Fluke 2625A, 2635A service manual Function Relay States Relay Position A3K17, A3K16, A3K15

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

Table 2-5. Function Relay States

	Relay Position
Function		A3K17
DC mV, 3V,Thermocouples		Reset
DC 30V, 300V		Set
ACV		Set
Ohms, RTDs		Reset
Frequency		Set

A3K16

Set

Set

Set

Reset

Set

A3K15

Set

Set

Reset

Set

Reset

2-54. DC Volts and Thermocouples

For the 3V and lower ranges (including thermocouples), the HI input signal is applied directly to the A3U8 analog processor through A3R11, A3K17, and A3R42. Capacitor A3C27 filters this input, which the analog processor then routes through S2 and other internal switches, through the passive filter, and to the internal a/d converter. The LO SENSE signal is applied to A3U8 through A3R35 and routed through internal switch A3U8-S19 to LO of the a/d converter.

Guard signals MGRD and RGRD are driven by an amplifier internal to A3U8 to a voltage appropriate for preventing leakage from the input HI signal under high humidity conditions.

For the 30V range, the HI signal is scaled by resistor network A3Z4. Here, the input is applied to pin 1 of A3Z4 so that an approximate 100:1 divider is formed by the 10-MΩand 100.5-kΩresistors in A3Z4 when analog processor switches S3 and S13 are closed. The attenuated HI input is then sent through internal switch S12 to the passive filter and the a/d converter. Input LO is sensed through analog processor switch S18 and resistor A3R34.

For the 300V range (Figure 2-4), the HI signal is again scaled by A3Z4. The input is applied to pin 1 of A3Z4, and a 1000:1 divider is formed by the 10-MΩand 10.01-kΩresistors when switches S3 and S9 are closed in A3Z4. The attenuated HI input is then sent through internal switch S10 to the passive filter and the a/d converter. LO is sensed through analog processor switch S18 and resistor A3R34.

2-55. Ohms and RTDs

Resistance measurements are made using a ratio ohms technique, as shown in Figure 2-

5.A stable voltage source is connected in series with the reference resistor in A3Z4 and the unknown resistor. Since the same current flows through both resistors, the unknown resistance can be determined by multiplying the ratio of the voltage drops across the reference and the unknown resistors by the known reference resistor value.

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.

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