SectionIV

Model3490A

(D

R8

+ I 7 V + I 7 V

I N T E G R A T I N G

-

 

 

OUTPUTOF

20K

 

 

 

 

 

S/H CIRCUII

@

 

 

 

 

 

 

OUTPUT

 

 

 

 

 

 

R34

 

R 5 8

R 3 9

D A

COMPENSATION

 

tooK

S 4 , S 5 , A N D 5 5 A R E

 

 

7 t K

7rK

 

 

 

c t 6

FET SWITCHES . THEY

 

-t7v

 

 

 

ARE ALL OFF (NOT

 

 

@

S 5

o.47

 

 

 

v

C O N D U C T I N G )F O R T H E

 

 

 

 

 

HOLD MODE .

 

 

t8

R36

 

R 6

 

 

 

Ja+z

roo

 

t o K

J 4 9 0A - B - 3 4 3 3

 

 

 

 

 

 

 

 

v

 

 

 

Figure4-25. SimplifiedDiagramof S/H Circuit B.

4-223. Circuit B. A simplified diagramof Sample/Hold Circuit B is shown in Figure 4-25. The operation of Sample/Hold Circuit B is similar to the operation of Circuit A. Circuit B respondsmore slowly than A and is able to hold a constantoutput level long enoughfor an accuratemeasurementto be completed.To allow time for Circuit B to respondto the output of A, the B Hold command occursabout 2 millisecondsafter the A Haid command.The purposeof the amplifiercircuit shownin Figure 4-25 betweenpoints D and F is to compensate for dielectric absorption in the integration capacitor,

c r 2 .

4-224. Dielectric Absorption. Dielectric absorption is the memory effect of a dielectric when the voltage acrossthe capacitoris changedsuddenly.This may be illustratedby the following example:

l. Chargea capacitorto a givenvoltage.

2.Quickly dischargethe capacitorto zero with a smallload resistance.

3.Disconnectthe load resistor.

4.The voltage acrossthe capacitorwill increase slowly from zero in the directionof the voltage applied in step l. This voltage will develop quite slowly, but could contribute several counts of error in the Sample/Holdmeasure- ment.

This phenomenon becomessignificant in the Sample/ Hold inreeratorcircuit when the voltageappliedto this

circuit changesfrom the input signal to the reference voltage,which is oppositein polarity.

4-225. Compensationfor Dielectric Absorption.To prevent deteriorationof the Sarnple/Holdoutput signal due to dielectricabsorption,an amplifier circuit is used betweenthe output of Circuit B and the non-inverting input to the integratorportion of this circuit (pqints D and F in Figure 4-25). The waveforms in Figwe 4-26 help to illustratethe actionof the DielectricAbsorption (DA) Compensationcircuit. During the Track Mode,55 and 36 (Figure 4-25) are closed. Any change in the output voltageat point D causescurrent to flow through the rc circuit R34, Cl6 and R6. This current decreasesas Cl6 assumesthe new voltage. The resulting voltage acrossR6 is appliedto Cll and the DA Compensation amplifier input. When a Hold B command occurs,55 opens and the voltage acrossCl I remainsat the level present at that moment. 56 also opens at this time, allowing Cl8 to beginchargingslowly toward a voltage proportional to the chargeon C11. The compensating voltage at point F is the voltagedevelopedacrossCl8. As the time increasesbetween a voltage changeat the integratingcapacitorand the Hold B command,the need for compensation decreases,and the compensation providedalsodecreases.

4-226. Ottset Amplifier. During Sample/Hold opera- tion, the Auto Zero circuit for the DC Input Amplifier is disabled; consequently, some offset voltage may be present in the amplifier output. This offset will be includedin the voltageheld by Sample/HoldAmplifien

Aand B. During both the run-upand run-downportions of the measurementperiod, the input to the DC Input

I r l