SectionIII

pose Interface Bus controi, the 3490A should be programmed for Next External Trigger (T2). WhenNext External Trigger is programmed, upon completion of a measurement the GPIB circuits wait for an External Trigger command. Either of the Sample/Hold Trigger inputs at Jll providesan External Triggercommandto initiate a measurementat the sametime a Hold mode is initiated.

3-172. AcquisitionTime. The maximum time required by the DC Input and Sample/HoldAmplifiersto respond to an input voltage varies with the range selected.as shownin Table3-10.The acquisitiontime-mustbe taien into consideration in Track/Hold operation. The delay incorporated into Acquire/Hold operation includes thl acquisition time. Figure 3-l I shows typical times re_ quired for the amplifiers to respond within a specified accuracyto a stepinput voltage.

Table3-10. MaximumAcquisition (Settingl

Time for Full-RangeStep Input.

 

Settling Time to Within

Range

0.01 % of Final Vatue

1 V

512 tts

1 0 v

128 ps

100v

512 ps

1000v

128 ps

NOTE

.1 V rangeis not specified(seeparagraph

3-1541.

uJ (9

z

tr 200

J

J 100

tt tJ-

a

=

t o

uJ

F at,

lrJ

o

F

J

1 . 0

F

=

t!

UJ 0 . 1

a

E

J

1 V R A N G E

6.6t ^

u . l ,,rs 1 . 0ps IOps IOOps lms

TYPICALTIME REOUIREDTO SETTLLTO O.OTX

Model3490A

3-173Using.theTrack/HoldMode.

3-174.Digitizing a Ramp. The output of the Sample/ Hold amplifiercircuitslagsthe 3490input yoltageby a

delay which is approximately equal to " " I , where L 1 3 d B

f:On ir the 3 dB bandwidth frequency shown in Table l-2. Whenmeasuringa stabledc input voltage,this time lag presentsno problem.However,if the input voltageis changing, as a ramp input for example, thii delay must be consideredin interpreting the 34gOA reading. In Track/Hold measurements,the voltagereadingis actu_ ally the input voltageat a point previousto the time the Sample/HoldTriggercommandwas received.The point of measurement,then, effectively precedesthe trigger

command b-ya length of time equal to the delay, shown in Figure 3-12 as the analog dähy. Digitization of a

repetitivestable ramp may be done as illustrated in Figure 3-12. The delay betweenthe start of the ramp and the Sample/HoldTriggermust be accuratelydetei_ mined by somemeans,suchasan intervaltimer. It is not necessarythat measurementsbe made on successive cyclesas might be inferred from Figure 3-12. Measurement may be made on every 2nd, 3rd or nth cycle as convenient,if the waveform is stable. The maximum dv/dt limitations shown in paragraph3-l5g and Table 3{ must be observed.

3-175. Reoonstructingan Input Waveform. An input waveform may be reconstructed from a series of Track/Hold measurementsby usingthe secondequation

.lLl

.z

(r 200

J

J 100

f

IL

o

öR

=

o- 1 0

LIJ

F

.tt

Ll,

(9

F.

J

o1 . 0

F f,

=

tJ.

o

uJn 0 . 1

t-

=

o-

0.o1

0.1 lrs 1.0rrs 10,/s 100rc

TYPICALTIMEREOUIREDTO SETTLETO1 %

Figure3-11. TypicatResponseto a StepInput Voltage.