SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATION - Design and Development of Medical Electronic Instrumentation - page 278

in IC37, the PIC16C77 microcontroller shown in Figure 6.20. The 12-bit values are output
serially through the microcontroller's SPI port to IC1, an LTC1451 D/A converter. The
output from each D/A converter is scaled by a programmable attenuator made up by IC3
and the resistor network, R2 and R5-R14. A thumbwheel switch connected to analog mul-
tiplexer IC3 selects the resistor used in the attenuator circuit. The signal from the attenua-
tor is bu ered by IC2A and changed from voltage to current by IC2B, IC2C, and IC2D.

This current signal

ff

resistor, and is seen by the device under test as
a voltage signal. Figure 6.21 shows the schematic of the right-atrial and right-ventricular
CENELEC signal generators.
A typical cardiac cycle would start with generation of the right-atrial CENELEC signal
followed by the right-ventricular CENELEC signal. The delay from the start of the right-atrial
signal to the beginning of the right-ventricular signal is

fl flows into R74, a 10- Ω

fixed in this simulator at 100 ms. The
left-ventricular signal would then start after the right-ventricular signal, with a time delay that

fi

+5V

12

11 10

9

R103
10K

R104
10K

R98E
47K

R98F
47K

R98G
47K

R98H
47K

IC37

Thumbwheel 1

2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

40

V_PACE_DETECT

MCLR
RA0/ANO
RA1/AN1
RA2/AN2
RA3/AN3/Vref
RA4/ TOC KI
RA5 /SS /AN4
RE0 /RD /AN5
RE1 /W R/AN6
RE2/CS/AN7
VDD1
VSS1
OSC1/CLKIN
OSC2/CLKOUT
RC0/T1OSO/T1CKI
RC1/T1OS1/CCP2
RC2/CCP1
RC3/SCK/SCL
RD0/PSP0
RD1/PSP1

RB7

5 6 7 8

39

RB6

A_PACE_DETECT

LV_DELAY_SW_1 LV PVC

SWITCH

38

LV_DELAY_SW_2

RB5

LV_P _TRIG

37

LV_DELAY_SW_3

LV_PVC_SW RB4

36

LV_DELAY_SW_4 RB3

35

RB2

34

RB1

+5V

33

+5V

RBO/INT

32

VDD2

C55

C54
0.1uF

31

+

C57

VSS2

C58
15pF

C56
4.7uF

30

4.7uF

RD7/PSP7

0.1uF

29

RD6/PSP6

28

RD5/PSP5

Y1
10.0MHZ

27

RD4/PSP4

26

RC7/ RX/ DT

Z_CS

25

C59
15pF

RC6/TX/CK

24

RC5/ SDO

SDO

23

RC4/SDI/SDA

SCLK

22

RD3/PSP3

A_CS

21

RD2/PSP2

V_CS

LV_CS

PIC16C77

ECG_CS

150

R105

A_SENSE_LED

R106 150 A_PACE_LED

V_SENSE_LED

R107 150

V_PACE_LED

R108

150 LV_SENSE_LED

LV_P _OUT_LED

R109 150

LV_P _LED

R110 150

R111 150

Figure 6.20 The responsive cardiac simulator is controlled by a PIC16C77. In addition, this PIC contains digitized CENELEC, ECG, and
impedance waveforms that are played back via LTC1451 D/A converters to simulate the heart's electrical and mechanical activities.