SectionIV

that the data has been accepted, the Outguard ASM signals to the Inguard ASM that new data may be transferred acrossguard. The output of the six digits of measurement magnitude is timed by the same scan signals (HSA, HSB, HSC) that control the 3490A display.

4-211. Talk Only. When the rear panel slide switch is set to the TALK ONU/ position, the 34904 may be operatedwith a printer without the useof a controller. This switch sets the internal Talk Only line LOW, enabling the I/O circuits to output data after each measurement.Operation of the 3490A is programmed by the front panelcontrols.

+212.SAMPLE/H0tD0PTl0Nll40or045.

4-213. When the SampleiHoldoption is installedin the 3490^, it occupiesa position between the DC Input Amplifier and the Analog-to-Digital conversion circuits as illustrated in Figure 4-21. Because Sample/Hold follows the Input Amplifier, it is limited by that amplifier'scharacteristics.Either of two modes of operation may be used;Acquire/Hold (delayedtrigger), or Track/Hold (non-delayed).

Figure4-21. Sample/HoldCircuitPosition.

4-214. Twk/Hold Mode.

__-"

4-215. The Track/Hold Mode is useful in determining the value of a varying input voltage at a specifictime. This is illustratedin Figure4-22.Following a complete

A N A L O G T O

D I G I T A L

C O N V E RS I O N

C I R C U I T S

VOLTAGE

L E V E L M E A S U R E D

A T T H I S T I M E

+ T R A C K M O o E

O M O D E E E G I N S W I T H I N

N A N O S E C O N O SA F T E R

E l P T O F T H E S / H T R I G G E R

M E A 5 g R E M E N T p E R r o D - - /

i X ] E R ^ i A L S A M P L E / H O L D T R I G G E R

1490a'B3430

FiEure4-22. WaveformslllustratingTrack/HoldMode.

1-ll

Model3490A

measurement,the Sample/Hold amplifiers are again allowedto track the input signal.

HoLo MooE--._J

MEASUREMEN,pERroD_Jf

E X T E R N A L S A M P L E / H O L D

T R I G GE R

3 4 9 0 4 B ' 3 4 1 1

Figwe 4-23. WaveformslllustratingAcquire/Hold Mode.

4-216Acquire/Hold.Mode.

4-217. Acquire/Holdoperationis essentiallythe sameas Track/Hold operation, except that a delay is added between receipt of an external Sample/Hold Trigger commandand the time the Hold Mode begins,asshown in Figure 4-23. This delay permits the DC Input and Sample/Holdamplifiersto respondfully to a stepinput voltage before holding and taking a measurement, making it possible to apply a Sample/Hold Trigger commandsimultaneouslywith a step input voltage.The lengih of the delayis dependentupon the rangeselected, asshown inTable 44.

Table44. Acquire/HoldDelay.

Range

 

Delav

 

 

Option 050

Option

060

1 V

6 1 5 . 0 r t s 1 4 0 0 n s

512.6ps t

400 ns

1 0 v

154.0ps t 400 ns

128.4 tts t

4p0 ns

100v

615.0rrs t

400 ns

512.6trs i

zl00ns

1000v

154.0ps t

400 ns

128,4 tts + 400 ns

4-218. Sample/HoldMea$rementSequence.

4-219. Figure 4-31 shows a simplified diagram of the Sample/Hold circuits and describesthe measurement sequenceSample/Hold.Circuit A canrespondquickly to a change in voltage at point 2. However, this circuit cannot hold a fixed level long enough for a complete measurementSample/Hold.Circuit B cannot respondas quickly as A, but can hold a fixed levellong enoughfor

acomplete measurement.In a Sample/Hold measure- ment, the voltageis sampledby Circuit A, then held by A until Circuit B has time to respond.Then the voltage is held by B while the measurementis completed.