GE MAC 5500, Service Manual

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Revision E MAC

5500 resting ECG analysis system 2-27

2020299-020
Equipment Overview: Theory of Operation
transistors are of the same type their Vbe’s track well enough to provide acceptable
output offset.
Diode clamps to ground and +3V-EMI provide ESD protection for the VGA video
and sync signals. The +3V-EMI rail is isolated from ESD transients by FB106.
Acquisition Module Transceiver / Power Switch
MAC 5500 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.
Transceiver
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.
Acquisition Power Regulator / Switch
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 (U18) 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.
COMM Port Power Switch / Current Limiter / KISS Power
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 R6 (0.1

). U9 is used as a differential amplifier to boost this current
sensed signal. U7 is used as an integrator which integrates the amplifier current limit
signal before entering comparator U8. When the current exceeds the comparator