HP Car 3490A manual L44. x 20Amplifier, ZercDetectAmplifier, 52AC.CONVEBTEB, ACAttenuators

SectionIV Model3490A

ELECTRONICINTEGRATING XzO AMPLIFIER ZERO

SWITCH AMPLIFIERDETECT

AMPLIFIER

INPUT FROM

OC AMP

ANALOG Z E R O D E T E C T TO

LOGIC

	Figure4-5. SimplifiedDiagram,IntegratorCircuits.
L44. x 20Amplifier.			450. IntegratorAuto Zero.
445. The output of the IntegratingAmplifier is applied			4-51. Any chargeremainingon the integating capacitor
			at the end of rundown must be removed before the
to an inverting amplifier having a gain of 20, whose
			beginning of the next measurement.The Auto Zero
output is limited to approximatelyt 0.6 V by a parallel
			Circuit accomplishesthis by connecting one input of the
complementarydiode connectionbetweenits input and
output. This output is then appliedto the Zerc Detect			Integrating Amplifier to ground and the other to the
			x 20 Amplifier output. Usingits own invertedoutput as
Amplifier. A feed-forwardconnectionis made from the
Integrator input to the non-invertinginput of the x 20			a reference, the Integator then dischargesthrough an
Atnplifier.	This reduces noise and switching transieq{s		RC circuit. Since the Integrating Amplifier is gounded
generatedat the integrator,sinceany noiseis appliedto			during Auto Zero, wry residualoffset in the Integrating
both inputs of the x 20 Amplifier simultaneously.		n	andx 20 Amplifiers will be stored in the Auto Zero 2 pF
			capacitor(seeFigure4-5). Duringthe following measure-
446. ZercDetectAmplifier.			ment, this.voltage stored in the capacitor cancelsthe
			amplifier offset.
447 . The	Zero Detect Amplifier is a high-gain opera-		4 . 52AC.CONVEBTEB .
tional amplifier. If the Integratorinput is positiveduring			z1-53.The AC Converterusedin the 3490A is an average
run-up, the Analog Zero Detect Output goes HIGH
			respondingcircuit, calibrated to the rms value of a
during run-up and returns to LOW at the Zero Detect
			sinusoidalinput. A simplified diagramis shown in Figure
point. If the input is negative,the Analog Zero DeLect
			4-6.
Output goesLOW during run-up and HIGH at the Zero
Detectpoint. A frxedpercentageof the referencevoltage			4-54.ACAttenuators.
is applied to the inverting input of the Zero Detect
Amplifier	to determine the level at which Zero Detect		4-55. Rangingin the AC function is accomplishedby
will occur. The Analog ZeroDetect output goesto the			attenuatingthe input sigral and adjustingthe amplifier
Zero Detectlogic circuits.			gain, which is inverselyproportional to the feedback
			voltage. Table 4-1 shows the attenuator and amplifier
448. ElectronicSwitch.			gainsfor eachrange.The converteroutput is + I Vdc for
			full-rangeinput on all ranges.
4-49. Two FET switchesat the input to the Integrator			4-56.ConverterAmplifier.
act as a sinele-pole
	double-throwswitch. During run-up
and rundown. the Integratorinput is connectedto the			4-57. A dual FET is used in the input stageof the
DC .tmplit-rer output, and at all other times it is			AC-to-DCConverterAmplifier to maintain a high input
coflreu-tcdto ground.			impedance.Two ac feedbackpathsareprovided,so that

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