2A-27

Theory of Operation (2635A) 2A

Detailed Circuit Description

	OHMS
	VOLTAGE
	SOURCE
	IX
					LOW
	+	A3Z4	RREF		A/D
	+	A3Z4	RREF		INTEGRATE
VR REF		REFERENCE			INTEGRATE
VR REF		REFERENCE			REFERENCE
	–	RESISTOR			REFERENCE
		RESISTOR
				A3R34
					HIGH
A3K16
A3RT1 & A3R10
	A3R11
	HI			A3R42	HIGH
	HI	A3K17		PASSIVE	HIGH
		A3K17		FILTER
				FILTER
	+	R X
VR X		R X			A/D
VR X		UNKNOWN			A/D
	-	UNKNOWN			INTEGRATE
	-	RESISTOR			INTEGRATE
		RESISTOR			UNKNOWN
					UNKNOWN
	LO				LOW
					LOW
VR X	IX•RX		RX
VR REF	= IX•RREF =		RREF
					s15f.eps

Figure 2A-5. Ohms Simplified Schematic (2635A)

For the RTD, 300Ω, 3-kΩ, and 30-kΩranges, the ratio technique is implemented by integrating the voltage across the unknown resistance for a fixed period of time and then integrating the negative of the voltage across the reference resistance for a variable time period. In this way, each minor cycle result gives the ratio directly.

For the 300-kΩ, 3-MΩ, and 10-MΩranges, the ratio is determined by performing two separate voltage measurements in order to improve noise rejection. One fixed-period integration is performed on the voltage across the unknown resistance, and the second integration is performed on the voltage across the reference resistance. The ratio of the two fixed-period voltge measurements is then computed by Microcontroller A3U9. The resistance measurement result is determined when A3U9 multiplies the ratio by the reference resistance value.

When an input is switched in for a measurement, the ohms source in Analog Processor A3U8 is set to the correct voltage for the range selected and is connected to the appropriate reference resistor in network A3Z4. A measurement current then flows through A3Z4, relay A3K16, thermistor A3RT1, resistor A3R10, the unknown resistance, A3R43, ground, and the ohms source.

The resulting voltage across the unknown resistance is integrated for a fixed period of time by the A/D Converter through the HI SENSE path of A3R11, A3K17, A3R42 and

2635A, 2625A specifications

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Fluke 2625A, 2635A service manual 2A-27

Models: 2635A 2625A

Figure 2A-5. Ohms Simplified Schematic (2635A)

2A-27

2635A, 2625A specifications