Fluke 5720A service manual Calibration Hardware, ADC Amplifier, ADC Input Selection

Theory of Operation 2

Analog Section Detailed Circuit Description

2-112. Calibration Hardware

The main components of the calibration hardware are the adc amplifier and the adc (analog to digital converter). This adc circuitry converts dc analog voltages into 22-bit binary numbers which the software interprets.

2-113. ADC Amplifier

ADC amplifier circuitry is located on sheet 4 of the DAC schematic. The adc amplifier is used like a null detector. It has two inputs (inverting and noninverting) and a single output with ADC AMP OUT and ADC AMP SENSE connected together.

The noninverting input (+INPUT) is switched between ADC COM, RCL, or DAC SENSE CAL by relays K6 and K7. DAC SENSE CAL is the output of the DAC and RCL is the calibration line which other assemblies use during their calibration.

The inverting input (-INPUT) is switched between DAC SENSE CAL and REFCAL by relay K5. ADC amplifier inputs are high impedance. The output voltage is the voltage difference between the inputs multiplied by the overall adc amplifier gain of 11.

To determine adc amplifier output, the following formula is used: (Noninverting input - inverting input) x 11 = adc amplifier output. For example, if the noninverting input is a 5.0V and the inverting input is at 5.1V, the output would be -1.1V. (The calculation for this example is (5.0V - 5.1V) x 11 = -1.1V.) Op amp U20A configured as an amplifier with an inverting gain of 1 is used to cancel the current in ADC COM generated from op amp U19A.

Zener diodes VR19 and VR20 keep the output of the adc amplifier from exceeding ±4.0V. Similarly, this protection is provided for the adc amplifier inputs by VR17, VR18, VR21 and VR22.

2-114. ADC Input Selection

The input to the adc chip, U25, is selected by a quad FET analog switch array, U23. A large filter (R74 and C84) and a buffer (U24) are put on the adc input line to filter out 60 Hz and 190 Hz before it is connected to the adc chip input (pin 22).

During calibrator diagnostics, control line PC2 selects the SDL (system diagnostic line) line, which is used by other analog assemblies to monitor their diagnostic voltages.

During diagnostics of the DAC assembly control, line PC3 selects DAC HI DIAG which is divided by R79 and R84. Control line PC4 selects DCAMP HEATER and REF HEATER which are summed and divided by R80, R81, and R83.

Since DCAMP HEATER and REF HEATER are referenced to FR1 COM, DAC LO DIAG is buffered by U22 to provide the proper current return.

During calibrator calibration, control line ADC OUT SEL selects the output of the adc amplifier.

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