789 10 11 12 13

SCLK

D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

DOUT

END OF
CONVERSION

>8.5 µ s

Figure 3.16 Data acquisition and serial protocol timing for the MAX187. A/D conversion is initiated by a falling edge on the *CS line. After
conversion, data are read out in serial format, shifted from the sequential-approximation register on each falling-edge transition of SCLK.

expansion bus, it plugs into one of the parallel printer ports, which is used as a serial I/O
for an eight-channel A/D converter. As shown in the diagram, at the heart of the circuit
is a MAX187. This IC is one of Maxim's single-chip A/Ds featuring a 12-bit 8.5- s

successive-approximation converter, 1.5- s track-and-hold, on-chip clock, precision

1
2 LSB nonlinearity, and high-speed three-wire serial interface.
One of eight isolated analog signals to be measured is presented to the analog input line
AIN of the MAX187 by way of a DG508A analog multiplexer IC. Voltages within the
range 0 to 4.096 V can be converted by the A/D into distinct digital codes for every 1 mV
of change. MAX187's A/D conversion initiation and data-read operations are controlled by
the *CS (chip select) and SCLK (serial clock) lines. As shown in Figure 3.16, an A/D con-
version is initiated by a falling edge on the *CS line. At this point, the track-and-hold holds
the input voltage and the successive-approximation process begins. The start of conver-
sion is acknowledged by the MAX187 changing the state of the DOUT line from high
impedance to the low state. After an internally timed 8.5-

4.096-V reference,

s conversion period, the end of
conversion (EOC) is signaled by the DOUT line going high.
Once a conversion is completed, data can be obtained in serial format, shifted from the
sequential-approximation register on each falling-edge transition of SCLK. Since there are
12 bits, a minimum of 13 falling-edge pulses are required to shift out the A/D result. Isolation
between the PC's printer port and the MAX187 is provided by IC4-IC7. Bits 1 and 2 of the
PC's LPT 8-bit output port (hex address 378 for LPT1:) are toggled by software to imple-
ment the control portion of the MAX187 serial protocol. Bit 6 of the printer status port reg-
ister (hex address 379 for LPT1:) is used to receive the serial data from the MAX187. Bits
3-5 of the output port are used to control the analog signal multiplexer. Bit 0 of the output
port is an auxiliary line that may be used in the control of the applied part's circuitry.
Power for the MAX187 must be supplied from a patient-contact-rated isolated power
supply capable of delivering filter formed by C1-C4 and L1 ensures a clean sup-

ply to the A/D. In addition, you may notice that two separate isolated ground planes, one
analog and one digital, are shown in Figure 3.15. Ideally, the signal ground plane, used as
the reference for the analog input signal, should be constructed to shield the analog portions
of the A/D and signal multiplexer: namely, the input network and voltage reference

5 V. A pi fi

filtering
and decoupling capacitors. The analog and digital ground planes should be connected at a
single point, preferably directly to the isolated ground line supplying the circuit.

fi

SOFTWARE FOR THE ISOLATED A/D

The sample program that follows is for driving the isolated A/D converter from the printer
port of a PC. The program fl flow starts by initializing the ports. Notice that use of the standard