DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

120 DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES*CSSTARTCONVERSION>0.25µsSCLKDOUT1 2 3 4 5 6 7 8 9 10 11 12 13D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0END OFCONVERSION>8.5 µsFigure 3.16 Data acquisition and serial protocol timing for the MAX187. A/D conversion is initiated by a falling edge on the *CS line. Afterconversion, 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/Ofor an eight-channel A/D converter. As shown in the diagram, at the heart of the circuitis a MAX187. This IC is one of Maxim’s single-chip A/Ds featuring a 12-bit 8.5-ssuccessive-approximation converter, 1.5-s track-and-hold, on-chip clock, precision4.096-V reference, 1 LSB nonlinearity, and high-speed three-wire serial interface.2One of eight isolated analog signals to be measured is presented to the analog input lineAIN of the MAX187 by way of a DG508A analog multiplexer IC. Voltages within therange 0 to 4.096 V can be converted by the A/D into distinct digital codes for every 1 mVof change. MAX187’s A/D conversion initiation and data-read operations are controlled bythe *CS (chip select) and SCLK (serial clock) lines. As shown in Figure 3.16, an A/D conversionis initiated by a falling edge on the *CS line. At this point, the track-and-hold holdsthe input voltage and the successive-approximation process begins. The start of conversionis acknowledged by the MAX187 changing the state of the DOUT line from highimpedance to the low state. After an internally timed 8.5-s conversion period, the end ofconversion (EOC) is signaled by the DOUT line going high.Once a conversion is completed, data can be obtained in serial format, shifted from thesequential-approximation register on each falling-edge transition of SCLK. Since there are12 bits, a minimum of 13 falling-edge pulses are required to shift out the A/D result. Isolationbetween the PC’s printer port and the MAX187 is provided by IC4–IC7. Bits 1 and 2 of thePC’s LPT 8-bit output port (hex address 378 for LPT1:) are toggled by software to implementthe control portion of the MAX187 serial protocol. Bit 6 of the printer status port register(hex address 379 for LPT1:) is used to receive the serial data from the MAX187. Bits3–5 of the output port are used to control the analog signal multiplexer. Bit 0 of the outputport 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 powersupply capable of delivering 5 V. A pi filter formed by C1–C4 and L1 ensures a clean supplyto the A/D. In addition, you may notice that two separate isolated ground planes, oneanalog and one digital, are shown in Figure 3.15. Ideally, the signal ground plane, used asthe reference for the analog input signal, should be constructed to shield the analog portionsof the A/D and signal multiplexer: namely, the input network and voltage reference filteringand decoupling capacitors. The analog and digital ground planes should be connected at asingle point, preferably directly to the isolated ground line supplying the circuit.SOFTWARE FOR THE ISOLATED A/DThe sample program that follows is for driving the isolated A/D converter from the printerport of a PC. The program flow starts by initializing the ports. Notice that use of the standard