SectionIII

3-149. Sample/HoldAC Trigger. This ac coupledcommand has the sameeffect as the Sample/HoldTrigger command (TTL). This signal must be a negative-going pulse at least 30 nanosecondswide with an amplitude between 2Y and 200 V. The signalmust be static at least2 ps prior to the negativetransition.

3-150. Hold Command. This is an intemal commandto

aSample/Holdamplifier,resultingin a Hold mode.Hold A command switchesAmplifier A and Hold B switches Amplifier B. Both commands are generated in the Sample/Holdlogic circuits.

3-151. External Encode. This command initiates a

3490A measurementwhen the SampleRate control is set to HOLD. This signalmust be from HIGH to LOW for a minimum of 240 ps.

3-152Special.S/H0peratingGonsiderations.

3-153. Display. When the 3490A is operatingin either the Track/Hold or Acquire/Hold mode, the fifth digit in

the display is normally blanked, leaving a display of 4 full digits plus the overrange"1". This is done because

of the uncertainty in the fifth digit due to noise. However, if the instrument is equipped with the Data Output (BCD) Option 021 or the GPIB Option 030, the output data includes the fifth digit. The fifth digit can be restored to the display if desired by changing the connection of the white/black wire from the display assembly. For a 4 - digit S/H display, this wire is connected to Test Point L on the Main Circuit Assem- bly, Al, and for a 5 - digit display,it shouldbe movedto Test Point M. Thesetest points are located on the Main Circuit Assemblyjust to the rear of P2, which is the display cable connector.

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3-154. .1 V Range.Sample/Hold measurementaccuracy is not specifiedfor the .l V range.On this range,the output of the Input amplifier contains an appreciable amount of widebandnoisedue to the broad bandwidth of the amplifier and the amplifier gain of X100. The rapid responsetime of the Sample/Hold circuits allows the Hold mode to occur anywhere rvithin the envelope of the noise-

3-155. Autoranging. Autoranging requires successive readings (initiated internally) when changing ranges. Consequently,the final readinglosesits time relation-

Model34904

ship to the extemal trigger and therefore is not useful Sample/Holdinformation.

3-156Guard.ConnectioninSample/HoldMeaurements.

3-157. The Guard terminal should alwaysbe connected to the Low input terminal when making Sample/Hold measurements,unless the guard can be properly driven by a low-impedanceand low-noisesource.

3-158Input.SignalLimitations.

3-159.The analog-to-digitalconversionprocessrequiresa certain amount of time, and any changein the voltage input to the A-to-D conversioncircuits during this time

degradesthe accuracyof the measurementThe. purpose of Sample/Hold is to "fresze", or hold, a changinginput

voltage at a specific point in time and accurately measure the voltage. The bandwidth of the Input Amplifier and the responsetime of the Sample/Hold Amplifiers restrict the rate at which the voltage to be measuredcan change.The Sample/Holdcircuits areable to maintain tracking only if the rate of change of the input voltageis within the follo,winglimits:

2.5%rangelpson the l0 V and 1000V ranges.

5 % of rmgelpson the I V and 100 V ranges.

The rate of changein input voltage (dv/dt) affects the ability of the instrument with SampleiHold to digitize a ramp or sine wave. Table 3-8 showsthe maximum dV/dt for a ramp and the maximum frequency for a full-range sine wave to be measuredwithin the accruacygiven. The input signal is also limited to a maximum dV/dt during digitizationof 50 V/ss.

3-160Sample/Hold.TriggerSignalRequirements.

3-161. Either of two signal inputs may be used to initiate a Hold mode. The Sample/HoldTrigger input

(TTL) is dc coupled, and the AC Trigger input, of course,is ac coupled.The term "Sample/Hold Trigger"

is usedas a generalterm in this manualto refer to either signal. tn some option combinations,a Sample/Hold Trigger does not initiate a 3490A measurement(see Paragraph3-165).

AccJracy*

(% of Rangef

Table3-8. Ability of Sample/Hotdto Digitizea Rampor SineWave.

1 0 v

B A N G E

1 V, 100V, 1000v

 

 

Sine Wave

Sine Wave

 

Sine Waw

Sine Wave

 

Ramp

lZero Crossing)

(Peak Reading)

Ramp

(Zero Crossing)

{Peak Reading)

.01 %

30v/s

5 H z

75OHz

12.5V lS

2 H z

3OOHz

1 %

300v/s

5OHz

275OHz

1 2 5 V / S

20 Hz

90OHz

1 %

3000v/s

5O0Hz

75OOHz

1250V/S

2OOHz

3ü)O Hz

*Accuracy specificationsare to be added to (% of range) dc specification.

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