HYDRA

Service Manual

2-59. A/D Converter

Figure 2-7 shows the dual slope a/d converter used in the instrument. The unknown input voltage is buffered and used to charge (integrate) a capacitor for an exact period of time. This integrator capacitor is then discharged by the buffered output of a stable and accurate reference voltage of opposite polarity. The capacitor discharge time, which is proportional to the level of the unknown input signal, is measured by the digital circuits in the Analog Measurement Processor. This time count becomes the conversion result.

+ REFERENCE

 

 

 

(– INPUT)

 

 

 

+

 

 

+

 

 

 

REFERENCE

 

 

COUNTER

 

 

 

_

_

 

A3C13

A/D

 

 

 

COMPARATOR

 

INTEGRATE

S77

 

 

 

 

–REFERENCE

REFERENCE

 

 

 

 

 

(+ INPUT)

 

 

 

 

+

A3Z2

 

 

 

 

INPUT HI

_

_

 

 

 

 

BUFFER

+

 

 

 

INTEGRATOR

 

INPUT LO

INTEGRATE

 

 

INPUT

 

 

 

 

 

 

 

 

s7f.eps

Figure 2-7. A/D Converter Simplified Schematic

In both the slow and fast measurement rates, the a/d converter uses its ±300 mV range for most measurement functions and ranges. The primary exceptions are that the 3V dc range is measured on the a/d converter 3V range, thermocouples are measured on the ±100 mV range, and the temperature reference is measured on the 1V a/d converter range. The typical overload point on a slow rate 30000 count range is 32000 display counts; the typical overload point on a fast rate 3000 count range is 3200 display counts.

During the integrate phase, the a/d buffer in the A3U8 Analog Measurement Processor applies the signal to be measured to one of the four integrator input resistors in network A3Z2. As shown on the A/D Converter schematic diagram in Section 8, the choice of resistor selects the a/d converter range. Switch S69 connects the buffer output through pin B.1 for the 100-mV range, S71 connects the output through B.32 for the 300 mV range, S73 connects to pin B1 for the 1V range, and S75 sets up the 3V range through pin B3.2.

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Fluke 2635A, 2625A service manual 59. A/D Converter

2635A, 2625A specifications

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