Acquisition Module Transceiver / Power Switch; Transceiver; Acquisition Power Regulator / Switch; Comm Port Power Switch / Current Limiter / Kiss Power - GE MAC 5000 Service Manual

Acquisition Module Transceiver / Power Switch

Transceiver

Acquisition Power Regulator / Switch

COMM Port Power Switch / Current Limiter / KISS Power

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Equipment Overview: Theory of Operation
MAC 5000 acquires ECG data with a new generation CAM acquisition
module. The FPGA provides the interface logic. Clocks and commands
are transmitted to the acquisition module on a balanced RS485 line.
Data is received similarly. Power to the acquisition module is provided
by a software controlled linear regulator.
To reduce EMI and susceptibility to noise, the acquisition module link is
implemented using RS-485 differential signaling. An RS485 interface
device provides the single ended to differential conversion in both
directions. Ferrite beads, capacitors and resistors are used to reduce EMI
on both sides of the transceiver.
To reduce standby power consumption, acquisition module power is
switchable. To protect the acquisition module from temporary brownouts
on the main 12V supply, power is obtained from a parasitic winding on
the main 5V regulator. This voltage is not well regulated, so a linear
regulator (U16) is used to provide regulation. This regulator also sports
an enable input which is used to disable power to the acquisition module
when not in use. The regulator also has build in current limit and over
temperature shutdown for protection.
Power for external peripherals such as a modem or the KISS vacuum
electrode pump is available on the COMM connectors. Power may be
turned on/off under software control and current limiting is employed to
protect internal operations from excessive external loads. The current
requirements and startup conditions of the KISS pump require very high
currents. U.L. limits power to external devices to 15 Watts for reducing
the likelihood of fire during overload. The KISS and U.L. requirements
conflict to a degree that a simple current limiter will not satisfy both
needs therefore a special current limiter circuit had to be devised.
Six Sigma project #27118 Mac3000 Com Port Power Circuit project
addressed this issue and is implemented in this design.
Since currents exceed 1 Ampere and the supply is 12 Volts a linear
current regulator is impractical since the pass element would need a
heatsink. The method chosen here was to use a FET (Q2) as a switch
(a switch is either on or off and in both cases dissipates little power).
In normal operation the ENIOPWR signal is driven high by software to
activate the power switch. This signal saturates transistor Q103 which
provides the gate drive for the dual FET Q2. Both P channel FETs of Q2
are used and therefore are connected in parallel. Return current from the
load is sensed by shunt resistor R4 (0.1
amplifier to boost this current sensed signal. U5 is used as an integrator
which integrates the amplifier current limit signal before entering
comparator U6. When the current exceeds the comparator threshold the
MAC™ 5000 resting ECG analysis system
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Ω
). U7 is used as a differential
Revision B