SectionIV

Model 3490A

circuit fumishes the positive referencewhich is used in negative dc voltage measurements.A feedback circuit which sensesthe reference voltage at the FET switch input to the DC Amplifier ensuresthe correct voltageat that point, even though there may be switch and connector contact resistancein the circuit. The input to the Inverting Amplifier, which suppliesthe - Reference, is alsoconnectedto the sensepoint.

4-120. Plus Reference.The differentialamplifierof this supply is referenced to a zener diode which is in the emitter circuit of a feedback amplifier transistor. The diode and transistor are in the same package, for improved temperature stability. The amplifier gain is adjustedto providean accurate+ l0 V output.

4-121. Minus Reference.The - reference,usedin ac and positive dc measurements,is supplied by an inverting amplifier whose input is the + 10 V reference.The gain of this amplifieris adjustedto providean accurate- l0 V output.

4-122.RatioMeasrements(0ption080).

4-123. A 34904 equippedwith Option 080 is capable of making dc-to-dcand ac-to-dcratio measurementsThe. External ReferenceAmplifier has a gain of l0 when the Ratio switch is set to the I V range.This is necessaryto make the external referencecompatiblewith the reference switching logic, since in voltagemeasurements* and - l0 V referencesare used. Figure4-14 is a simplified diagramof the externalreferenceamplifiercircuits.

4-124. Extetnal Reference Amplifier. A microcircuit operational amplifier, connected in the non-inverting

I N P U T

PROTECTION

+ I 7 V

EXTERNAL

REFERENCE

AMPLIFIER

I N P U T

COMPENSATION

AMPLIFIER

mode, is used as a buffer amplifier for the external referenceinput. The gain of this amplifieris selectedby adjustingthe output of the FeedbackAmplifier. A third microcircuit amplifier provides current to compensate for bias current drawn by the input amplifier. The External Reference Amplifier input is clamped to approximately! 14 V to protect againstexcessiveinput voltage up to + 250 V. Greater input voltages may damage either the protection circuit or the Reference Amplifier. The amplifier output is limited to approximately t 14 V for the protectionof subsequentcircuits.

4-125. FeedbackAmplifier. An amplifier is usedin the feedback circuit so that the External ReferenceAmpli- fier gain on the 10 V Ext. Ref. range can be adjusted slightly above or below unity as required. On the I V Ext. Ref. range, the feedback is attenuated to give the Reference Amplifier a gain of 10, becausein ratio measurements,this referenceis substitutedfor the l0 V intemal reference.

4-126. Input CompensationAmplifier. Bias current at the input of the External Reference Amplifier could causean offset voltagedrop acrossthe input protection resistorif this current were not compensatedfor by the Input CompensationAmplifier. Figure4-15 showshow this is accomplished.The External ReferenceAmplifier maintains both its inputs at approximately the same voltage, so that the voltage at the inverting (feedback) input is essentially equal to the External Reference input. This voltage is connected to the non-inverting input of the input CompensationAmplifier, U2, which has a gain of l. The output of U2, then, is essentially equal to the External Referenceinput voltage, and may be r 0.1 V to + 12 V. The amplifier offset is adjusted

Figure4-14. SimplifiedDiagram,ExternalReferenceCircuits.