8of10
4. UsingtheSR555/SR556withaLockIn
PowerissuppliedtotheSR555/SR556viathe9pincablewhichissuppliedwiththeunit.Ifitisusedasa
pre‐amplifierforaSRS’lock‐inamplifier,attachoneendofthecabletotheconnectorontherearofthe
SR555/SR556.Withthelock‐inpoweroff,connecttheotherendofthecabletothe“PREAMP”
connectorontherearofthelock‐in.Bothendsofthecableareequivalent.Ifalongercableisrequired,
anystandardcablewithmale9‐pinD‐subconnectorsonbothsides,wiredpin‐to‐pin,willsuffice.
TheOutputAoftheSR555/SR556shouldbeconnectedtothe(A)Inputofthelock‐in.TheOutputB
shouldbeconnectedtothe(B)Inputofthelock‐in.Equal‐length,BNC‐terminatedcoaxcablesshouldbe
usedforbothconnections,andforbestresultsthetwocablesmaybetwistedtogethertominimize
inductivepickup.Thelock‐inInputSelectorshouldbesetto(A–B).Notethattheoutputofthe
SR555/SR556isabalanced,differentialsignalpresentedacrosstheAandBoutputs,makingitimportant
toreceivetheoutputsignaldifferentially.
Forapplicationsinwhichtheinputcurrentnodeshouldbeleftunbiased(0Vpotential),the“Bias
Enable”switchshouldbeleftinthedownpositionontheSR555/SR556.
Forapplicationsinwhichanon‐zeropotentialistobeestablishedattheinputcurrentnode,the“BIAS
IN”BNCconnectorisused.ThisisaDCinputthatprogramsthenon‐invertingnodeoftheinputopamp,
andsetsthevoltageofinputcurrentsignal.TheDCbiasvoltageissubtractedbythedifferential
amplifierandwillnotbesensedbythelock‐inamplifier.
Theoverallsensitivityofthelock‐inplusthepreamplifierisnotdisplayedonthelock‐infrontpanel.Itis
necessarytodividethedisplayedsensitivitybythepreamplifiergain:10V/AfortheSR555,or10V/A
fortheSR556.
5. UsingtheSR555/SR556withoutaLockIn
TheSR555/SR556canbepoweredwithanexternalpowersupply.Powerisappliedthroughthe9pin
connectorasdescribedinthetablebelow:
PinVoltageCurrent
1+20VDC±10%80mA
6–20VDC±10%50mA
7,8Ground
BothDCvoltagesarerequired.EitherPin7orpin8,orbothmaybeusedasground.Allotherpinsare
notused.Therearetwoprotectiondiodesconnectedbetweentheinternalregulatorsandthe+20Vand
‐20Vinputpins.
