Fluke 35040 Basic Operating Equations, Electrometer, Bias Supply, Multiplexer and A/D Converter

Theory of Operation

Functional Description 2

2.2.5 Electrometer

The electrometer is a single range, resettable, integrating amplifier. The high speed solid state reset is activated either by the operator pressing the RESET/MEASURE button or automatically when the instrument detects the end of an exposure. Errors resulting from temperature drift of the components and from dielectric absorption are dynamically corrected by the controlling firmware.

2.2.6 Bias Supply

The Model 35040 Therapy Dosimeter has a fully regulated electronic bias supply. The user may specify up to 11 settings within the range -500.0 to +500.0 volts, in any order, during customization of the instrument. Typical settings are +300.0, +150.0, 0, -150.0, and -300.0 volts, which conform to the recommendations of the American Association of Physicists in Medicine Task Group 21 (AAPM TG21). Other locality specific protocols can be accommodated within the 11 settings.

During operation, the bias supply output level is periodically measured by the A/D through a resistive divider. If the output is more than ± 1.0 volt from the set voltage, an error message (LoBias, or HiBias) will appear on the display. The bias supply has been designed to drive a capacitive load of less than 5000 pf.

CAUTION

Attaching a larger capacitance may create a potential shock hazard.

For operator safety, the output of the bias supply is energy and current limited. Before attempting to connect or disconnect an ion chamber, the operator should either set the bias voltage to 0 volts or turn off the instrument. Setting the bias voltage to 0 volts pulls the bias to a safe level within one second. The output of the bias supply is de-energized at power down. De-energizing may take 15 to 30 seconds.

2.2.7 Multiplexer and A/D Converter

The output of the electrometer is sampled at approximately 20 times per second by an analog to digital converter. The temperature of the electrometer is measured whenever the electrometer is reset. The program uses the temperature measurement to correct for temperature-induced drift in the electrometer components. The internal temperature may be different from the ambient temperature, especially during the battery high charge rate stage.

2.3 Basic Operating Equations

The instrument makes extensive use of the microcontroller’s processing capabilities to directly implement the operating equations. The variables used in the equations are detailed in the table below. The equations in this section have measurement units enclosed in square brackets as a reminder of the required units, e.g. [C] for Coulombs.

2-3