Model3490A

SectionIII

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SEETABLE 3.7

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Figure3-10. Exampleof MarkedCardprogramming.

2.The line on the marked card immediately fol- lowing the last line in which MRE is pulled LOW (,,200- space marked) should be left blank. This enablesthe 3490A to be in the programmingroutine before the first programming character is sent. If this line is not left blank, the characteron that line will be ignored.

3-139SAMPLE/H0LD.MEASUREMENTS(0ption 040or045).3-140Definition.ofSample/HoldTerms

3-141. Track/Hold. This term describesthe mode of operation in which the "Hold" mode (see paragraph

3-14,beginswithin 400 nanosecondsafter receiptoi a Sample/HoldTrigger command.Prior to this time, the Sample/Hold amplifiers follow, or track, the output of the 34904 DC Input Amplifier. Upon receipf of a SampleiHold Trigger command, the amplifiers ..hold" for measurementthe voltage presenton the Amplifier A

integrating capacitor at that time. After the 3490A completesa measurement,the .track,'moderesumes.

3-142.Acquire/Hold. This mode differs from the Track/Hold mode in that a precisedelayis addedin the Sample/Hold trigger path. This delay allows the DC

lnput and Sample/Hold amplifiers to respond to a full-rangestep input voltage before the Äold mode begins.

3-143.Hold Mode. Following a Sample/Hold Trigger command, the Sample/HoldAmplifierJ A and B retain

Ior measurementthe voltage present at Amplifier A when the Hold command was received.During this retention period. the Sample/Holdamplifiersaresaidto

be in the Hold mode. Theseamplifiersarealsoplacedin

the Hold mode during the run-down, or discharge, portion of the measurementcycle.

3-1u14Track. Mode. After completion of a measure_ ment, the Sample/Holdamplifiersfollow, or track, the input voltageuntil they are placedin the Hold mode by

asubsequentSample/HoldTriggercommand.This con_ dition is referredto asthe Track mode.

3-145.Aperture Time. This is defined as the period betweenreceipt of a Sample/HoldTriggercommandand

the time at which the Sample/Hold switching circuits

.placeAmplifier A in the Hold mode.

F3-146.Maximum Acquisition Time. This is the time required for the DC Input and Sample/Holdamplifiers to respondto a full-rangestepinput voltage.

3-147.Delay. This term, as used in connection with Acquire/Hold operation,refersto the delayaddedin the Sample/Hoidtrigger path which extends the aperture time to includethe acquisitiontime.

3-148. Sample/HoldTrigger(TTL). This is the dc coup led commandto the Sample/Holdcircuitswhich initiates a Hold mode. The sigral must go from HIGH to LOW for a minimum of 30 nanosecondsIt. must go HIGH at least600ps prior to goingLOW.(SeeFigure2-l l.) This commandwill initiate a 3490A measurementonly under

certain conditions (see Paragraph3-165). The term "Sample/HoldTrigger" is usedas a generalterm in this

manua.l,referring either to a dc coupledor ac coupled triggersignal.

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