Model 34904

Attenuator is switched by reed relays which are controlled by signalsfrom the DC Switching Logic circuits' No attenuation is used on the .l V, I V and 10 Vdc Ranges.Attenuation of 100 is used on the 100 V and 100Ö V ranges.No attenuationis requiredfor resistance

measurements,becausethe full-rangevoltageacrosstheresistancebeing measuredis not greaterthan l0 V. In ac voltage measurements,the AC Converter output is applied to the DC Amplifier through a FET (Field Effect Transistor)switch, and the dc attenuatoris not used.

+23. DCAmPlifier.

424.The DC Amplifier is a differential amplifier circuit using a FET input stageto provide an input resistance sreater than l0ro ohms on the .l V, I V and lOV i*g.r. Input resistanceon the 100V and 1000V ranges is l0MO, as determinedby the attenuator. The push- pull output stage is protected by diodes which prevent excessiveoutput current. The amplifier output is f or - 10 V full range for aU rangesand functions. In normal operation, this output is applied to the Integrator circuit. If optional Sample/Holdoperation is selected, the DC Amplifier output is applied to the Sample/Hold Analogcircuits.

+25. OCAmplifierGain.

4-26. T\'rcgain of the DC Amplifier may be l, l0 or 100, and is determinedby the feedbackpath selected. FET switching circuits are used to select the fee{back resistanceratios, and the gain is equalto FRI-I-FR2,as

shown in Figure 4-3.

FR2

 

 

 

 

OUTPUT

 

 

 

 

 

aMPLTFIEFCA]N- -- i -

 

A R I

 

 

 

 

 

9 . 9 M

 

 

 

 

 

 

 

 

F R I

 

 

A R 2

 

 

 

 

 

O . I M

 

 

 

 

 

 

 

 

GAN

FRI FR2

 

$

$

xr

o

roox

 

xro

90K

roK

a T T E N U a T T o N , a R 2

xroo

99K

x

 

 

Figure4-3. SimplifiedDiagram,DC Amplifier.

4-27.SwitchingCircuits.

4-28. Figure 4-4 shows the 3490A measurementse- quence.During the charging,or run-upperiod,the input sigtal is applied to the DC Amplifier input. During the discharge,or run{own period, the proper referenceis applied. Switching of thesesignalsand others such as a FeedbackAttenuator and Auto Zero circuits.is accomplished by FET switchingcircuits. TheseFETs are all contahed in one microcircuit package,A2UZ, and are driven by signalsfrom the DC Switching Logic circuits.

SectionIV

4-29.BoottrapCircuit.

4-30. The gate-to-sourcebias for the switching FETs is provided by a voltagewhich is bootstrappedto the DC Amplifier input voltage to provide the proper turn-on and turn-off bias.The input of the unity-gainBootstrap Amplifrer, shown in Figure7-20,is connectedto the DC Amplifier feedbackline.

l[.31. ReferenceVoltages.

4-32. lnternal reference voltages are provided for the run-down portion of the measurementsequence.In dc measurements,if the input voltageis positive,the - l0 V referenceis used;and if the input is negative,the + l0 V referenceis used.In acmeasurements,the AC Converter output is positive requiring the negative reference for run-down. The Sl reference,usedin resistancemeasure- ments, is generated by the Ohms Converter and is approximately- I V.

4-33. OverloadProtection.

4-34. Protecfion againstexcessiveinput voltage is provided at the DC Amplifier input. On the 10 V, 100 V and 1000V ranges,the input is limited by breakdown diodesto approximately 1 13 V. On the .1 V and I V ranges,the protection circuits are switchedto limit the input to the amplifier to approximately x 2 V. Overload protection is alsoprovided at the amplifier output by a diode circuit in parallelwith the feedbackattenuator.If, for example,the amplifier is operatingwith a gain of l0 or 100 and the output voltage exceedsapproximately

+l6 V, the diode circuit beginsto conduct,reducingthe amplifier gain,with a correspondingreductionin output voltage.If the amplifier is operatingwith a gainof l0 or 100, the gateof the xl gain FET (K in Figure 44)isar

-17 Y.If the amplifier output goesexcessivelynegative,

asit

-

gain FET beginsto turn

approaches 17 V, the xl

on, reducingthe amplifier gain and its output voltage.

€onsequently,the amplifier output is limited to approxi- mately1 16 V.

4-35.LeakageGontrol.

4-36. A FET switch (B in Figure 44) is alsoprovided which disconnectsthe three referenceFETs (F, G, H), the AC Converteroutput FET (E), and the .01 Attenuator FET (D) from the amplifier input during run-up when the xl Attenuator line is being used.This reduces the posibility of leakage from these circuits into the high impedanceDC Amplifier input circuit.

4-37.DCAmplifierAutoZero.

4-38. During certain portions of the measurementse- quence,as shownin Figure44,lhe DC Amplifier circuit is automaticallyzeroed.At the completionof a measure- ment, the amplifier input is groundedthroughthe Input Short FET switch C. The Auto Zero signalthen turns on FET switch J, placing the amplifier in a xl gain

4-3