HP Car 3490A manual +128 .FBONTPANELS1VITCHING, Powersupplies, + t

Model3490A

			EXTERML
			REFERENCE
			AMPLIFIER
		+
EXT REF	BIAS CURRENT
INPUT		VOLTAGEAT	FEEDBACK
	976K
		AMPINPUTS
	tI	APPROXEQUAL	AMPLIFIER
		+ l 7 v

COMPENSATING

CURRENT

90K

t o K

VOLTAGE= INPUT VOLTAGE

SectionIV

TO+REF FET

A ? U ? F

34904- B - 3044

I N P U T

COMPENSATION

AMPLIFIER

Figure4-15. Input BiasCompensation.

with the Bias Adjustment so that the compensating current is equal to the bias current at the External

ReferenceAmplifier inPut.

4-127. ReferencePolarity Logic. In the dual-slopeinte- gration method of measurement,the polarity of the reference voltage used for run-down (integrator dis charge) must be opposite the polarity of the input voltage. The 3490A automatically selects the correct referencepolarity whether the External Referenceinput is positive or negative. Since the external reference voltage in ratio measurements replaces the positive internal reference, the logic signal which selects the rundown referencepolarity must be inverted when the extemal reference is negative. Figure 4-16 shows how this is accomplished.The External ReferenceAmplifier output is inverted and applied to one input of an Exclusive OR Gate. The other input of this gate is connected to the stored Input Polarity signal from the I"ogic circuits. The ExclusiveOR Gate output is HIGH if one and only one input is HIGH. If both inputs are either HIGH or LOW, the output is LOW. Becausethe output of the AC Converter is positive, a negative reference is selected in all ac measurements.The front panel display polarity symbol is + if the input signaland extemal referenceare of the samepolarity, and - if they are of opposite polarity. No polarity symbol is displayed

for acldc ratio measurements.Table 4-2 shows the referencepolarity required for the variouscombinations of input and referencevoltages.

+128 .FBONTPANELS1VITCHING .

4-129. Range, Function, and Sample Rate delay are controlled by HIGH true BCD logic signals from the front panel switches.HOLD and MAN Triggersignalsare LOW true. If the 3490A has a Remote Control option, Rangeand Function information are routed through the Remote Control circuits so that, if desired, these operationsmay be controlled remotely. Unless Auto- rangeis selected,the Rangeinformationis storedby the RangeCounter in the Logic circuits.

4130.POWERSUPPLIES.

4-131. Figure 4-17 is a block diagramof the inguard power supplies.Regulatedsuppliesof t 30 V, t 17 V, 'ändt 5 V provide power to the inguardcircuits of the

"3490A. An outguard + 5 V supply is a part of Options O2O,O2l andO22. The + 17V supply is regulatedby a microcircuit regulator, and its output is accurately adjusted.All the other inguard regulatorcircuits depend directly or indirectly upon the + l7 V supply, as indicatedin the block diagramof Figure4-17, since+ l7

+F O R+ E X T E R N A LR E F .

. FOR . EXTERNALREF .

+F O RI N T E R N A LR E F .

INVERTING

A M P L I F I E R

LOWFOR+ EXTERNALREF .

H I G HF O R. E X T E R N A LR E F .

LOWFOR INTERNALREF .

R A T I O P O L A R I T Y ( S E E T A B L E 4 - 1 )

I,

HtGHFOR+ tNpUT

LOWFOR. INPUT

H I G HF O RA C I N P U T

Figure4-16. ReferencePolarity Logic.

+ t 5