HP Car 3490A manual +23. DCAmPlifier, +25. OCAmplifierGain, SwitchingCircuits, BoottrapCircuit

Model 34904

Attenuator is switched by reed relays which are con- trolled 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 accom- plished by FET switching circuits. These FETs 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 Figure 7-20,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 pro- vided 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 Attenua- tor 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 groundedthrough the Input Short FET switch C. The Auto Zero signalthen turns on FET switch J, placing the amplifier in a xl gain

4-3