Section IV

Model3490A

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oTr Prior ro the run{p portion ot the m@su.erent sequence,the 3490A inerts any delay requi.ed by the smple rate control sening, and any delav rhat may be required by the fundion and range selections. During lhis time. the input to the DC Amplifier is grounded and this amplifier and the Integrator arc in lhe Auto-Zero state (see Para$aphs 4-37 and 4-5O).

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Following any delay required, the input signal is applied to the DC Amplifier and sufficient time is allowed lo. the amplitier to settle belore the run{p period begins.

The Integrator charges tor a fi xed p6iod of ti me to a rcltage proponional lo the input rignal .

Signal pola.iry is determined and the prop€r referen@ wltage is plected- The relerence is applied to rhe DC Amplilier in plee of the input signal.

The liltegrator discharge. at a tixed rate which is determined by the reference wltage.

llthe instrument is in autoJange, it determin6 whether the reading is on the @rrect range. It not, it upranges or downranges ooe range aod.initiat€s another reading.

This is an Auto-Zero period of 65 ms. (See Paragfaphs 4-37 "nO q-SO.l

s 4 9 0 - a - 5 5 ! 3

Ftgure4-2. BasicDiagramand Operation.

circuits, and is dependent upon the samplerate switch setting, as well as the function and range selected.In auto-rangeoperation, the logic circuits selectthe correct range. The input polarity information is utilized by the logic circuits to selectthe correct referencepolarity for run-down.

415. Display.

,1-16.The display consistsof six digits; however,the most significantdigit is either zero or an overrange"1" during normal measurements.During the logic test operation (Test No. 1), other numbersare displayedin

this dist. The display also contains a polarity symbol and an "overload" annunciatorlight. If the instrument

hasa RemoteControl option, a "REM" annunciatoralso lights*tren remoteoperationis selected.

4-17.RangeandFunctionControl.

4-18. Rangeand function may be selectedby the front panelswitchesor remotely,if the instrumentis equipped with one of the remotecontrol options. Remotecontrol is providedby Option 022 paragraphLl74), or by the General Purpose Interface Bus I/O Option 030 (Para-

$aph4-196).+19 .DCANAL0GCIRCUITS .

ar20. In general,the following explanationsof both the analog and digital circuits describethe circuits outlined in the block diagramshownin Figure7-18.

421. lnputAttenuator.

4-22. A simplified diagam of the Input Attenuator and DC Amplifier circuits is shown in Figure 4-3. The Input

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