Fluke 2625A, 2635A service manual Input Protection

Theory of Operation (2620A/2625A)	2
Detailed Circuit Description

The Analog Measurement Processor (A3U8) is a 68-pin CMOS device that, under control of the A/D Microcontroller (A3U9), performs the following functions:

∙Input signal routing

∙Input signal conditioning

∙Range switching

∙Passive filtering of dc voltage and resistance measurements

∙Active filtering of ac voltage measurements

∙A/D conversion

∙Support for direct volts, true rms ac volts, temperature, resistance,and frequency measurements

Two separate signal paths are used, one for dc/ohms/temperature and one for ac. The volts dc (3V range and below) and temperature voltages are coupled directly to the a/d converter, while higher voltages are attenuated first. For ohms, the dc circuitry is augmented with an internal ohms source voltage regulator controlled through an extra set of switches. For volts ac, inputs are routed through the ac buffer, which uses the gain selected by the Measurement Processor (A3U8).

The a/d converter uses a modified dual-slope minor cycle method. The basic measurement unit, a minor cycle, consists of a fixed time integrate period for the unknown input, a variable time reference integrate period, a variable time hold period, and various short transition periods. A minor cycle period lasts for 25 ms or until a new minor cycle is begun, whichever comes first.

2-51. Input Protection

The instrument measurement circuits are protected when overvoltages are applied through the following comprehensive means:

∙Any voltage transients on channel 0 HI or LO terminals areimmediately clamped to a peak of about 1800V or less by MOVs A3RV1and A3RV2. (This is much lower than the 2500V peaks that can beexpected on 240 VAC, IEC 664 Installation Category II, ac mains.)

∙Fusible resistors A3R10 and A3R11 protect the measurement circuitryin all measurement modes by limiting currents.

∙A3Q11 clamps voltages exceeding 0.7V below and approximately 6.0Vabove analog common (LO) or LO SENSE, with A3R35 limiting the inputcurrent.

∙A3Q10 clamps voltages during ohms measurements with A3RT1, A3R34,A3R10, and A3Z4 limiting the input current. With large overloads,thermistor A3RT1 will heat up and increase in resistance.

∙A3U8 also clamps voltages on its measurement input pins that exceedthe VDD and VSS supply rails. Resistors A3R42, A3R11, A3R10, A3RT1,A3Z4, A3R35, and A3R34 limit any input currents.

∙Any excessive voltages that are clamped through A3U8 to VDD or VSS,are then also clamped by zener diodes A3VR3 and A3VR2.

∙The open thermocouple detect circuitry is protected against voltagetransient damage by A3Q14 and A3Q15.

∙When measuring ac volts, the ac buffer is protected by dual-diodeclamp A3CR1 and resistor network A3Z3.

∙Switching induced transients are also clamped by dual-diodeA3CR4 and capacitor A3C33, and limited by resistor A3R33.

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