Fluke 8842a instruction manual Display, 23. A/D Amplifier, Bootstrap Supplies

8842A

Instruction Manual

The bit-switches determine the output voltage of U102B by controlling the binary ladder network. The gain of U102B is set by the ratio of a precision feedback resistor (Z101-7, - 8) and the equivalent output resistance of the ladder network.

5-23. A/D Amplifier

The A/D Amplifier is composed of a comparator/amplifier (U103), two remainder- storage capacitors (C103 and C102), an autozero storage capacitor (C101), and several digitally controlled analog switches contained in U101.

The A/D Amplifier has three modes of operation: autozero mode, where any offsets in the A/D input are stored on C101 so as to be cancelled later; compare mode, where the A/D input is compared to the DAC output; and remainder-store mode, where U103 amplifies and stores the difference between the A/D input and the DAC output on one of the two remainder-storage capacitors (C102 or C103). The autozero mode is shown in Figure 5-11. The other modes are shown in Figures 5-9 and 5-10.

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Figure 5-11. Autozero Period

5-24. Bootstrap Supplies

The bootstrap supplies are composed of U102A, Q101, Q102, CR103, CR104, and associated components. The bootstrap supplies enhance the gain accuracy of U103. During compare periods, the bootstrap supplies limit the output of U103 to minimize the time it takes to recover from being driven to a supply rail. Both functions are achieved by manipulating the supplies of U103 (BS1 and BS2).

5-25. DISPLAY

The vacuum fluorescent display is similar to a vacuum tube, containing eight control grids and 69 phosphor-coated plates which form the display segments and annunciators. (See Figure 5-12.) The filament voltage is 4.5V ac, with a +5V dc bias. Each plate is controlled by a G line and a P line. The G lines go to the control grids, and the P lines go to the plates.

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