Siemens SC 7000 Service Manual page 39

SC 7000 and SC 9000XL Patient Monitors
Respiration
ASK-T898-03-7600
7k9kXLTM.c2.CD_ROM.fm/04-99/kaupp
+ Clamp
RF Filter
RA
- Clamp
+ Clamp
RF Filter
LL
- Clamp
+ Clamp
LA RF Filter
MUX
- Clamp
+ Clamp
Chest RF Filter
- Clamp
+ Clamp
RL RF Filter
- Clamp
Figure 2-9 Lead Forming Network
The Wilson point, "W", the average of the LA, RA, and LL electrode
potentials, serves as the negative reference potential for the V and V' lead-
pairs and is also used as a measure of the common-mode potential of the
patient Figure 2-9. By driving the isolated common of the front end at the
same potential as the Wilson point, the common-mode voltage across the
electrodes is reduced nearly to zero and the effective common-mode
rejection is improved. As most of the common-mode current is now forced
through the neutral electrode, it becomes noisier and hence is not used as
part of another signal path. Switches are provided to select other electrodes
to be neutral if the RL electrode is off or missing. If the V' electrode is
present, then it can be selected to be neutral so that the three Einthoven
and the V lead pairs can still be used. However, the V' lead-pair will be
corrupted due to neutral current noise. Similarly the V electrode can be
selected to be neutral. Now that the RL is disconnected from the neutral
driver, its potential can be monitored to determine whether it has been
reconnected to the patient and thus is able to be reselected to be neutral.
If only the three Einthoven (LA, RA, and LL) electrodes are connected, one
is selected as neutral leaving the remaining two electrodes to form one valid
lead-pair. The "W" now contains the neutral drive signal which bypasses
the neutral electrode and reduces the gain of the neutral driver amplifier. To
improve the resulting poor common-mode rejection, a Wilson Grounding
"WG" switch is activated to selectively disable the offending input to the
"W".
Refer to Figure 2-10.
• Respiration is both ac- and dc-coupled in hardware. DC is used for
high Z sensing; ac is used for signal acquisition.
• Respiration may be monitored on leads I and II.
• Detection sensitivity has low dependence on base resistance or
electrode unbalance.
Siemens Medical Systems, EM-PCS Danvers
NOT A CONTROLLED DOCUMENT
RA
RA
Augmented
aVL, aVR,
Leads
aVF
LL
Wilson
Ref
Star
Normal Leads
LA
Chest
RA
Resp
Demod
RL
Augmented Leads
Service Manual
I
LA
V
W
Chest
III
II
LL
LA
aVR aVL
aVF
LL
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