Teledyne M300EM Motherboard, A to D Conversion, Sensor Inputs, Thermistor Interface

Teledyne API – Technical Manual - Model 300E Family CO Analyzers Theory of Operation

11.5.6.MOTHERBOARD

This printed circuit assembly provides a multitude of functions including, A/D conversion, digital input/output, PC- 104 to I2C translation, temperature sensor signal processing and is a pass through for the RS-232 and RS-485 signals.

11.5.6.1.A to D Conversion

Analog signals, such as the voltages received from the analyzer’s various sensors, are converted into digital signals that the CPU can understand and manipulate by the analog to digital converter (A/D). Under the control of the CPU, this functional block selects a particular signal input (e.g. BOX TEMP, CO MEAS, CO REF, etc.) and then coverts the selected voltage into a digital word.

The A/D consists of a Voltage-to-Frequency (V-F) converter, a Programmable Logic Device (PLD), three multiplexers, several amplifiers and some other associated devices. The V-F converter produces a frequency proportional to its input voltage. The PLD counts the output of the V-F during a specified time period, and sends the result of that count, in the form of a binary number, to the CPU.

The A/D can be configured for several different input modes and ranges but in the M300E/EM is used in uni- polar mode with a +5 V full scale. The converter includes a 1% over and under-range. This allows signals from –0.05 V to +5.05 V to be fully converted.

For calibration purposes, two reference voltages are supplied to the A/D converter: Reference Ground and +4.096 VDC. During calibration, the device measures these two voltages, outputs their digital equivalent to the CPU. The CPU uses these values to compute the converter’s offset and slope and uses these factors for subsequent conversions.

See Section 7.4.3 for instructions on performing this calibration.

11.5.6.2.Sensor Inputs

The key analog sensor signals are coupled to the A/D through the master multiplexer from two connectors on the motherboard. 100K terminating resistors on each of the inputs prevent cross talk from appearing on the sensor signals.

CO MEASURE AND REFERENCE

These are the primary signals that are used in the computation of the CO concentration. They are the demodulated IR-sensor signals from the sync demodulator board.

SAMPLE PRESSURE AND FLOW

These are analog signals from two sensors that measure the pressure and flow rate of the gas stream at the outlet of the sample chamber. This information is used in two ways. First, the sample pressure is used by the CPU to calculate CO concentration. Second, the pressure and flow rate are monitored as a test function to assist the user in predicting and troubleshooting failures.

11.5.6.3.Thermistor Interface

This circuit provides excitation, termination and signal selection for several negative-coefficient, thermistor temperature sensors located inside the analyzer. They are as follows:

SAMPLE TEMPERATURE SENSOR

The source of this signal is a thermistor located inside the sample chamber of the Optical Bench. It measures the temperature of the sample gas in the chamber. This data is used to during the calculation of the CO concentration value.

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