Power Supply-Protection Description

Preamp

Isolated Power Supply

The Isolated Power Supply provides the isolated +15Vdc, +12Vdc, -15Vdc, +5Vdc, and -12Vdc supplies required by the Preamp circuitry. The drive for transformer Ti is generated by pulse-width modulator U5. Capacitor C12 and R28 set the frequency between 12kHz and 24kHz. The secondary of T i is applied to full-wave bridge CR20. The positive half of the ac input waveforms are stored at C87, forming the isolated +15V supply. The negative ac waveforms are stored at C86, forming the isolated -15V supply. Regulators U38 and U39 isolate more sensitive circuitry from the heavier loads. Since the +5Vdc loads are synchronous and small, U37 doubles as a regulated supply and a reference source.

Front-End Protection.

To protect against voltage surges, VSP 1 through VSP3 limit the input current to 2mA. Isolation for the paddles input is provided by R131 and R132 which guard against excessive charge input to the Preamp. Diodes CR18, CR19, and CR28 clamp the differential paddle voltage when KI is switched to the leads mode.

Lead Select.

To select between paddles and the different patient leads, latch U33 is loaded with the decoded data from counter U32. Flip-flop U33 controls relay K1, U43A, and analog switches U42 and U41. To select between patient leads or paddles, U33 causes Q8 or Q9 to conduct, switching the state of K1 to the desired position. Patient leads are selected when Q8 conducts; paddles are selected when Q9 conducts. If patient leads are selected, analog switch U42 selects the lead (1,11,111, or STD) requested by the output of U33.Optical Interface.

Command pulses from the System microprocessor pass through the the optical interface to perform three different functions: synchronize the ECG conversion, select 1 of 5 patient cable leads, and induce a +1mV calibration signal. The System microprocessor uses an internal timer to transmit a pulse of variable duration (approximately 80ms to 1.2ms) and a fixed-frequency period (540Hz). The variable-duration pulse from the System microprocessor enables U4 to start counting, creating a 16kHz pulse train at U4 pin 4.

The System microprocessor controls the count of the pulse train by interrupting the output with a CLR signal as shown at TP6 in Figure 1-5. The 16kHz pulse drives the infrared-emitting diode in U7 providing pulse isolation. Comparator U34D performs waveform conditioning for the pulse train as it passes to the Instruction Decoding circuit, U32 and U33.

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