Welch Allyn Atlas Service Manual page 91

R624
51.1K
f+1.24V
C633
.01uF
E
-Clamp +Clamp
D602
BAV99L
1 2
J601
Temp-In
2
1
Temperature
CON2
Thermistor
2.7 Impedance Respiration
2.7.1 Overview
Impedance respiration is accomplished by measuring a change of resistance across the patient's chest
during breathing. The RA and LA electrodes (Lead I) are uses to acquire the respiration signal. A current
source is driven through the patient, and the voltage developed across the patient is measured. The change
in resistance caused by respiration is small, in the order of 1 Ohm. This small change must be measured
on top of a large baseline impedance, typically 100 to 1000 Ohms.
2.7.2 Signal Source – Current Drive
An AC current source is used as the input signal, a 34.8KHz square wave. The Power supply sync signal,
at 76.8KHz, is transmitted across opto isolator U710. Flip Flop U701, configured as a divide by 2,
converts this signal to a 34.8KHz. Current through the patient is about 150uA Pk-Pk, set by the output of
U710 (5V) and the circuit series impedance, 69K including cable resistance. The impedance of the patient
is small (<2K) compared to the series impedance and has minimal affect on the magnitude of the drive
current. Diode clamps are used to protect the respiration circuits in the event of a Defibrillation pulse.
-Clamp
D701
MMBD1503A
1
R701
1.96K
D701-3
Right-Arm
D702
MMBD1503A
1
R702
1.96K
D702-3
Left-Arm
Appendix A
fVcc
Temperature
Amplifier
U608-3 3
+
1 U608-1
2
-
U608A
AD712
fVee
C634 .01uF
R625
1K
R627 1.21K
0.1%
C635 .01uF
C636
2200pF
E
+Clamp
Respiration
Drive
Circuit
2
C701 R703
1000pF 31.6K
C701-1
2
C702 R704
1000pF 31.6K
C702-1
R626
1K
Patient-Temp
f+5V
+
E
U701A
14 74HC74
VCC
RespDr+ 5 2
Q D
3
CLK
6
Q
7
GND
E
RespDr-
10/14/99
C719
4.7uF
Resp-Clk
Resp-Off