The purpose of the driver section of the CPD board is to individually select the different system
coils and to provide the distinction between transmit and receive modes on these coils by
biasing PIN diodes located at the coils to either pass or prohibit the flow of RF energy.
Pulsed drive signals (amplifier unblank signals) are directed to the various drive elements:
Multicoils, TRs, DD, and body coil drivers as specified by the communications section of the
CPD board. Additionally, open-circuit and short-circuit error detection signals (faults) for each
of the drivers are stored in fault buffers which are read via select lines originating from the
CPD microprocessor. The CPD board reports any faults to the remainder of the system where
appropriate actions are taken.
The multicoil drivers consist of four separate drivers that each output a voltage of –5 volts at
nominally 0.5 amp of current. Each driver receives its own drive signal from the EPLD. The
drive signal goes through a level shift circuit consisting of a comparator and dual rail precision
line driver to obtain +5V and 5V levels. A high power operational amplifier (op-amp)
accurately converts the low-current –5V drive level to a high-current –5V level. External
circuitry around the op-amp determines its gain and frequency response and sets current limits
to 1 amp.
A low value precision current sense resistor and high gain instrumentation amplifier accurately
measure the output current of the op-amp. A window comparator arrangement triggers a fault
signal if the measured value of current does not fall within a small, precise range.