Welch Allyn Atlas Service Manual page 92

2.7.3 Differential Amplifier and Peak Detector
The AC current driven though the patient develops a voltage across the RA to LA electrodes. This signal
is measured and amplified with the Differential Amplifier formed by U702A/B. This amplifier converts
the differential voltage across the patient to single ended signal, with a gain of 16.8. Inverting this signal
(U703A), and summing the original signal and inverted signal through the dual Diode D705 then forms a
synchronous peak detector. Then, the voltage on C711 is the DC resistance seen across the RA to LA
buffer. This baseline resistance is digitized, and if the signal is too large (>2.5K), a Respiration Lead Fault
message is displayed. Note that this circuit measurement includes the two 1K resistors in the EKG cable
set. This 2K resistance is subtracted in software before determining a leads off condition.
-Clamp +Clamp
D703
MMBD1503A
1 2
R707
1.96K
D703-3
Right-Arm
D704
MMBD1503A
1 2
R708
1.96K
D704-3
Left-Arm
2.7.4 Gain and Filtering
The DC content (baseline resistance) of the respiration signal is not needed, so the signal is next AC
coupled, and additional gain applied before digitization. Low pass filtering is performed at this stage to
reduce high frequency noise outside the respiration signal bandwidth. A baseline-reset circuit (transistor
switch Q701) is used to quickly restore the high pass pole if excessive DC voltage is present, for example
following a Leads-Off condition.
Appendix A
fVcc Respiration
Diff Amp
C706
330pF
U702-3 3
+
1
2
-
U702A
R709 AD712 R711
464K 15.8K
fVee U702-2
E
R712
1K
fVcc
U702B
C707
330pF
AD712
U702-5 5
+
7 U702-7
6
-
R713
1K
R710
464K fVee
U702-6
E
R714
15.8K
E
U702-1
D705
MMBD354LT1
R715 R716 1
3.16K 3.16K
2
fVee
2
1
3
U703A
AD712
E
fVcc
10/14/99
R724
196K
Resp-DC
C720
.047uF
E
3
D605-C
R717
3.16K C711
0.1uF
E E