Fluke 2635A, 2625A service manual A/D Converter PCA

HYDRA

Service Manual

2-6. Serial Communication (Guard Crossing)

This functional block provides a high isolation voltage communication path between the Digital Kernel of the Main PCA and the microcontroller on the A/D Converter PCA. This bidirectional communication circuit requires power supply voltages from the Power Supply block.

2-7. Digital Inputs and Outputs

This functional block contains the Totalizer, Totalizer Debouncer, eight bidirectional Digital I/O channels, four Alarm Outputs, and the Input Threshold control circuits. These circuits require power supply voltages from the Power Supply, a reset signal from the Display PCA, and signals from the Digital Kernel.

2-8. A/D Converter PCA

The following paragraphs describe the major blocks of circuitry on the A/D Converter PCA.

2-9. Analog Measurement Processor

The Analog Measurement Processor (A3U8) provides input signal conditioning, ranging, a/d conversion, and frequency measurement. This custom chip is controlled by the A/D Microcontroller (A3U9). The A/D Microcontroller communicates with the Main PCA processor (A1U4) over a custom serial interface.

2-10. Input Protection Circuitry

This circuitry protects the instrument measurement circuits during overvoltage conditions.

2-11. Input Signal Conditioning

Here, each input is conditioned and/or scaled to a dc voltage for measurement by the a/d converter. DC voltage levels greater than 3V are attenuated. To measure resistance, a dc voltage is applied across a series connection of the input resistance and a reference resistance to develop dc voltages that can be ratioed. DC volts and ohms measurements are filtered by a passive filter. AC voltages are first scaled by an ac buffer, converted to a representative dc voltage by an rms converter, and then filtered by an active filter.

2-12. Analog-to-Digital (A/D) Converter

The dc voltage output from the signal conditioning circuits is applied to a buffer/integrator which charges a capacitor for an exact amount of time. The time required to discharge this capacitor, which is proportional to the level of the unknown input signal, is then measured by the digital counter circuits in the Analog Measurement Processor.

2-13. Inguard Microcontroller Circuitry

This microcontroller (and associated circuitry) controls all functions on the A/D Converter PCA and communicates with the digital kernel on the Main PCA. Upon request by the Main PCA, the inguard microcontroller selects the input channel to be measured through the channel selection circuitry, sets up the input signal conditioning, commands the Analog Measurement Processor to begin a conversion, stops the measurement, and then fetches the measurement result. The inguard microcontroller manipulates the result mathematically and transmits the reading to the digital kernel.

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