SectionIII

Table3-1. MaximumVoltages.

 

Maximum

 

Voltage

Between Input HIGH and LOW

100OVrms

Between fi Signal HIGH and LOW

250 Vrms

Between either LOW terminal and Guard

2O0V peak

Between Guard and Chassis

500 V peak

3 . I7 . GENERALOPERATINGINSTRUCTIONS .3-18.Turn-OnandWarm-Up.

3-19. Before connectingac power to the 3490A, make sure the rear panel line selector switchesare set to correspondto the voltageof the availablepowerline as shownin Paragraph2-5 and Figure2-1.

3-20.Guarding.

3-21. Common-ModeVoltages.Common-modevoltages are those existingbetweenthe power line ground point of the source circuitry and that of the 34904, and between the Low measurementpoint and power line ground of the sourcecircuit. When current due to these voltages flows into the 3490A input terminals, some error in measurementresults becauseof the voltage drop in the measuringcircuit. In systemsmeasurements, the resistanceof long input leadsmay becomesignifi- cant. Wide separationbetween the 3490A power line ground point and the ground point of the sourcecircuiü, may result in high common-modevoltage.

3-22. Guard Connection.Figure 3-2 illustrates three ways of connectingthe 3490A Guardterminalto reduce errors causedby common-modevoltages.In exampleA, Guard is at practically the samepotential as the Low measurementpoint, so any common-modecurrentflows through Guard and not through the measurement circuit. In example B, the 3490A Guard strap is connectedto the Low terminal,placingboth at the same potential. This allows common-modecurrent to flow through the input lead resistanceR6, causing some measurementerror. This connection may be used if common-mode voltages are not expected to be a problem or if R6 is small (short leads).Example C is similar to A. except that connecting Guard in this manner allows any common-modecurrent generated between the Low measurementpoint and power line gound of the sourcecircuit to flow in the measurement circuit- Guard shouldalwoysbe connected,eitherto the Lou'terminal or to a point in the source circuit as rrdicated. If the guard terminal is left open, common- modevoltagesmay exceedthe Low-to-Guardbreakdown r3ringand damagethe 34904.

Model3490A

The Guard Terminal must always be connected to Low or to a corresponding point in the source circuit or damage to the instrument moy result.

3-23. GuardingInformation. More detailedinformation

on purpose and methods of guardingmay be found in

-hp-Application Note No. 123, "Floating Measurements and Guarding." This application note is available throughyour nearest-hp-Salesand ServiceOffice.

3-24.FloatingMeasurements.

3-25. The Model 3490A is capableof making flöating measurementsThat. is, the input Low terminal is not connectedto chassis(power line) ground. The voltage between the guard terminal and chassismust not be geater than 500 V peak.

A. Best Connection - Guard Connected To Low At Source.

r ---

--------

- -

 

4 4 9 0 4

 

EESTCONNECITON

SAME VOLTAGE

NO COMMONMOOECURRENI

GOES IHROUGH Rb

9!9!'!-9 _ _

B. Guard Connected To Low At Voltmeter.

sÄME VOLIACE

COMMONMOOECURRENT 6 O E S T B R O ü G HF b ,

CAUSIN6 ERRORS

9E0!{q__ _

C. Guard Connected To Earth Ground.

FLOATING

- -l

r'-

i*.J

DOITEO CONNECIION

DEFEAIS GIAROI MAY

OAMACEINSIqUMENT

SOLIOCONN € CION SHUNTS

MUCH OF COMMONMOOE

CURRENTAWAYFROM Rh,

PROVIDEDTHAT ECM 15 -

MOSILY 8 € TWEEN GROUNOS

GROUNO

Figure3-2. Connectingthe Guard.