2A-51.Input Signal Conditioning

Theory of Operation (2635A) 2A

Detailed Circuit Description

∙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.

Each input is conditioned and/or scaled to a dc voltage appropriate for measurement by the a/d converter. DC voltage applied to the a/d converter can be handled on internal ranges of 0.1V, 0.3V, 1V, or 3V. Therefore, high-voltage dc inputs are scaled, and ohms inputs are converted to a dc voltage. Line voltage level ac inputs are first scaled and then converted to a dc voltage. Noise rejection is provided by passive and active filters.

2A-52.Function Relays

Latching relays A3K15, A3K16, and A3K17 route the input signal to the proper circuit blocks to implement the desired measurement function. These relays are switched when a 6-millisecond pulse is applied to the appropriate reset or set coil by the NPN Darlington drivers in IC A3U10. The A/D Microcontroller A3U9 controls the relay drive pulses by setting the outputs of port 6. Since the other end of the relay coil is connected to the VDDR supply, a magnetic field is generated, causing the relay armature and contacts to move to (or remain in) the desired position. Function relay states are defined in Table 2A-5.

Table 2A-5. Function Relay States (2635A)

	Relay Position
Function		A3K17
DC mV, 3V,Thermocouples		Reset
DC 30V, 300V		Set

ACV		Set

Ohms, RTDs		Reset

Frequency		Set

A3K16

Set

Reset

Set

A3K15

Set

Reset

Set

Reset

2A-53.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.

2A-25

2635A, 2625A specifications

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Fluke 2625A 2A-51.Input Signal Conditioning, Function Relay States 2635A Relay Position A3K17

Models: 2635A 2625A