Model3490A

numerical display; the length of time that the Data Counter is allowed to count, and the Data Counter output to the Timing Counter (divide by 10,000 or divide by 100,000 counts). In each range, one of six Timing Counter outputs determines the length of the count, and consequently,the most significantdigit. The position of the most significant digit is the result of the Data Counter output selected.If Main Timing Bit B is HIGH, the divideby 100,000output is selected,andif B is LOW, the divide by 10,000 output is used. If Main Timing Bit C is HIGH, the instrument upranges,and if C is LOW, it downranges.Both timing bits are set HIGH when the instrument reachesthe lowest range,and set LOW when the highestrangeis reached.

+080.024

+O4.0024

+o.2N24

+. 0 1 0 0 2 4

+0.09032

+t0.oo24

+200.024

+4000.24

+80002.4

+6000.xx OL

4-136. During the last measurementlisted above,the Transfer signalis switched on for 8 counts and off for 8 counts during the entire counting period, which lastsfor 1,600,000+ counts.Becauseof the countingfrequency, nearly all dots in the display appearto be on during this period. The I (in 1,600,000)is lost becausethe display does not provide a seventh digit. In all except the last reading, the two least significant digits are predictable becauseof known transition times during the reading.

+137.10V RangeZero,TestNo.2.

4.138. Test No. 2 selectsa DC Amplifier gain of I for both run-up and run-down, which is the l0 V range configuration. During run-up, the amplifier input 1s grounded through the Input Short FET switch (C in Figure 44). Either the + or - Reference is used for

run-down just as in a normal dc measurementwith zero input. If the amplifier and its auto zero circuits are operating correctly, the display should be zero.

4139.TurnoverError,TestNo.3.

4-140.Test No. 3 checksfor turnover errors, such as differences in value between the + and - Reference voltages, or amplifier offsets which result in differences between positive and negative readingswith inputs of

the same absolute value. For this tesi, the instrument usesthe + Ref for run-up and the - Ref for run_downin one measurement, and _Ref for run_up and + Ref for

run-downin the following measuremänt. The stored pola.rity of the run-up voltage in one measurementis usedto selectthe run-upvoltageof the oppositepolarity

for the next measurement.ti tne RAiustmenf proce_ dures,after the + Referencehas beenadjusted,TestNo.

3 is usedto adjustthe - Reference.

SectionIV

4141.+ ReferenceCheck,TestNo.4.4-142Tlnslest.comparesthevoltageat the+ RefFET

switch (F in Figure 44) Io a preciseinput voltage. The voltage through the + Ref FET is used for run-up, and the input voltage is usedfor run-down. If the instrument doesnot havethe ratio option, or if the RATIO switch is set to INT REF, this checks the internal + Reference voltage. With the RATIO switch set to EXT and the EXT REF input terminals shorted, Test No. 4 can be used in adjusting the External ReferenceAmplifier zero for both the I V and l0 V referenceranges.

+143.0.1VRangeZero,TestNo.b.

4-144. The 0.1 V Range is simulated for this test by setting the DC Amplifier gain at x 100 for run-up, and using the normal reference voltagesfor run-down. The amplifier input is grounded through the Input Short FET during run-up, and either the + or - Referenceis

usedfor run-down asin a normal voltagemeasurement. Test No. 5 checksoperationof the auto zerocircuits in the DC Amplifier.

4145.x .01Atten.x, 100Gain,TestNo.6.

4146. In Test No. 6, a precise- l0 V input voltageis first divided by 100 in the attenuator,then amplified by 100 in the DC Amplifier. The + Referenceis used for run-down as in a normal measurement.An incorrect readingin this test could indicate error in the attenua-

tion, the DC Amplifier gain adjustment,or a thermal offset. Test No. 6 is usedin the Adjustmentprocedures

for adjustingthe x 100 gain.

4147.0hmsReference,TestNo.7.

,l-148.In TestNo. 7, the signalat the Q Ref FET switch (G in Figure44) is amplifiedby l0 andcomparedto the

.tlOv Ref. The A Signal input terminals must be shorted to close the feedback loop of the e cunent source amplifier, thus generatingthe Referencevoltage acrossthe referenceresistor(seeFigure4-7). Becausea precisevoltage is not required for the fl Reference,the tolerance for this test reading is I 7000 counts. A correctreadingverifiesproper operationof most of the OhmsConvertercircuits.

+149.BCDREMOTEEXPAND0PTt0N020(Figure7-3r).4150.0utguardPowerSupply.

4-151. A microcircuit regulator suppliesa voltage of approximately + 5.3 V to the outguard Data Output, Remote, Trigger and Sample/Hold Trigger circuits. This voltage is shown on the schematic diagramsas + 5 VI, and is isolated from all inguard circuits. The outguard groundis alsoisolatedfrom the inguardcommon circuits and from chassis(earth) ground, and may be floated a maximum of 40 V abovechassissround.

+ 1 7