Directed Audio 6550 Service Manual download pdf (Page 26)

Electronic Theory of Operation

Main Board

Model 515B/C Service Manual Rev. 01

signals control the sensor LED drive and the photodiode’s return path circuitry during normal

operation and calibration.

The RESET and SC1 lines control when the outputs of IC13 are active, both these lines must be

low in order for IC13 to operate normally. The RESET line controls IC13 during power up,

while the SC1 line is under processor control and will toggle when a probe off patient alert exists

and during the power up self test.

5.2.9

Sensor Status Parameters

The sensor (and battery) status parameters input to the multiplexer IC25 (page 3 on schematic)

are described below.

ADCFEDC: Photodiode DC Level.

Resistors R66, R67 and capacitor C33 form a voltage divider and low pass filter that provide a

measure of the mean DC level at the output of the photodiode current-to-voltage amplifier

IC21b pin 7. This signal is used in determining ambient light interference. If this line is

examined while the sensor’s Red and Infrared LEDs are turned off, then any DC level at IC21b

pin 7 must be the result of ambient light impinging on the photodiode. If the DC shift is in excess

of limits set in the software, a light interference message appears on the monitor’s display.

ADCLPWR: Sensor LED Supply Voltage.

This channel, at IC25 pin 12, monitors the sensor LED supply voltage through a voltage divider

consisting of R113 and R114. If a fault occurs that causes the LED supply fuse F201 to blow,

or if the sensor wires are shorted, this channel reports the condition and the monitor will indicate

an error condition.

ADCIRLED: Infrared LED Cathode Voltage.

A low pass filter consisting of R60, R61 and C51 provides a means to measure the cathode

voltage of the sensor’s Infrared LED. If the channel at IC25 pin 5 is sampled the monitor can

determine if the LED is open circuit (zero volts at IC25 pin 5) or operational (approximately 2.5

volts at IC25 pin 5).

Signal Description

INSIG*

Input Signal

This line will enable signals from the photodiode, or prevent sig-

nals from the photodiode from reaching the detection circuitry.

RDLED*

Red LED

Controls the signals for the Red Led in the sensor.

IRLED*

Infrared LED

Controls the signals for the Infrared Led in the sensor.

SIGND*

Signal Ground

This is used to short out the inputs of the detection circuitry so that

the system can compensate for offsets.

ASAMP*

Analog Sample

This line is used to short out the capacitor used in the sample and

hold circuitry to avoid having residual charge interfere with data

sampling.

SYNC

Synchronization

Synchronization signal used for the -VA supply.

IRSAMP*

Infrared Sampling

Used for sampling the Infrared signal response from the photo-

diode.

RDSAMP*

Red Sampling

Used for sampling the Red signal response from the photodiode.

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